Dr S.J. Heyes

Lanthanides & Actinides

Textbook version by Nikolaos A. Parisis

3

Lanthanides & Actinides Dr S.J. Heyes

Four Lectures in the 2ndYear Inorganic Chemistry Course

Hilary Term 1998

If you have any comments please contact stephen.heyes@chem.ox.ac.uk

Introduction 1 Some Definitions and Viewpoints

2 Occurrence and History of the Elements

3 Nature of f-electrons

4 Electronic Properties

5 Periodicity in the f-Block

Lanthanides 1 Abundance & Distribution

2 Extraction

3 Separation

4 The Metals

5 Examination of the predominance of the 3+ Oxidation State

6 General Features of Lanthanide Chemistry

4

7 Solution Chemistry

Hydrated Ions Coordination Compounds Redox Chemistry - Oxidation States other than +3

8 Magnetism

Paramagnetism Ferro- and Ferri-Magnetism

9 Electronic Spectroscopy

Luminescence Phosphors Bioinorganic Chemistry Lasers

Solids

~ Includes: Structures, Magnetism, Spectroscopy, Electronic & Optical Properties, Technological Importance

10 Halides

LnX3 LnX2 SmI2 ~ a versatile organic reagent Lower Halides

11 Hydrides

LnH2 Hydrogen Storage

12 Oxides

13 Borides

14 Carbides

5

15 Organometallic Chemistry

Cyclopentadienides Cyclooctatetraenides Arenes Mixed Alkyl Cyclopentadienides

16 Comparisons/Contrasts

Is Y a rare earth? Should Sc be considered with the Lanthanides, Transition Metals or

as Main Group 3? Contrasts between Lanthanides & Transition Metals Similarities between Lanthanides & Other Elements

Actinides 1 Periodicity

2 Occurrence

3 Synthesis of Trans-Uranium Elements

4 Uses of Trans-Uranium Elements

5 The Metals

6 General Observations on Actinides Including Comparisons with Lanthanides & Transition Metals

7 Oxidation States

8 Aqueous Chemistry

9 Coordination Stereochemistries

6

10 Uranium Chemistry

Halides Hydrides Oxides Aqueous Chemistry

o especially the uranyl ion

11 Nuclear Technology

12 Organometallics

13 Super-Heavy Elements?

Problems Set

BIBLIOGRAPHY

Standard Textbook Accounts of increasing degree of sophistication:

J.E. Huheey, E.A. & R.L. Keiter, Inorganic Chemistry, 4th ed., HarperCollins, N.Y., 1993 p. 599-617

A.G. Sharpe, Inorganic Chemistry, 3rd ed., Longmans, London, 1992 Ch. 1, 26, 27

N.N. Greenwood & A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinman, London, 1997 Ch. 30, 31

F.A. Cotton & G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley, N.Y., 1988 Ch. 20, 21

C.S.G. Phillips & R.J.P. Williams, Inorganic Chemistry, OUP, Oxford, 1966 Vol. 2, Ch. 21, 22

7

Books on lanthanides and actinides

S.A. Cotton, Lanthanides and Actinides, Macmillan, London, 1991 pp. 191

Open University, The Chemistry of the Lanthanides and the Transactinium Elements, Open University Press, Milton Keynes, 1977 S304 (The Nature of Chemistry) Unit 27 pp. 61

G.T. Seaborg & W.D. Loveland, The Elements beyond Uranium, Wiley, N.Y., 1990 (An enjoyable account of trans-uranium elements by the discoverer of many)

J.J. Katz, G.T. Seaborg & L.R. Morss, The Chemistry of the Actinides, 2nd ed., Chapman & Hall, London, 1986 (a major reference work, see especially Ch. 14 Summary & Comparative Aspects)

Books with useful sections

D.A. Johnson, Some Thermodynamic Aspects of Inorganic Chemistry, 2nd ed., CUP, Cambridge, 1982 p. 158-193 (a very good account of the thermodynamic underpinning of lanthanide/actinide chemistry)

S.F.A. Kettle, Physical Inorganic Chemistry, Freeman, N.Y., 1996, Ch 11. (f-Electron Systems)

R.D. Cowan, Theory of Atomic Structure & Spectra, UCP, Berkley, 1981 p. 598-613 (Lanthanide/Actinide Configurations)

J.A. Cowan, Inorganic Biochemistry, VCH, Weinheim, 1993 p. 300-307 (Ln3+ probes in biology)

C. Elschenbroich & A. Salzer, Organometallics, a Concise Introduction, 2nd ed., VCH, Weinheim, 1992 p. 363-5 (Uranocene) p. 442-445 (organolanthanides)

T. Imamoto, Lanthanides in Organic Synthesis, Academic Press, London, 1994 parts of Ch. 4, 5, 6

P.A. Cox, The Electronic Structure and Chemistry of Solids, OUP, 1987, p.137-145 (Lanthanides)

A.R. West, Solid State Chemistry, Wiley, London, 1984 Ch. 16 (Magnetism), Ch 17. (Optical Properties)

K. Kosuge, Chemistry of Non-Stoichiometric Compounds, OUP, 1994 p. 219-230 (Hydrogen-absorption)

8

Article of Interest

W.B. Jensen, The Positions of Lanthanum(Actinium) and Lutetium(Lawrencium) in the Periodic Table, Journal of Chemical Education, 1982, 59, p. 634-636

9

Terminology Definitions?

Rare Earths

La-Lu and Y & Sc occur together in minerals ~ hard to separate from each other

{Th which occurs in many rare earth minerals is sometimes included in the definition}

Lanthanides

Usually taken to indicate the 15 elements from La-Lu or the 14 from Ce-Lu

Actinides

The 15 elements from Ac-Lw or the 14 from Th-Lw

Mnemonics:

Lanthanides

Special Feature - The ICL's 1998 Mnemonic Competition Winner

by Fergus Baillie, Peter Jelfs & Matthew Powell

Lanthanide Chemistry Presents No Problems Since Everyone Goes To Doctor Heyes's Excrutiatingly Thorough Yearly Lectures

10

Actinides

Some Viewpoints

"Lanthanum has only one important oxidation state in aqueous solution, the +3 state. With few exceptions, this tells the whole boring story about the other 14 Lanthanides"

G.C. Pimentel & R.D. Sprately,

"Understanding Chemistry",

Holden-Day, 1971, p. 862

"These elements (rare earths) perplex us in our researches, baffle us in our speculations and haunt us in our dreams. They stretch like an unknown sea before us - mocking, mystifying and murmuring strange revelations and possibilities"

Sir William Crookes

Address to the British Association, 1887

Due to the increasing technological importance of the rare earths William Crookes's view seems to be returning to fashion!

11

Where/how do rare earths/actinides fit into the PERIODIC TABLE? 1751

Swedish Mineralogist A.F. Cronstedt discovers a new heavy mineral

1789

Klaproth shows pitchblende contains a new element, uranium

isolated as a metal in 1841 by Peligot

1794

J. Gadolin isolates "yttria",

but thought it was the pure oxide of Y

1794-1907

ca. 100 claims of elements in the rare earth group, due to:-

problems of separation (very similar properties)

lack of conclusive tests as to whether a mixture was involved

12

pre-1905

Mendeleevian Periodic table could only accommodate 1 element for this group, La

at one stage W. Crookes thought that apparently different rare earths were "simply different modifications of the same element."

Alfred Werner produced a Periodic table in 1905 of essentially current form, (though he did put spaces for 15 elements between La & Hf - there was no viable atomic theory at that time!)

1913

X-ray spectroscopy

W. Moseley shows from characteristic X-ray patterns conclusively that there are only 14 elements between La & Hf

1918-1921

Bohr interprets this as expansion of 4th quantum group from 18 to 32 e-

Lanthanides are therefore identified as the first f-series

1939-45

Lanthanides recognised in fission products of uranium

Techniques to obtain lanthanide separation greatly improved

pre-1940

of the actinides only U(1789), Th (1829), Ac(1889), Pa (1913) are known

post -1940

The nuclear age ~ uranium and plutonium chemistry

13

1940-1961

Trans-uranium elements synthesized by "bombardment" expts.

Actinides developed as the 5f series equivalent of the lanthanides (Seaborg's Actinide Concept), & proven not to be a 4th transition series

post-1955 Lanthanides obtained in increasing amounts ~ increasing technological importance

14

f-orbitals (l=3) 4f Orbitals (General Setting)

fz3 = fz(2z2 - 3x2 - 3y2) [ml = 0]

fxz2 = fx(4z2 - x2 - y2) & fyz2 = fy(4z2 - x2 - y2) [ml = 1]

,

fxyz & fz(x2-y2) [ml = 2]

,

fx(x2-3y2) & fy(3x2-y2) [ml = 3]

15

,

The different f-orbitals are shown here with their representation as general and xy-plane projections

16

Why are 4f orbitals filled when they are? Thomas-Fermi explanation

The Effective Electron Potential:

Large angular momentum for an f-orbital (l = 3) large centrifugal potential

tends to keep the electron away from the nucleus Aufbau order

Increased Z increases Coulombic attraction to a larger extent for smaller n

due to a proportionately greater change in Zeff reasserts Hydrogenic order

This can be viewed empirically as due to differing penetration effects

Radial Wavefunctions Pn,l2 for 4f, 5d, 6s in Ce

4f orbitals (and the atoms in general) steadily contract across the lanthanide series

17

Effective electron potential for the excited states of Ba {[Xe] 6s 4f} & La {[Xe] 6s 5d 4f}

show a `sudden' change in the broadness & depth of the 4f "inner well"

for Ba (Z = 56) 4f is an outer orbital with 4f close to its value for the H atom

for La (Z = 57) 4f is an inner orbital with 4f ca. 0.7ao

though only for the next atom, Ce (Z = 58) is the 4f electron of sufficiently high binding energy to appear in the ground state configuration

18

Which Elements are d-block or f-block? Most current Periodic tables have:-

La as first 5d transition element Ac as first 6d transition element

Reasons?

possibly erroneous (early) interpretation of atomic spectra misleading electronic configurations?

Some ground state electronic configurations Calcium [Ar]4s2 Scandium [Ar]4s23d1

Strontium [Kr]5s2 Yttrium [Kr]5s24d1

Barium [Xe]6s2 Lanthanum [Xe]6s25d1

Ytterbium [Xe]6s24f14 Lutetium [Xe]6s24f145d1

Radium [Rn]7s2 Actinium [Rn]7s26d1

Nobelium [Rn]7s25f14 Lawrencium [Rn]7s25f146d1

In each case: Differentiation by (n-1)d1 ~ as expected for the start of a transition series

Lutetium and Lawrencium are just as good candidates to be the first elements of the 3rd and 4th transition series as Lanthanum and Actinium

Some suggestions why Lu might best regarded as the first 5d transition element. Periodic Trends in Various Properties

19

Structures of Metal, Metal Sesquioxide (M2O3) and Metal Chloride (MCl3)

Similarities for Sc, Y, Lu

Differences from La

Revised Medium-Block Format Periodic Table See the article:- W.B. Jensen, The Positions of Lanthanum(Actinium) and Lutetium(Lawrencium) in the Periodic Table, Journal of Chemical Education, 1982, 59, p. 634-636

21

LANTHANIDES Abundance & Distribution Not especially Rare!

except Prometheum (147Pm, t1/2 = 2.6 years) which is produced artificially

e.g. La, Ce & Nd are more common than Pb

"Rare Earth" label not really justified today

Most-common minerals:

monazite & xenotime (mixed La, Th, Ln phosphates) widely-distributed, concentrated in sand & river beds due to relative insolubility

bastnaesite (a La, Ln fluorocarbonate MIIICO3F) a vast deposit in Sierra Nevada, USA ~ discovered in 1949 ~ supplies much of world's needs

60-70% of the metal content of these minerals is rare earth oxide

Elemental Proportions of Rare Earth-Content of Minerals

% of Ln as: Th Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lumonazite ca 6 3 22 45 5 17 4 0.1 2 0.2 1 0.1 0.4 - 0.2 -

xenotime ca 5 61 0.5 5 0.7 2.2 1.9 0.2 4 1 8.6 2 5.4 0.9 6.2 0.4basnaesite 0.05 0.1 32 49 4.4 13.5 0.5 0.1 0.3 - - - 0.1 - - -

Abundance of lanthanides in nature

abundance shows even-odd alternation with atomic number mirrored by several/few alternation of number of stable isotopes with

even/odd Z

22

Due to nuclear shell structure (see : P.A. Cox, The Elements, OUP, 1989, p. 17, 36-44)

Extraction The extraction of the Lanthanides from minerals is illustrated here by the:-

Alkali Digestion of Monazite/Xenotime

23

Monazite and Xenotime can also be opened-out through an Acid Route

Acid Dissolution of Bastnaesite

The website of Molycorp explains the mining and processing of Bastnaesite at their Mountain Pass Mine in the Sierra Nevada, USA.

24

Separation 2/3 of world production is actually used mixed in the proportions occuring naturally in the ore

1. Cerium & Europium may be extracted Chemically:

Oxidize only Ce to M4+ by HClO or KMnO4, then precipitate as CeO2 or Ce(IO3)4

On action of Zn/Hg only Eu forms a stable M2+ that doesn't reduce H2O, then isolate by precipitation as EuSO4

2. Separation by Fractionation: Small Scale methods used originally:

Fractional Crystallization of e.g. Ln(NO3)3.2NH4NO3.4H2O or Ln(BrO3)3

Fractional Thermal Decomposition of e.g. Ln(NO3)3

Current Small Scale Lab. Separation:

Ion-Exchange Displacement Column

Ln3+(aq) are strongly adsorbed by a cation-exchange resin add an eluant ligand

25

typically chelating ligands

e.g. EDTA , or 2-hydroxy-EDTA

e.g. HIB{[[alpha]]-hydroxyisobutyric acid}

Ligand binds most strongly to smallest ion

e.g. the binding constants of the Ln(EDTA) complexes

Elution order is Lu La The process of separation is indicated graphically

26

greater detail on these columns may be found in:-

Greenwood & Earnshaw, p. 1427-1428 Open University S304 Unit 27, p. 20-22

Current Large Scale Industrial Separation:

Solvent Extraction

Ln3+(aq) is extracted in a continuous counter-current process into a non-polar organic liquid (e.g. kerosene)

the kerosene contains ca. 10% of o bis(2-ethylhexyl)phosphinic acid (DEHPA)

or

o tri-n-butylphosphine oxide (TBPO) (nBu3O)3PO

1. solubility of Ln3+ in organic solvent increases with its RAM 2. separation factor for adjacent rare earths = 2.5 3. automatic multistep, counter-current conditions 99.9% purity Ln

27

The Metals

Production of Elemental Metals

La, Ce, Pr, Nd, Gd:

2MCl3 + 3Ca 2M + 3CaCl2 (T > 1000 C)

Tb, Dy, Ho, Er, Tm, Y:

2MF3 + 3Ca 2M + 3CaF2 (MCl3 is too volatile)

Pm:

PmF3 + 3 Li Pm + 3LiF

Eu, Sm,Yb:

M2O3 + 2La 2M + La2O3 (MCl3 reduced to MCl2 by Ca)

Mischmetall (mixed light Ln) :

electrolysis of fused LnCl3/NaCl with graphite anode & graphite or steel cathode

Structures of Elemental Metals

Example Structures:- View using CrystalMaker

Ce, CCP Structure

28

Gd, HCP Structure

Pr, hc(4H) Structure

Eu, BCC Structure

Properties of the Metals

Silvery white, but tarnish in air Rather soft (later M are harder) High mpt & bpt Very reactive (I1 + I2 + I3) comparatively low

o (reactivity as atomic radius ) + H2O M2O3 or M(OH)3 & liberates H2 ~ slowly in cold, rapidly

on heating + H+ (dilute acid) Ln3+ & liberates H2 ~ rapid at room

temperature burn easily in air ~ slowly in cold , burn at T > 150 C

o sesquioxides, M2O3

(but Ce CeO2) & (Y is passivated below 1000 C due to its oxide coating)

o nitrides, MN exothermic reaction with H2 MHn (n = 2,3, often results in defect

states) react readily with C, N2, Si, P, halogens & other non-metals

form binaries on heating with most non-metals (e.g. LnN, Ln2S3, LnB6, LnC2, ...)

Uses of Metals

29

Chemistry is principally of Ln3+ Why the prevalence of oxidation state III (Ln3+)?

Examine Thermodynamic Parameters:

I1/2/3/4 atmH hydH(Ln3+) LH(LnX3) these values are available in a Table(import DHatm from larger table for web!)

Ionization For any given Lanthanide

As successive electrons are removed from neutral Ln the stabilizing effect on the orbitals is related to their principal quantum number, 4f > 5d > 6s.

For Ln2+ {except for La & Gd} the configuration is [Xe]4fn

For Ln3+ the configuration is always [Xe]4fn

the 4f binding energy is so great that remaining 4f electrons are regarded as "core-like" (i.e. incapable of modification by chemical means) (except Ce)

Note that as a rule of thumb: I4 ~ 2 I3 ~ 4 I2 ~ 8 I1

I4 > (I1 + I2 + I3 )

Therefore in almost all cases Ln3+ provides the best energetics

30

Observing trends across the Lanthanide Series

The general trend is for increasing ionization energies with increasing Z (i.e. with increase in Zeff)

Marked Half-Shell Effects - magnitude as n in In

Also Quarter/Three-Quarter Shell Effects

(compare with transition metals - these are not seen clearly with dn configurations)

Explanation?: interelectronic repulsion is related not just to electron pairing but also to angular momentum of the electrons

o e.g. in Pr2+ (4f3) Pr3+ (4f2) ionization removes repulsion between e- of like rotation, whereas Pm2+ (4f4) Pm3+ (4f3) removes the stronger repulsion between e- of unlike rotation ( latter Ionization Energy is correspondingly lower - hence the local minimum in the I3 graph at Pm)

The three-quarter effect is the bigger: interelectronic repulsion is bigger in smaller Lnn+

31

Atomization atmH follows the inverse trend to I3 {and therefore also to (I1 + I2 + I3 ) }

metallic bonding is correlated with ease of ionization to Ln3+ state

this trend is modified slightly due to the different structures of the Ln metals

Some Thermodynamic Observations (Ionic Model style)

The trends in the formation of LnIII

Formation of Compounds {fH(LnX3(s))} or Ln3+(aq) {E(Ln3+(aq)/Ln(s))}

depend on the balance between:

Energy Supplied to effect Ln(s) Ln(g) Ln3+(g) + 3e- [atmH + I1 + I2 + I3]

versus

Energy gained from Ln3+(g) + 3X-(g) LnX3(s) [LH(LnX3(s))] or Ln3+(g) Ln3+(aq) [hydH(Ln3+)]

The energies determining trends in E(Ln3+(aq)/Ln(s)) are graphed below:

32

Production of Ln3+(g) shows

a smooth trend based on size effects (trend based on Zeff) shell structure effects superimposed with clear maxima at half-shell

(f7) and full-shell (f14)

also smaller quarter and three-quarter shell effects

Hydration Energy of Ln3+ (also Lattice Energies of LnX3(s)) shows

only a smooth ionic-size-based trend (the trend based on Zeff) and no shell structure effects

Balance of trends in Ionization + Atomization Energies with Hydration (Lattice) Energy

removes size effects leaves only the Shell effects - see values of fH(Ln3+(aq))

Overall: The most important energy correlations are with I3

"exceptions to +3 rule" can also be rationalized

Occurrence of +4 oxidation state

predicted from [atmH + I1 + I2 + I3 + I4] which follows trends in I4

33

Ce, Pr Ce4+ [4f0], Pr4+ [4f1] ~ early in series 4f orbitals still comparatively high in energy

Tb Tb4+ [4f7 valence shell] ~ half shell effect

Occurrence of +2 oxidation state

predicted from [atmH + I1 + I2] which follows trends in atmH, which is reverse of trend in I3

Eu, Sm, Yb Eu2+ [4f7], Sm2+ [4f6], Yb2+ [4f14]

~ clear influences of electronic shell structure & from atmH

34

General Features of Lanthanide Chemistry The general features of lanthanide chemistry are reviewed before they are examined in more detail

1. similarity in properties, with gradual changes occurring across the lanthanide series

a size effect from the Lanthanide Contraction

Causes: o Poor screening of nuclear charge by 4f electrons

steady increase in Zeff o Relativistic effects influence the shielding

characteristics of inner electrons

see: K.S. Pitzer, Accounts of Chemical Research, 1974, 12, p. 271-276

2. Primarily the 3+ oxidation state adopted for all elements

Redox chemistry is commonly encountered only for Eu (3+/2+) and Ce (4+/3+)

Some solids formulated as LnII compounds actually contain Ln3+ & delocalized e-

35

3. Coordination chemistry is not especially extensive Chelating ligands are preferred

4. Bonding on coordination is primarily ionic in character

complexes undergo rapid ligand exchange

Why is the bonding so ionic? 4fn electrons are contracted into the core and unable to participate in bonding

Other implications from lack of covalent bond-forming orbital-availability

no -backbonding occurs

no simple carbonyl species (except in Ar matrix at 10 K)

cyclopentadienyls are ionic in nature [c.f. Ln(C5H5)3 vs Fe(C5H5)2]

lanthanide organometallics have different properties from transition metal equivalents

5. Ln3+ cations display typical a-class (hard) properties

preference for O-donor ligands Qu. Why not N too? Ans. O-donor ligands are more likely to be

charged(importance of ionic bonding to lanthanides!)

6. Binding to water is common such that H2O is often found included in products isolated from (aq)

36

7. Coordination numbers are high >6, typically 8, 9,... (up to 12 found)

8. Coordination polyhedra are often ill-defined determined by ligand requirements, not by bonding requirements no confirmed examples of isomerism Solid state structures of binaries are often rather different from

those of other metals

9. Ligand Field Effects are very small Pale Colours from weak, narrow forbidden f f optical transitions

M3+ ions are :-

Colourless (La, Ce, Gd, Yb, Lu) Green (Pr, Tm) Lilac (Nd, Er) Yellow Pink (Pm, Ho) Yellow (Sm, Dy) Pale Pink (Eu, Tb)

Magnetic properties have spin-orbit coupled contributions

(spin-orbit coupling >> ligand field splittings)

Ln3+ magnetic moments: 0 B [1S0] to 10.55 B [5I8]

Magnetic & Optical properties are largely independent of environment (e.g. similar spectra in gas/solution/solid)

Renewed Technological interest in Lanthanides is mainly in optical/magnetic materials

37

Solution Chemistry Solubility

Ln3+ are not especially soluble in water no simple relationship of solubility to cation radius

o depends on small difference between large solvation and lattice energies

o depends on entropy effects oxalates/double sulfates/double nitrates lighter Ln more soluble basic nitrates heavier Ln more soluble

basis of classical separation procedures

Hydrated Lanthanide Ions

Primary hydration numbers in (aq) are 8, 9

from Luminescence & NMR measurements

Primary hydration number with Lanthanide Contraction Secondary hydration number with Lanthanide Contraction

o increased polarization of 1ry hydration sphere by a smaller cation enhances hydrogen bonding to water in the secondary hydration sphere

Aqua ions hydrolyze ~ increasingly so from La to Lu as they become smaller

o Ln(H2O)n3+ + H2O Ln(H2O)n-12+ + H3O+ Salts with common anions frequently contain Ln(H2O)93+

with tri-capped trigonal prismatic (ttp)

geometry

Coordination Compounds

38

Strongly complexing, chelating ligands necessary to yield isolable products from (aq)

Some O-donor chelating ligands that form complexes include:

NO3-

o binds in its chelate mode o notable for high Coordination Numbers

! Ce(NO3)52- 10-coordinate bicapped dodecahedron

Ce(NO3)63- 12-coordinate icosahedron

Oxalate

Citrate

Tartrate

-diketonates R(CO)(CH-)(CO)R

39

Classic bidentate complexes formed as Ln(L-L)3L' (L' = H2O, py, etc...) or Ln(L-L)4

o dehydrate Ln(L-L)3(H2O) in vacuo -> Ln(L-L)3 now coordinatively unsaturated!

o Ln(L-L)3 with bulky R ! thermally stable ! volatile & sublimable ! soluble in non-polar solvents ! Widely used as NMR shift reagents

polar molecules may coordinate {Ln(L-L)3}, their NMR resonances are perturbed by the paramagnetism of Ln

e.g. Eu(facam)3

"Anti-knock" activity as petroleum additives

Macrocyclic Ligands

Crown Ethers

o differing cavity/M sizes allow range of C.N & stoichiometry

e.g. with 18-crown-6

La-Gd [Ln(18-C-6)(NO3)2]3[Ln(NO3)6] 10-coordinate

La-Nd [Ln(18-C-6)(NO3)3] 12-coordinate

Tb-Lu [Ln(NO3)3(H2O)3](18-C-6) 18-C-6 is not coordinated ~ cavity too large

40

Complexes with o uncharged monodentate ligands o ligands with donor atoms other than O

MUST be prepared in the absence of H2O

e.g. Ln(en)43+ (8-coordinate) from polar organic solvents (e.g. acetone)

(18-crown-6) complexes dissociate instantly in water!

Coordination by halides is weak

but LnX63- (unusually octahedral) are isolable from non-aqueous solution

Solution Chemistry of Other Lanthanide Oxidation States Ln(IV)

Cerium is the only Ln4+ with significant aqueous or coordination chemistry

E (Ce4+(aq)/Ce3+(aq)) = 1.72 V (others est. 2.9 V)

prepared by the action of a strong oxidizing agent, e.g. S2O82-, on Ce3+(aq)

widely used as an oxidant itself:- e.g. quantitative analysis / organic chemistry

E (Ce4+/Ce3+) is markedly dependent on complexation and hydrolysis

o strong oxidizing agent in perchloric acid solution o in other acids coordination occurs

e.g. Ce(NO3)62- is generally used for oxidations as its NH4+ salt

o on pH: 1. hydrolysis to Ce(OH)3+ occurs 2. then polymerization 3. ultimately precipitation of yellow gelatinous CeO2.xH2O

41

4+ charge stabilizes halogeno-complexes e.g. CeF84-

{aside: CeF62- is 8-coord through fluoride-bridging, but CeCl62- is octahedral}

Ln(II)

Significant solution chemistry of Ln2+ is essentially confined to SmII, EuII, YbII

Preparation:

electrolytic reduction of Ln3+(aq) Eu2+ (the most stable LnII) is prepared by reduction of Ln3+(aq) with

Zn/Hg

Properties

Ln2+ Aquo-ion colours o Sm2+ blood-red o Eu2+ colourless o Yb2+ yellow

Ln2+(aq) are readily oxidized by air o BUT Eu2+(aq) is easily handled

Sm2+(aq) & Yb2+(aq) reduce water Eu2+(aq) is relatively stable in the dark Carbonate and sulfate salts have been isolated Sm2+ and Yb2+ salts are susceptible to oxidation by their water of

crystallization Eu & Yb dissolve in l-ammonia to give intense blue, highly

reducing solutions o contain [Ln(NH3)x]2+ and solvated electrons? o Solutions decompose on standing, precipitating the amide

Ln(NH2)2 Properties of Ln2+ are closely-related to those of the alkaline earths

In particular Eu2+ is often likened to Ba2+

1. Similar Salt Solubilities (like Ba, sulfates are insoluble, hydroxides are soluble)

2. Behaviour in l-NH3 is very similar 3. Similar Coordination Chemistry (Not extensive / Hard

ligands) 4. But Very different redox chemistry!

42

Magnetism & Spectra config. Ground No. of Observed

Ln Ln3+ State unpaired e- Colour eff/B La 4f0 1S0 0 colourless 0 0 Ce 4f1 2F5/2 1 colourless 2.54 2.3 - 2.5 Pr 4f2 3H4 2 green 3.58 3.4 - 3.6 Nd 4f3 4I9/2 3 lilac 3.62 3.5 - 3.6 Pm 4f4 5I4 4 pink 2.68 - Sm 4f5 6H5/2 5 yellow 0.85 1.4 - 1.7 Eu 4f6 7F0 6 pale pink 0 3.3 - 3.5 Gd 4f7 8S7/2 7 colourless 7.94 7.9 - 8.0 Tb 4f8 7F6 6 pale pink 9.72 9.5 - 9.8 Dy 4f9 6H15/2 5 yellow 10.65 10.4 - 10.6 Ho 4f10 5I8 4 yellow 10.6 10.4 - 10.7 Er 4f11 4I15/2 3 rose-pink 9.58 9.4 - 9.6 Tm 4f12 3H6 2 pale green 7.56 7.1 - 7.6 Yb 4f13 2F7/2 1 colourless 4.54 4.3 - 4.9 Lu 4f14 1S0 0 colourless 0 0

Magnetic Properties

Paramagnetism see R.L. Carlin, Magnetochemistry, Springer, N.Y., 1986 Chapter 9 for a detailed account

Magnetic properties have spin & orbit contributions

(contrast "spin-only" of transition metals)

Magnetic moments of Ln3+ ions are generally well-described from the coupling of spin and orbital angular momenta ~ Russell-Saunders Coupling Scheme

spin orbit coupling constants are typically large (ca. 1000 cm-1)

ligand field effects are very small (ca. 100 cm-1)

43

only ground J-state is populated

spin-orbit coupling >> ligand field splittings

magnetism is essentially independent of environment

Magnetic moment of a J-state is expressed by the Land formula

Sample Land Calculation

e.g Pr3+ [Xe]4f2 Find Ground State from Hund's Rules

o Maximum Multiplicity S = 1/2 + 1/2 = 1 2S + 1 = 3

o Maximum Orbital Angular Momentum L = 3 + 2 = 5 H state

o Total Angular Momentum J = (L + S), (L + S) - 1, L - S = 6 , 5, 4

! Less than half-filled sub-shell Minimum J J = 4

{Greater than half-filled sub-shell Maximum J}

44

o Experiment? 3.4 - 3.6 B

Ln3+ Magnetic Moments compared with Theory Experimental _____ Land Formula --- Spin-Only Formula - - -

45

Land formula fits well with observed magnetic moments for all but SmIII and EuIII

Moments of SmIII and EuIII are altered from the Land expression by temperature-dependent population of low-lying excited J-state(s)

Uses of Ln3+ Magnetic Moments?

NMR Shift Reagents - paramagnetism of lanthanide ions is utilized to spread resonances in 1H NMR of organic molecules that coordinate to lanthanides (see account of Eu(fcam)3)

Ferromagnetism / Anti-Ferromagnetism / Ferrimagnetism

see C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, CUP, 1986 p. 394-398

West, Solid State Chemistry p. 565-566, 575-578

Lanthanide metals and alloys have interesting ordered magnetism effects

SmCo5 permanent magnets - FERROMAGNETIC o light weight o high saturation moments, o high coercivity o high magnetocrystalline anisotropy o Superior performance magnets for magnetic bearings /

couplings / wavetubes & d.c. synchronous motors

46

Garnets o Complex oxides A3B2X3O12

A site distorted cubic environment

B /X sites octahedral & tetrahedral sites

[unit cell of Garnet contains 128 atoms!]

o Rare Earth Garnets e.g. Ln3Fe5O12 and Y3Fe5O12 (yttrium iron garnet, YIG)

FERRIMAGNETISM shows an unusual temperature-dependence

as T

o moment to zero at the Condensation Temperature

o above Condensation Temperature moment rises in the opposite direction to a maximum

o moment then to zero at the Curie Temperature.in the normal manner

47

Reason?

the magnetic moments of the rare earth and iron ions oppose each other

the rare earth moments dominate at low temperature

the rare earth moments randomize at a lower temperature than the iron moments

"Magnetic Bubble" Memories o magnetic stripe domains which shrink to smaller cylinders in

an applied field o Called bubbles because they follow the same equations as

soap bubbles o Magnetic Memory since bubble = 1, and lack of bubble = 0

(binary) o 'Bubbles' move towards regions of lower field bias and don't

coalesce ! magnetic medium doesn't need to move (unlike disks

or tapes!) ! typically the bubbles are moved along Ni-Fe tracks

o Best bubble memory materials are 20 m films of rare-earth Garnets, Ln3Fe5O12

! Smallest bubble sizes (2-3 m) in (Sm0.51Lu0.42Y1.21Ca0.86)(Ge0.70Si0.16Fe4.14)O12

48

How a Magnetic Bubble Memory Works

49

Electronic Spectroscopy Transitions which involve only a redistribution of electrons within the

4f orbitals (f f transitions) are orbitally-forbidden by the Selection Rules

pale colours of LnIII compounds are usually not very intense

Crystal/Ligand field effects in lanthanide 4f orbitals are virtually insignificant

4f electrons are well shielded from external charge by 5s2 & 5p6 shells

f f absorption bands are very sharp (useful fingerprinting and quantitation of LnIII)

[dd transitions in transition metal compounds are also orbitally forbidden, but gain intensity from and are broadened by the effects of molecular vibrations in distorting the crystal field]

optical spectra are virtually independent of environment

- similar spectra in gas/solution/solid (sharp lines like typical gas atom spectra)

Insensitivity of f f transitions of limited use in study of lanthanide materials

50

CeIII and TbIII have high intensity bands in the UV

due to 4fn 4fn-15d1 transitions i.e. f d and therefore not orbitally forbidden

why? n-1 =0 (empty sub-shell) for CeIII = 7 (half-filled sub-shell) for TbIII

Fluorescence / Luminescence of certain lanthanides e.g. Tb, Ho & Eu [see West, Solid State Chemistry Chapter 17]

Luminescence: emission of light by material as a consequence of its absorbing energy

Photoluminescence: use of photons for excitation

Photoluminescent materials generally require a host [H] crystal structure, doped with an activator [A] {sometimes a second dopant is added to act as a sensitizer [S]}

Fluorescence: short time lapse (~ 10-8 s) between excitation & emission

Phosphorescence: long decay times luminescence continues long after excitation source is removed

51

Applications for Fluorescence/Phosphorescence

e.g. red phosphor in TV tubes from Eu3+ doped into Y2O3 other luminescence colours:-

o green: Ce3+ in CaS Eu3+ in SrGa2S4 o blue: Eu2+ in (Sr,Mg)2P2O7

mixed red/green/blue effectively white luminescence

used in fluorescent lamp coatings to convert blue/UV discharge to white light

Anti-Stokes Phosphors have the remarkable property of emitting photons of higher energy than the incident exciting radiation!

e.g. YF3 host doped with Yb3+ as a sensitizer and Er3+ as an activator can convert incident IR radiation into green luminescence

Luminescence of Eu3+ is used to probe its environment

ligand charges / binding constants / ligand exchange rates / site symmetry

Ln3+ as a Probe for Ca2+ Sites in Bioinorganic

52

Chemistry

Ln3+ may replace Ca2+ in its binding sites in proteins

o Similar ionic radii o Coordination number of Ln3+ (7-9) close to Ca2+

(6-8) o Hard metal ions / Prefer oxygen ligation (Ca2+

bound by O of Glu, Asp, Thr, Ser, H2O...) o Ln3+ binds ligands ca. 105x more strongly than

Ca2+ o Ligand exchange rates on Ln3+ are ca. 102x

slower than on Ca2+ o When Ln3+ replaces Ca2+ at a catalytic site

reaction rates decrease

(explains mild toxicity of rare earth ions!)

Use of Luminescence Spectra

Eu3+ (green) & Tb3+ (red) luminesce strongly at ca. 296 K after laser excitation

because excitation may occur strongly to excited ligand states which are just above the Ln3+ excited states involved in the luminescence for these ions, which are therefore easily populated

o Determine Number of H2O molecules bound to the active-site metal ion

e.g. for the protein thermolysin

! Eu3+ luminescence 1H2O for Ca2+ site 1

53

and 3H2O at sites 3 & 4

o The different sites may be replaced independently with different Ln3+

Energy transfer expts. (e.g. Eu3+Tb3+) intersite distances

Use of Paramagnetism

o Ln3+ bound in a metallo- site acts as NMR shift/relaxation agent

active site protein geometry from 1H NMR spectra

Use of Electron Density

o Ln3+ Heavy atom derivatives to assist solving X-ray diffraction structures

Lasers

One of the most common high power lasers is the Neodymium YAG laser

o Host material is Yttrium Aluminium Garnet (YAG), Y3Al5O12, doped with Nd3+

o Examine the garnet structure (also considered for its magnetic properties in e.g. YIG)

o a '4'-Level Laser System

54

Change of host material makes small differences in laser radiation frequency

Change of dopant ion makes large changes in laser radiation frequency

Halides Halides of the form LnX2, LnX3 & LnX4 {only (Ce,Pr,Tb)F4} exist

LnX4

Only (Ce, Tb, Pr)F4 are known o correlation with I4 of Ln o fluorides only - most oxidizing halogen!

CeF4 is comparatively stable e.g. crystallizes as a monohydrate from (aq)

TbF4, PrF4 are thermally unstable and oxidize H2O o i.e. prepare dry!

55

MF4 all white solids with the UF4/ZrF4 structure

o Dodecahedral coordination of M

LnX3

All LnX3 are known (except Pm {not attempted} & possibly EuI3) Typically crystalline / high mpt. / deliquescent Typically obtained as hydrates from (aq) e.g. La - Nd 7H2O

Nd - Lu 6H2O

On heating, react with water oxyhalides LnX3 + H2O LnOX + 2HX

at high temperatures react even with glass 2LnX3 + SiO2 2LnOX + SiX4

Preparation of anhydrous LnX3

LnF3

1. Ln(NO3)3(aq) + 3HF LnF30.5H2O (very insoluble) 2. heat LnF30.5H2O (under an HF atmosphere for heavy Ln)

anhydrous LnF3

LnCl3

3. Ln2O3 / Ln2(CO3)3 + HCl(aq) LnCl36-8H2O (rather soluble) 4. heat LnCl36-8H2O (under HCl atmosphere for heavy Ln)

anhydrous LnCl3

or

Heat at 300C, Ln2O3 + 6NH4Cl 2LnCl3 + 3H2O + 6NH3

56

LnBr3 / LnI3

5. Best by direct combination (susceptible to hydrolysis to oxyhalides)

6. purify by sublimation (but avoid contact with hot silica!)

Structures: Ln coordination varies from 9 for light trifluorides to 6 for heavy iodides

e.g. LnCl3 La - Gd

UCl3 structure

9-coordinate Ln

tri-capped trigonal prism (ttp)

Tb, Dy(form I) PuBr3

structure 8-coordinate

Ln bi-capped trigonal

prism (btp)

Dy-Lu AlCl3 structure 6-coordinate

Ln octahedron

Coordination Environments from LnX3 structures

LnX3 structures (and colours)

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

F LaF3

white

LaF3

white

LaF3

green

LaF3

violetLaF3?

YF3

white

YF3

white

YF3

white

YF3

white

YF3

green

YF3

pink

YF3

pink

YF3

white

YF3

white

YF3

white

Cl UCl3

white

UCl3

white

UCl3

green

UCl3

mauveUCl3?

UCl3

yellow

UCl3

yellow

UCl3

white

PuBr3

white

AlCl3

white

AlCl3

yellow

AlCl3

violet

AlCl3

yellow

AlCl3

white

AlCl3

white

Br UCl3

white

UCl3

white

UCl3

green

PuBr3

violet

PuBr3

?

PuBr3

yellow

PuBr3

grey

FeCl3

white

FeCl3

white

FeCl3

white

FeCl3

yellow

FeCl3

violet

FeCl3

white

FeCl3

white

FeCl3

white

I PuBr3

white

PuBr3

yellow PuBr3

PuBr3

green

PuBr3

?

FeCl3

orange

FeCl3

?

FeCl3

yellowFeCl3

FeCl3

green

FeCl3

yellow

FeCl3

violet

FeCl3

yellow

FeCl3

white

FeCl3

brown

LnX2

57

Preparation o Typically from comproportionation:- Ln + 2LnX3 3LnX2 o {(Sm,Eu,Yb)I2 are obtained from thermal decomposition of

LnX3 o (Sm,Yb)I2 from Ln + ICH2CH2I LnI2 + CH2=CH2 }

LnX2 are easily oxidized o liberate H2 from H2O {Except for EuX2 }

Occurrence of dihalides o parallels high values for I3 o depends upon the oxidizing power of the halogen (iodides

most numerous!)

Trends in the Stability of MX2 o Consider 3MX2(s) M(s) + 2MX3(s)

! the reverse of the preparation of MX2 ! the most likely decomposition route for MX2

mH = 3LH(MX2) - 2LH(MX3) + 2I3 - atmH(M) - (I1 + I2)

! Irregularities should follow [ 2I3 - - atmH(M) ]

i.e. effectively follow I3 [since variation in atmH follows closely variation in I3

More? see D.A. Johnson, Some Thermodynamic Aspects of Inorganic Chemistry, p. 160-162, 165]

58

explains occurrence of MCl2

! La, Ce, Pr MCl2 unknown ! (Sm, Eu, Yb)Cl2 are the most stable MCl2

~ may be prepared from LnCl3(s) + 1/2H2 LnCl2(s) + HCl

! Gd, Tb low I3 MCl2 unstable to disproportionation

! Nd, Dy, Tm MCl2 known ! Ho, Er MCl2 unknown

Occurrence: o all X (Sm,Eu, Yb) o X=Cl,Br,I (Nd,Dy,Tm,) o X=I only (La,Ce,Pr,Gd)

Structures o Coordination numbers from 9 to 6 (see: Wells) o Fluorides are Fluorite (CaF2) [C.N. = 8] o Nd,Sm,Eu chlorides are PbCl2type [C.N. = 7 + 2] o Nd,Sm,Eu bromides and iodides are SrBr2 type [mixed C.N.

= 7 & 8] o (Dy,Tm,Yb)I2 have layer structures (CdCl2,CdI2) [C.N. = 6]

polarization effects

Two Classes of Dihalide

1. Most LnX2 are regarded as "salt-like" halides (insulators)

2. (Ce,Pr,Gd)I2 have metallic lustre, high conductivity

o formulation as Ln3+(I-)2(e-) with the electron in a delocalized conduction band

! see Cox, Electronic Properties of Solids, {p. 142-145 explanation of metallic LnII compounds}

59

LnII Compounds are finding increasingly more uses

Lower Halides

LnX3/Ln melts yield phases of reduced halide formulae e.g. Ln2X3 & LnX

"Reduced Halides" contain "condensed metal clusters" Black & metallic delocalization of electrons through the metal-

metal bonded networks

Gd2X3 single chains of edge-sharing metal octahedra with M6X8-type environment (i.e. face-capped by X)

60

Lowest Halides are stabilized by H, C or N atoms encapsulated in Ln6 cluster octahedra

e.g. Gd2Cl2C2 Layers of edge-sharing M6C units

e.g. Gd3Cl3C Framework of M6C units

61

Hydrides Preparation: Heat at 300-350C, Ln + H2 LnH2

Properties of LnH2

black, reactive, highly conducting

fluorite structure

62

most thermodynamically stable of all binary metal hydrides formulated as Ln3+(H-)2(e-) with e- delocalized in a metallic

conduction band further H can often be accommodated in interstitial sites

o frequently non-stoichiometric ! e.g. LuHx where x = 1.83-2.23 & 2.78-3.00

high pressure of H2 LnH3 o reduced conductivity: salt-like Ln3+(H-)3

except for Eu and Yb (the most stable LnII)

The Hydrogen Storage Problem see e.g. C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, CUP, 1986 p. 399-405

K. Kosuge, Chemistry of Non-Stoichiometric Compounds, OUP, 1994 p. 219-230

The use of H2 as a fuel is most attractive

Problem: Difficult to store/transport as a liquid

low bpt & low density H2 forms explosive mixtures with air explosion risk on storage!

Solution: Store hydrogen as a solid compound (hydride) from which it can be re-extracted

Metals show two common types of hydride-forming behaviour:- o Absorb Hydrogen reversibly but in small amounts

e.g. Pd, V, Nb, Ta

o Absorb large quantities of hydrogen, but, essentially irreversibly

e.g. rare earths, alkaline earths, Ti, Zr

Intermetallic Alloys between the two classes can be useful for hydrogen storage

63

o e.g. LnNi5 class of alloys

Possible Applications of Rare Earth Intermetallic Hydrides

1. Production of ultrapure hydrogen

2. Isotope Separation of deuterium and hydrogen

3. Source of fuel for motor vehicles

4. Electrodes in Protonic Batteries/Fuel Cells

5. Load Levelling in Power Stations

6. Chemical heat-pump systems

7. Useful hydrogenation agents in organic chemistry

LaNi5

crystallize in the CaCu5 structure provides 9 interstitial sites for H

64

adsorption-desorption of hydrogen occurs topotactically o without drastic change in structure o however, lattice expands by ca. 25%

At a certain pressure, uptake of H2 begins A large amount of H2 is adsorbed at nearly constant equilibrium

pressure ~ the Plateau Pressure When the stoichiometry reaches LaNi5H6 further increase in

pressure yields very little extra hydrogen-adsorption {at highest pressures LaNi5H8.35 is characterized}

Plateau Pressure ca. 2.5 atm at 298 K o (ideal case ca. 1 atm!)

Desorption is endothermic faster at ca. 140C Different LnNi5 changes plateau pressures

65

~ log plateau pressure correlates linearly with unit cell volume of the LnNi5 phase

o mixtures allow tuning of equilibrium pressure `designer' compounds

Dependence of Hydrogen Plateau Pressure on

Unit Cell Volume for LaNi5 compounds

{open circles LnCo5, closed circles LnNi5, open triangles LaCo5-xNix}

Hydrides Binary Oxides

Ln2O3 The most common lanthanide oxide (notable exceptions; CeO2,

Pr6O11, Tb4O7) Sesquioxides, Ln2O3, adopt three structure types Strongly Basic Absorb water / CO2 from air hydroxide / carbonate salts Pale colours Properties strongly resemble alkaline earth oxides

A-type

66

light Ln unusual LnO7 capped-octahedra

B-type

middle Ln LnO7 units of three types:

o 2 x capped trigonal prisms o 1 x capped octahedron

B-M2O3 structure is very complex Densest of M2O3 structure types favoured by increased pressure

67

C-type

heavy Ln LnO6 units, but not octahedra (face & body - divacant cubic)

C-type M2O3 is related to Fluorite (MO2) with 1/4 of anions removed

68

Polymorphism

A (high T)

B (medium T)

C (low T)

See: Wells, Structural Inorganic Chemistry, p. 543-547 for structural details

Ln(OH)3 obtained by action of conc. NaOH on Ln2O3 under hydrothermal

conditions 9-coordinate Ln with tricapped trigonal prismatic (ttp) geometry basicity increases with Z - correlates with decrease in r(Ln3+)

LnO2 CeO2 (most stable) Fluorite (CaF2 ) structure Pr6 O11 , Tb4 O7 (formed at high PO2 and high temperature) Range of non-stoichiometric phases between Fluorite LnO2 and C-

type LnO1.5

~ intermediate phases were the first known examples of shear structures See: Wells, Structural Inorganic Chemistry, p. 226-228 for an introduction to shear structures

LnO known for some Ln Preparation: Comproportionation Ln + Ln2O3 3LnO NaCl structure NdO, SmO lustrous golden yellow, conducting formulated as

Ln3+(O2-)(e-) EuO (dark red), YbO (greyish-white) insulating genuine Ln2+O2-

69

EuO is ferromagnetic and an insulator when pure

[C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, 2nd ed, CUP, 1996 p. 302-3]

Borides A variety of binary borides exist e.g. YB2, YB4, YB6, YB12, YB66 Most important are LnB6

o contain B6 octahedral clusters o isomorphous with CaB6 o black, metallic conductivity

(c.f. CaB6 white, insulator)

o formulated as Ln3+(B62-)(e-)

except EuB6, YbB6 which are Ln2+(B62-)

MT4B4 compounds o M = Sc, Y, Ln, Th, U o T = Ru, Os, Co, Rh, Ir o of current interest for their superconductivity

e.g. CeCo4B4 structure

70

YRh4B4 Tc = 11.3 K

ErRh4B4 Tc = 8.7 K

also ferromagnetic (TCurie = 0.93 K)

Carbides Class III

Interstitial Carbides Close-Packed M with C in Octahedral Interstices

e.g. (Tb,Ho,Y)2C anti-CdCl2 structure

Class II

Occurrence with La - Ho M3C C randomly distributed in 1/3 of Octahedral holes in ccp M M2C3 Pu2C3 structure ~ C2 groups MC2 CaC2 structure ~ C2 groups Metallic lustre & conductivity not salt-like Hydrolysis of (La,Ce)C2 at ambient temperature ethyne, C2H2

Organometallic Chemistry Organolanthanide chemistry is not as extensive as organotransition

metal chemistry, Currently receiving a lot of attention, especially inC-H Bond

Activation studies Primarily ionic in their bonding contracted nature of the 4f valence

orbitals

71

Lanthanides cannot act as -bases Ln-CO compounds are not stable

Organolanthanides are extremely air & moisture sensitive

~ reflects highly carbanionic character of organic ligand & oxophilicity of Ln2+/3+

Cyclopentadienides (C5H5- ligand)

Preparation:

Structure:

Also in+2 Oxidation State

Alkyls & Aryls

72

-bonded alkyls & aryls from metathesis in ether/thf

o R = phenyl probably polymeric o R = CH2CMe3 stable as LnR3(thf)2

[LnMe6]3- have been isolated for most Ln

Mixed Alkyl Cyclopentadienides

C5Me5 (Pentamethyl-cyclopentadienyl) is a common organo-Ln ligand

o large bulk only 2 C5Me5 may be bound to Ln o causes major change in structural & chemical properties o especially novel chemistry in mixed alkyl cyclopentadienides

e.g. (C5Me5)2LuCH3 catalyzes alkene polymerization (Ziegler-Natta chemistry)

reacts with C-H bonds of extremely low acidity e.g. CH4 !!

Mechanism of Methane Activation?

Cycloctatetraenide M3+(C8H82&endash;)2 Sandwich Compounds

Preparation:

73

Structure:

pale green crystals paramagnetic air & moisture sensitive planar COT rings staggered D8d conformation solvated K+ interacts symmetrically with one COT ring

Arenes

Lanthanide Bis-Arene 'sandwich' compounds

prepared by metal vapour synthesis (MVS) techniques stable at ambient temperature

Ln0 - such compounds could not be expected for LnIII with contracted 4f orbitals

For more on Lanthanide Organometallics see: R.D. Khn et al, in Encyclopedia of Inorganic Chemistry, ed. R.B. King pp. 3618-3666

Comparisons and Contrasts Yttrium - Why consider it with the Lanthanides?

74

Occurs with lanthanides in rare earth minerals, e.g. monazite Y occurs effectively exclusively in +3 oxidation state

combines with non-metals YHal3 Y2O3 , Y3+(H-)2(e-), YH3, etc...

Y3+ has same radius as Ho3+ and is difficult to separate from it Forms complexes of high coordination number with chelating O-

donors, e.g. Y(acac)3(H2O) Typical organometallics include: Y(C5H5)3 (polymeric in the solid

state)

dimeric Y(C5H5)2Cl, monomeric form is a thf adduct

Scandium too?

6 Reasons why Scandium could be considered with the Lanthanides

1. Sc occurs effectively exclusively in +3 oxidation state

combines with non-metals ScHal3 Sc2O3, etc...

but coordination octahedral (small size)

2. Sc forms reduced halides e.g. Sc7Cl12 which is Sc3+(Sc6Cl12)3- with Sc6 clusters (but c.f. Nb)

3. Scandium Hydride ScH2 is highly conducting Sc3+(H-)2(e-)

4. Forms complexes of high coordination number with chelating O-donors

e.g. Na+[Sc(CF3COCHCOCF3)4-] with C.N. = 8

but forms octahedral complexes with monodentate ligands e.g. mer-ScCl3(thf)3

5. Nitrate & Sulphate are obtained as hydrated salts Sc(NO3)34H2O & Sc2(SO4)35H2O

6. Typical organometallics include: Sc(C5H5)3 (polymeric in the solid state)

75

dimeric Sc(C5H5)2Cl, monomeric form is a thf adduct

6 Reasons why Scandium could be considered as main group IIIA

1. Sc3+ (r = 74 pm) is appreciably smaller than any of the rare earths

behaviour intermediate between the Lanthanides & Aluminium

2. Sc2O3 is more like Al2O3 than Ln2O3: amphoteric Sc(OH)63- in excess OH-

3. ScF3 disssolves in excess F- ScF63- (N.B. scarcity of halogeno complexes for Lanthanides)

4. Anhydrous ScCl3 is easily obtained by P2O5-dehydration of hydrated halide

but unlike AlCl3, ScCl3 is not a Friedel-Crafts catalyst

Scandium may with similarly few exceptions be viewed as a 1st Row Transition Metal

Six-coordinate complexes are typical Aqua-ion is Sc(H2O)63+ and is susceptible to hydrolysis O-H-

bridged dimers...

Some CONTRASTS between Lanthanides & Pre-Transition & Transition Metals

Pre-Transition Metals Lanthanides Transition Metals

Essentially Monovalent

- show Group (n+)

Essentially Monovalent (+3)

+2/+4 for certain

Show Variable Valence

(extensive redox chemistry)

76

oxidation state configurations control by environment ~ ligands, pH etc

Periodic trends

- dominated by (effective nuclear)

charge

at noble gas config.

(i.e. on group valence)

Lanthanide Contraction of Ln3+

Size changes of Mn+ less marked

Similar Properties for a given group

(differentiated by size)

Similar Properties

(differentiated by size)

Substantial Gradation in Properties

widespread on earth common mineralogy diverse mineralogy

No Ligand Field Effects

Insignificant Ligand Field Effects

Substantial Ligand Field Effects

Always 'hard' (O, Hal, N donors)

(preferably negatively charged)

Always 'hard' (O, Hal, N donors)

(preferably negatively charged)

Later (increasingly from Fe&endash;Cu)/heavier metals

may show a 'soft' side

'Ionic' 'Covalent' Organometallics

'Ionic' Organometallics 'Covalent' Organometallics

No -Ligand Effects Paucity of -Ligand Effects -Acceptor Ligands Extensive Chemistry

Poor Coordination Properties

(C.N. determined by size)

High Coordination Numbers

(C.N. determined by size)

Extensive Coordination

C.N. = 6 is typical maximum

(but many exceptions) Flexibility in

Geometry Flexibility in

Geometry Fixed (by Ligand Field effects)

Geometries No Magnetism from

the metal ions

- noble gas configurations of ions

Free Ion-like Magnetism

ground state magnetism

Orbital Magnetism 'Quenched' by Ligand Fields

excited J-states populated

'Ionic' compound formulations

Weak, Narrow Optical Spectra

Stronger, Broader Optical Spectra

77

large HOMO-LUMO gaps

UV CT spectra

forbidden, unfacilitated transitions

forbidden transitions vibronically-assisted

79

ACTINIDES Periodicity

Actinides is taken here to mean the 15 Elements from Actinium (Ac) to Lawrencium (Lr)

q.v. argument as to whether Ac or Lr is the first 4th Transition Series element

Electronic subshells are filled in a manner analagous to the lanthanide series

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Z 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

M(g) 6d7s2 6d27s2

5f26d7s2

5f36d7s2

5f46d7s2

5f67s2

5f77s2

5f76d7s2

5f97s2

5f107s2

5f117s2 5f

127s2 (5f137s2

) (5f147s2)

(5f146d7s2)

M+(g) 7s2 6d7s2 5f27s2 5f37s2 5f57s? 5f67s 5f77s 5f77s2 5f97s 5f107s 5f117s (5f127s

) (5f137s) (5f147s) (5f147s2)

M2+(g) 7s 5f6d 5f

26d 5f36d? 5f5? 5f6 5f7 5f8 5f9 5f10 5f11 (5f12) (5f13) (5f14) (5f147s)

M3+(g) 5f 5f

2 5f3 5f4 5f5 5f6 5f7 5f8 5f9 5f10 (5f11) (5f12) (5f13) (5f14)

M4+(g) 5f 5f

2 5f3 5f4 5f5 5f6 5f7 5f8 (5f9) (5f10) (5f11) (5f12) (5f13)

i.e. the Actinides are the second row of the f-block (5f series)

The Actinide Concept of Seaborg - proposed only in 1944

o prior to the synthesis of trans-uranium elements only the chemistries of Th and U were much studied ~ leading to assignment as a 4th transition series elements

Naturally Occurring Actinides Only Actinium, Thorium, Protactinium & Uranium occur

naturally (i.e. Z 92) o Actinium & Protactinium occur only in trace amounts

80

o Neptunium & Plutonium occur in uranium minerals in minute amounts

~ not appreciated until after they had been synthesised that the synthesis route might occur naturally!

All isotopes of all the actinides are radioactive Most of the longer-lived isotopes decay by -emission Half-lives Only quantities of 232Th, 235U, 238U {and possibly 244Pu}

could have survived since the formation of the solar system Both Thorium and Uranium are far from rare

Thorium

o Widely dispersed, accounts for >3 ppm of the earth's crust o Natural Thorium is essentially 100% 232Th o Occurs in monazite [with the rare earths] and in uranothorite

[a mixed Th,U silicate] o Obtained as ThO2, thoria, from mineral extraction process o Used as 99% ThO2 / 1% CeO2 in thoria gas mantles

Uranium

o Widely distributed - found scattered in the faults of old igneous rocks

o Natural Uranium is 99.27% 238U & 0.72% 235U o Obtained usually as UO2 o Used for nuclear fuel, and on a smaller scale for colouring

glass/ceramics Basic Features of Nuclear Structure & Chemistry and

Radiochemistry o excellent, straightforward introduction: Sharpe, Inorganic Chemistry,

Chapter 1 o aspects of nuclear structure and stability: P.A. Cox, The Elements,

OUP, 1989, Chapter 2 3 Decay series explain features of the occurrence, distribution &

discovery of actinide elements

Uranium Decay Series (from 238U)

Actinium Decay Series (from 235U)

81

Thorium Decay Series (from 232Th)

All Radon isotopes are short half-life -emitters (but give rise to short-lived -emitters)

Radon gas is derived from Thorium content in Granite minerals hazard in igneous areas

Actinium and Protactinium occur in uranium ores in trace amounts, because of their participation in Actinium Decay series (from 235U)

In fact Protactinium was originally found by Fajans & Ghring (1913) as Brevium, 234Pa (T1/2 = 6.8 h), in the uranium (238U) decay series before Hahn & Meitner and Soddy & Cranston (1916) discovered the much longer-lived 231Pa (T1/2 = 33,000 y) in the actinium (235U) decay series. Named Protactinium because it is the parent of actinium in the decay series from 235U

Synthesis of Trans-Uranium Elements 1932 Chadwick discovers the neutron

1930s Fermi realises that neutron capture by heavy elements is often followed by -emission (& -ray production) leading to (Z+1) element.

However neutron-bombardment of 238U yields mainly fission products

1940 McMillan & Abelson identify tiny amounts of a short-lived isotope of element 93

82

1940-60 The Golden Age of Element Synthesis through various "bombardment" techniques

Principal investigators: G.T. Seaborg & A. Ghiorso Bombardment particles include:-

Neutrons, , Deuterons, ,-particles, ,

Carbon nuclei,

Production of elements beyond Pu requires successive neutron capture

e.g.

reasonable yields need high neutron fluxes

where are the highest neutron fluxes? - in a thermonuclear explosion!

hence the discovery of Es & Fm from debris of such an explosion

Heaviest elements are (more conveniently!) made by heavy ion bombardment

e.g.

Problems with heavy ion bombardments include:- o

Principal Difficulties associated with Heavy Element Isolation & Characterization

1. Powerful accelerators needed for appropriate velocities (inertia mass)

2. Products are produced only an atom at a time!

3. Individual elements are not produced cleanly in isolation

separation from other actinides & from lanthanide fission products

83

4. Radioactivity

remote-handling often necessary (Actinides are also highly toxic)

damage to solutions

e.g. generation of radicals, H, OH in H2O leads to reduction of higher Actinide oxidation states

heating problems (e.g. 242Cm gives out 122 Wg-1)

problems with crystallography

o fogging of X-ray film o creates defects in crystals

5. Instability of most nuclides ~ e.g. No (T1/2 = 1 hr) & Lw (T1/2 = 3 min)

Heavier elements ( Bk) are produced only in the minutest amounts

o e.g. typical yields of 258Md (T1/2 = 3 m) are 1 to 3 atoms per expt.!

o only a few atoms of No and Lr have ever been isolated

Timespan available for experiments can be very limited

6. Difficulty in identification of a few atoms

o Separation by ion-exchange techniques (c.f. lanthanides)

even after purification cumulative daughter product contamination may be a problem

o Using nuclear decay statistics to detect and count the atoms

prediction of behaviour from utilization of decay systematics

o Chemical Tracer methods

84

need for accurate prediction of properties

o Pure chemical properties (where accessible) performed on oxide samples confined in quartz capillaries attached to high-vacuum systems

computer-controlled apparatus for study of transuranic chemistry

For a fascinating account of heavy actinides, including personal recollections of the experiments

see: G.T. Seaborg & W.D. Loveland, The Elements beyond Uranium, Wiley, N.Y., 1990

Are there any uses for trans-Uranium elements? Principal Actinide Isotopes Available in Macroscopic Quantities

Isotope Half-Life Source Quantities

232Th 1.39 x 1010 y Natural (100% abundance) k. tonnes

231Pa 3.28 x 105 y Natural (0.34 ppm in uranium ores) grams

85

235U 7.13 x 105 y Natural (0.7204% abundance) k. tonnes

238U 4.5 x 109 y Natural (99.2739% abundance) k. tonnes

237Np 2.2 x 106 y multi kg 238Pu 86.4 y kg

239Pu 24,360 y k. tonnes

241Am 433 y kg 243Am 7650 y kg 243Cm 18.12 y kg

249Bk 325 days mg 252Cf 2.57 y mg

Other long-lived isotopes are known, but generally only obtained in traces

Plutonium: 239Pu

produced from 238U by neutron capture in all nuclear reactors acts as nuclear fuel in fast-breeder reactors processed for nuclear weapon applications

238Pu

produced by used as a compact energy source due to the heat from -decay

N.B. -emission is harmless, unless the emitter is ingested

deep-sea diving suits are heated by ca. 750g of 238Pu combined with PbTe thermoelectric totally reliable

electricity

86

used in Apollo space missions / Human heart pacemakers

Americium: 241Am is used as the -emission source in smoke alarms

Actinide Metals Preparation

General method for all Actinides:

Reduction of AnF3 or AnF4 with vapours of Li, Mg, Ca or Ba at 1100 - 1400C

Highly Electropositive

Typically react with:

air tarnishing boiling water or dilute acid releasing Hydrogen most non-metals in direct combination

Structures

Very dense metals (e.g. U 19 gcm-3) with distinctive structures

e.g. Plutonium has at least 6 allotropes and forms numerous alloys

Plutonium

Symmetry monoclinic monoclinic orthorhombic fcc bc tetragonal bcc

Stability < 122C 122-207C 207-315C 315-457C 457-479C

479-640C

/ gcm-3 19.86 17.70 17.14 15.92 16.00 16.51

87

General Observations (comparisons with Lanthanides)

Electronic Configurations of Actinides are not always easy to confirm

o atomic spectra of heavy elements are very difficult to interpret in terms of configuration

Competition between 5fn7s2 and 5fn-16d7s2 configurations is of interest

o for early actinides promotion 5f 6d occurs to provide more bonding electrons

much easier than corresponding 4f 5d promotion in lanthanides

o second half of actinide series resemble lanthanides more closely

5f orbitals have greater extension wrt 7s and 7p than do 4f relative to 6s and 6p orbitals

e.g. ESR evidence for covalent bonding contribution in UF3, but not in NdF3

5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers

o especially U - Am

tendency towards variable valency

greater tendency towards (covalent) complex formation than for lanthanides, including complexation with -bonding ligands

electronic structure of an element in a given oxidation state may vary between compounds and in solution

often impossible to say which orbitals are being utilized in bonding

Ionic Radii of ions show a clear "Actinide Contraction" o Actinide 3+ or 4+ ions with similar radii to their Lanthanide

counterparts show similarities in properties that depend upon ionic radius

88

Electronic Spectra o narrow bands (compared to transition metal spectra) o relatively uninfluenced by ligand field effects o intensities are ca. 10x those of lanthanide bands o complex to interpret

Optical Absorption Spectrum of U4+(aq)

Magnetic Properties o hard to interpret o spin-orbit coupling is large

& Russell-Saunders (L.S) Coupling scheme doesn't work

o ligand field effects are expected where 5f orbitals are involved in bonding

89

Survey of Actinide Oxidation States The known oxidation states of the actinides are indicated and surveyed:

[importance: > > ]

7 6 5 4 3 2 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

+2

Unusual oxidation state Common only for the heaviest elements No2+ & Md2+ are more stable than Eu2+ Actinide An2+ ions have similar properties to Lanthanide Ln2+ and to

Ba2+ ions rationalization of stabilities: Open University Course Book p. 54-56

+3

The most common oxidation state The most stable oxidation state for all trans-Americium elements

(except No?) Of marginal stability for early actinides Th, Pa, U (But: Group

oxidation state for Ac) General properties resemble Ln3+ and are size-dependent

o stability constants of complex formation are similar for same size An3+ & Ln3+

o isomorphism is common o later An3+ & Ln3+ must be separated by ion-exchange/solvent

extraction Binary Halides, MX3 easily prepared, & easily hydrolysed to MOX Binary Oxides, M2O3 known for Ac, Th and trans-Am elements

+4

90

Principal oxidation state for Th

Th4+ chemistry shows resemblance to Zr4+ / Hf4+ - is it a transition metal?

Very important, stable state for Pa, U, Pu Am, Cm, Bk & Cf are increasingly easily reduced - only stable in

certain complexes

e.g. Bk4+ is more oxidizing than Ce4+

MO2 known from Th to Cf (fluorite structure) MF4 are isostructural with lanthanide tetrafluorides MCl4 only known for Th, Pa, U & Np Hydrolysis / Complexation / Disproportionation are all important in

(aq)

+5

Principal state for Pa

Pa5+ chemistry resembles that of Nb5+ / Ta5+ - is it a transition metal?

For U, Np, Pu and Am the AnO2+ ion is known (i.e. quite unlike Nb/Ta)

Comparatively few other AnV species are known

e.g. fluorides, PaF5, NbF5, UF5; fluoro-anions, AnF6-, AnF72-, AnF83-

e.g. oxochlorides, PaOCl3, UOCl3; uranates, NaUO3

+6

AnO22+ ions are important for U, Np, Pu, Am

UO22+ is the most stable

Few other compounds e.g. AnF6 (An = U, Np, Pu), UCl6, UOF4 etc..., U(OR)6

+7

91

Only the marginally stable oxo-anions of Np and Pu, e.g. AnO53-

Actinide Aqueous Chemistry Frost Diagrams for Actinides

Latimer & Frost Diagrams for elements in acid & alkaline (aq) indicate

92

actinides are quite electropositive Pa - Pu show significant redox chemistry

e.g. all 4 oxidation states of Pu can co-exist in appropriate conditions in (aq)

stability of high oxidation states peaks at U (Np) An3+ is the maximum oxidation state for (Cf)Es - Lr No2+(aq) is especially stable ~ most stable state for No in (aq) redox potentials show strong dependence on pH (data for Ac - Cm)

o high oxidation states are more stable in basic conditions o even at low pH hydrolysis occurs formation of polymeric

ions

when hydrolysis leads to precipitation measurement of potentials is difficult!

e.g. Pa5+ hydrolyses easily; potentials that indicate it to be the most stable oxidation state are recorded in presence of F- or C2O42-

o tendency to disproportionation is particularly dependent on pH

e.g. at high pH 3Pu4+ + 2H2O PuO22+ + 2Pu3+ + 4H+

early actinides have a tendency to form complexes o ~ complex formation influences reduction potentials

e.g. Am4+(aq) only exists when complexed by fluoride (15 M NH4F(aq))

radiation-induced solvent decomposition produces H and OH radicals

which lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI

Stability of Actinide ions in aqueous solution:

Ion Colour Stability Preparation

Md2+ easy to oxidize, but stable to water Zn or Cr2+ on Md3+

No2+ stable normal oxdn state

in acid

93

Ac3+ colourless stable normal oxdn state

in acid

U3+ claret evolves H2 on standing; easily oxidized by air

Na or Zn/Hg on UO22+

Np3+ blue-purple stable to water; easily oxidized by air

Zn/Hg or H2(Pt) reduction

Pu3+ blue-violet stable to water & air; readily oxidized

SO2 or NH2OH reduction

Am3+ pink stable; difficult to oxidize I&endash;, SO2 , etc... on higher states

Cm3+ pale yellow stable; chemical oxidation not possible

normal oxdn state in acid

Bk3+ green stable; can be oxidized to Bk4+ normal oxdn state in acid

Cf3+ green stable normal oxdn state

in acid

Es3+ stable normal oxdn state

in acid

Fm3+ stable normal oxdn state

in acid

Md3+ stable, but easily reduced to Md2+ normal oxdn state in acid

No3+ easily reduced to No2+ CeIV or BrO3&endash;

on No2+

Lr3+ stable normal oxdn state

in acid

Th4+ colourless stable normal oxdn state

in acid

Pa4+ colourless stable to water; easily oxidized Zn/Hg on PaV(aq)

U4+ green stable to water; easily oxidized by air to UO22+

Zn/Hg on UO22+

Np4+ yellow-green

stable to water; slowly oxidized by air to NpO2+

SO2 on NpO2+ in H2SO4

94

Pu4+ tan-brownstable in 6M acid, disproportionates at higher pH

SO2 or NO2&endash; on PuO22+

Am4+ pink? only stable as fluoride complex; easily reduced

Am(OH)4 in 15 M NH4F

Cm4+ pale yellow

only as fluoride complex; stable only 1 hr at 25C

CmF4 in 15 M CsF

Bk4+ yellow marginally stable; easily reduced

Pa5+ colourless stable; readily hydrolyzed normal oxdn state in acid

UO2+ unstable to disproportionation (least at pH 2-4)

a transient species

NpO2+ green stable; disproportionates at high acidity

hot HNO3 on Np4+

PuO2+ pink-purple

tends to disproportionate (least at low pH)

NH2OH on PuO22+

AmO2+ yellow disproportionates in acid; reduced by its -decay

ClO&endash; or cold S2O82-on Am3+

UO22+ yellow stable; difficult to reduce

oxidation by air in HNO3

NpO22+ pink-red stable; easy to reduce oxidation by CeIV, MnO4&endash;, BrO3&endash; , etc.

PuO22+ orange-pink

stable; easy to reduce; reduced by its -decay

oxidation by CeIV, MnO4&endash;, BrO3&endash; , etc.

AmO22+ rum-brown

easy to reduce; rapidly reduced by its -decay

oxidation by CeIV, MnO4&endash;, BrO3&endash; , etc.

NpO53- deep only in alkaline O3 or S2O82- on

95

green solution NpO22+ + alkali

PuO53- deep green

only in alkaline solution; oxidizes water

O3 or S2O82- on PuO22+ + alkali

Colour omission concentrated enough solution to tell colour has not been obtained

Actinide Stereochemistries Actinide Stereochemistries show similarities with the Lanthanides

o High coordination numbers

e.g. [Th(NO3)6]2- has distorted icosahedral (C.N. = 12) geometry

e.g. C.N. = 8, 9 are very common UF82-, Th(S2CNEt2)4, Th(trop)4(H2O)

o Distortions from idealised stereochemistries

e.g. PuF62- is not octahedral

o but widest range of stereochemistries is for An(IV) rather than An(III)

{possibly because chemistry of early actinides has received most attention!}

Complexes o a wide range of complexes with monodentate and chelating

ligands o complexing ability:- [M5+] > M4+ > MO22+ > M3+ > MO2+

uni-negative: F- > NO3- > Cl- > ClO4-

bi-negative: CO32- > C2O42- > SO42-

o geometry may be strongly influenced by covalent bonding effects

96

e.g. MO22+ unit is always linear UO2(2-NO3)2(H2O)2 is hexagonal bipyramidal

Compounds Actinide Hydrides, Halides, Oxides, Oxyhalides,... o for a given oxidation state show similarly diverse C.N. to

Lanthanides o different accessible oxidation states even greater diversity of

structure Wide variety of oxidation states of ligands & number of oxidation

states

Extraordinary range of stereochemistry in actinide complexes and compounds

Stereochemistry of Actinides in Well-Defined Complexes and Crystalline Solids

C.N. Geometry O.N. e.g. 4 distorted +4 U(NPh2)4

5 distorted tbp +4 U2(NEt2)8

6 octahedral +3 An(H2O)63+, An(acac)3 +4 UCl62- +5 UF6&endash, -UF5 +6 AnF6 +7 Li5[AnO6] (An = Np, Pu) distorted octahedral +6 Li4UO5 , UO3 +5/+6 U5O8 +6 UO2(S2CNEt2)2(ONMe3)

8 cubic +4 (Et4N)4[U(NCS)8], ThO2, UO2 +5 AnF83- square antiprismatic +4 ThI4, U(acac)4, Cs4[U(NCS)8], +5 -UF5 dodecahedral +4 Th(ox)44-, Th(S2CNEt2)4 bicapped trigonal prismatic +3 PuBr3

97

hexagonal bipyramidal +6 UO2(2-NO3)2(H2O)2 ? +6 UF82-

9 tricapped trigonal prismatic +3 UCl3 capped square antiprismatic +4 Th(trop)4(H2O)

10 bicapped square antiprismatic +4 KTh(ox)4.4H2O

11? fully capped trigonal prismatic? +3 UF3

12 irregular icosahedral +4 Th(NO3)62- distorted cuboctahedral +4 An(3-BH4)4, (Np, Pu)

14? complex +4 An(3-BH4)4, (Th, Pa, U)

98

Uranium Chemistry

Halides

Fluorides

UF6

The most important fluoride preparation:

UO2 + 4HF UF4 + 2H2O

3UF4 + 2ClF3 (from Cl2 + 3F2 2ClF3) 3UF6 + Cl2

properties: o mpt. 64C, vapour pressure = 115 mmHg at 25C o made on a large scale to separate uranium isotopes

! gas diffusion or centrifugation separates 235UF6 from 238UF6

! uranium richer in 235U is termed enriched, richer in 238U depleted

! U.K. capacity (BNFL) = 6 kilotonne per year

99

o powerful fluorinating agent, e.g. + CS2 SF4

Other Fluorides

UF6 + Me3SiCl Me3SiF + 1/2Cl2 + UF5 (melts to an electrically-conducting liquid)

UF6 + 2Me3SiCl 2Me3SiF + Cl2 + UF4 500-600C UO2 + CFCl2CFCl2

Mixed-Valence fluorides such as U2F9 also form Reduction of UF4 with 1/2H2 yields UF3

o LaF3 structure o like LnF3 is insoluble in water

Chlorides

UCl4

is the usual starting material for the synthesis of other UIV compounds

preparation: liquid-phase chlorination of UO3 by refluxing hexachloropropene

properties: o soluble in polar organic solvents & in water o forms various adducts (2 - 7 molecules) with O and N donors

e.g. UCl4(CH3CN)4 ~ an ideal dodecahedron

but UCl4(DMSO)3 is actually [UCl2(DMSO)6]2+UCl62-

UCl3

Usually encountered as UCl3(thf)x (a rather intractable material) Unsolvated binary gives its name to the UCl3 structure! Actinide trihalides form a group with strong similarities (excepting

redox behaviour) to the lanthanides

UCl6

From chlorination of U3O8 + C Highly oxidizing Moisture-sensitive : UCl6 + 2H2O UO2Cl2 (Uranyl Chloride) +

4HCl In CH2Cl2 solution UCl6 decomposes to U2Cl10 (Mo2Cl10 structure)

100

Halogeno Complexes

All Halides can form halogeno complexes, but F- and Cl- are best-known

Preparation: from UXx + NaX in melts or solvents (e.g. SOCl2), but in water only for some fluorides

Occurrence: o UIII: UCl52-, U2Cl72- and UCl4- (a useful UIII reagent) o UIV: UF73- and UF84- are common, UF62- and UCl62- are also

known

also pseudohalide complexes, e.g. [U(NCS)8]4-

o UV: UV is usually unstable in (aq),

but UF5 in 48% HF M+UF6- (M+ = Rb+, Cs+, H3O+) salts

also UCl6- and UCl63-

o UVI: UF7- and UF82- are known, the latter is more thermally-stable

Hydrides Principal Uranium Hydride is UH3

~ important as a source material for UIII and UIV chemistry

101

Oxides Many binary phases UOx have been reported

o many are not genuine phases o genuine phases show range of O-content

The most important genuine phases are UO2, U4O9, U3O8, UO3

UO2 & U4O9 (UO2.25)

UO2 (black-brown) has the Fluorite structure stoichiometric material is best obtained from:-

Interstitial Oxide Ions may be incorporated into the structure

UO2+x

e.g. Octahedral hole at (1/2,1/2,1/2) is an obvious interstitial location

but Neutron Diffraction studies indicate:

1. oxide vacancies in the normal fluorite lattice

102

2. two types of interstitial sites, O' and O'' displaced from the nominal (1/2,1/2,1/2) position

3. 2O', 2O'' and 2 vacancies cluster together to form a defect 2 : 2 : 2 Willis Cluster

At UO2.25 (U4O9) (black) the interstitials are ordered forming a distinct phase in the phase diagram

U3O8 (UO2.67) & UO3

U3O8 (dark green)

conveniently made by heating uranyl nitrate or ethanoate in air

> 650C Higher uranium oxides decompose to U3O8

> 800C U3O8 loses oxygen

103

Structure: o Mixed oxide - average oxidation state U5.33 o Evidence suggests Class II/III mixed valence o i.e. each Uranium has a time-averaged configuration [Rn]5f0.67 o An orthorhombic, pillared-layer structure o All U atoms have essentially identical environments o Contains pentagonal bipyramidal UO7 units

UO3 (orange yellow)

produced by a variety of methods:-

Structure: o > 6 modifications have been characterised o Most contain O=U=O 'uranyl' groups linked by 4x

equatorial bridging O

distorted octahedral

environments

Uranates Fusion of uranium oxides with alkali or alkaline earth carbonates orange/yellow/brown mixed-oxides, Uranates

104

Aqueous Chemistry Complex aqueous chemistry due to:-

o extensive possibilities for complexation o hydrolytic reactions, often leading to polymeric ion species

Reduction Potentials appropriate for 1M HClO4 indicate:

U3+

powerful reducing agent, reduces H2O to H2 (solutions in 1M HCl stable for days)

obtained by reduction of UO22+ electrolytically or with Zn/Hg UF3H2O & U2(SO4)35H2O can be obtained from appropriate

solutions

U4+

only slightly hydrolysed in 1M acid solution U4+ + H2O U(OH)3+ + H+

but, it can give rise to polymeric species in less acid solutions

regarded as a 'stable' oxidation state of uranium in (aq)

UO2+

extremely unstable to disproportionation evidence for its existence in (aq) from stopped-flow techniques more stable in DMSO (half-life ~ 30 mins)

105

UO22+

the Uranyl ion very stable forms many complexes a dominant feature of uranium chemistry reduced to U4+ by e.g. Zinc, Cr2+

o re-oxidation by H218O2 U18O22+ o re-oxidation by 18O2 U(18O16O)2+

linear, symmetrical (O=U=O)2+ structure

Why is UO22+ trans linear, whereas WO22+ is cis, bent?

o WO22+ (6d0)is cis, bent because it allows -donation from the 2 O to 2 independent d-orbitals, with a single d-orbital shared

o ThO2 (6d05f0) is bent (122deg.) for similar reasons i.e. no f-orbital participation

o UO22+ (6d05f0) is trans, linear because of the participation of its 5f orbitals

U(5f) are of considerably lower energy than Th(5f) - see section 1

Details of the MO diagram for AnO22+ are controversial,

but f-orbitals have ungerade symmetry, d-orbitals are gerade

no d-f mixing in centrosymmetric AnO22+ unit

UO22+ readily adds 4-6 donors in its equatorial plane distinctive complexes

e.g. cyclic hexadentates strong complexes

e.g. Uranyl nitrate hydrates all contain the UO2(NO3)2(H2O)2 unit

106

extraction of uranyl nitrate from aqueous nitric acid into non-polar solvents is a classic separation method

UO22+ salts show characteristic (yellow) fluorescence

Nuclear Reactors, Atomic Energy & Uranium Chemistry Principles of Nuclear Reactors

Nuclear fission = large nucleus splitting into 2 highly energetic smaller nuclei + neutrons

Sufficient neutrons of suitable energy can induce fission of further nuclei Chain reaction

to sustain chain reaction a critical mass of uranium must be achieved (prevents neutron loss)

Kinetic energy of main fragments is converted to heat (106x energy of same mass of coal)

Only naturally-occurring fissile nucleus is 235U (0.72 % natural abundance)

' Fast' neutrons produced by fission are not very effective in producing further fission

either increase proportion of 235U in fuel

or slow down (moderate) the fast neutrons

107

~ necessary anyway to balance the chain reaction & prevent explosion

First self-sustaining nuclear 'reactor' built in 1942 at University of Chicago

NOT the first nuclear reactor on Earth!

The Oklo Phenomenon

in the Oklo Uranium mine in Africa it is observed that 235U levels are lower in those parts where the total uranium content of the rock is highest and fission product elements are found in the deposits

o when the rocks were formed 2 x 109 years ago 235U was 3% critical mass

o water in the clay mineral moderated the neutrons

o criticality was possibly maintained for 106 years

Modern Nuclear Reactors

Fuel

Current nuclear reactors use UO2 fuel ~ less reactive than U metal Enrichment is by fractional gaseous centrifugation of UF6 (easily

sublimed) Breeder reactors generate new fuel during operation.

Neutron capture by 238U results in formation of 239Pu, which is fissile. Significant amounts of Pu will only be produced in an unmoderated reactor (fuel reprocessing more dangerous!)

Cladding

Fuel container is usually stainless steel or zirconium alloy (resistance to corrosion)

108

Moderators

Best moderators are light atoms;

1H in water (so efficient at moderation that enriched fuel must be used)

2H in heavy water 10,11B in boron-steel control rods, 12C in graphite (must be highly purified)

Coolants

Water/Heavy Water ~ to keep it liquid it must be pressurized (PWR) CO2 gas ~ in the Advanced Gas-Cooled Reactor (AGR) Liquid-Na needed for the more severe cooling problems in breeder

reactors

Nuclear Fuel Reprocessing

Fission products 'poison' the fuel (by absorbing neutrons themselves) before it is spent

'spent' fuel = uranium / plutonium / trans-uraniums (small amounts) / ~30 fission products (inc. 2nd row transition metals / lanthanides as alloys or complex oxide phases)

reprocessing exploits the different redox/complexation chemistries of

uranium vs plutonium vs fission + capture products

109

Organometallics Organometallic chemistry of actinides is relatively recent Similar to lanthanides in range of cyclopentadienides /

cyclooctatetraenides / alkyls Cyclopentadienides are -bonded to actinides.

Compounds include:-

C5H5- does not behave ionically but Cl- is labile: formation of a wide variety of

(C5H5)3UX compounds The most notable Cyclooctatetraenide is Uranocene

Green crystals Paramagnetic Pyrophoric

110

Stable to hydrolysis Planar 'sandwich' Eclipsed D8h conformation UV-PES studies show that bonding in uranocene has 5f & 6d

contributions o e2u symmetry interaction shown can only occur via f-orbitals

See: C. Elschenbroich & A. Salzer, Organometallics, 2nd ed.VCH, Weinheim, 1992 p. 363-5

Trans-Actinide Elements Some atoms of elements 104 - 112 have probably been made

~ some are the subject of dispute:-

o have they been made? o who made them first? o what should they be called?

They are assumed to comprise the start of a 4th Transition Series, but chemical data is sketchy!

Any prospects for heavier elements? o Plots of Half-life of Longest-Lived Isotope vs Z don't hold

out much hope!

o Search for a "Magic Island"

Nuclear structure calculations possibly stable nuclei for Z ~ 114

but no confirmed reports synthetically or in nature

111

LANTHANIDES & ACTINIDES Problems Set

1. Compare and contrast the chemistry of the lanthanides with those of the first row transition metals and pre-transition metals.

2. Explain how you would prepare pure samples of the lanthanide elements, and indicate how you would assess the purity of your samples.

3. (a) Explain why YbS and YbI2 are semiconducting solids, but LaS and LaI2 show metallic behaviour.

(b) Compare and contrast the structures adopted by LaCl3, FeCl3 and AuCl3.

(c) Comment on the following statement: "Aqueous Eu2+ is thermodynamically less susceptible to oxidation than any other aqueous M2+ lanthanide cations because of the stability of the half-filled 4f7 shell".

(d) Comment in the variation of the exothermic enthalpies of formation of the lanthanide trichlorides, MCl3, given below (&endash;H in kJmol-1)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb 1075 1058 1061 1045 1030 939.5 1012 1001 991 1009 1002 990 949

112

(e) Predict the magnetic moments you would expect for Pr3+ and Gd3+ salts. Would you expect U4+ salts to have the same magnetic moments as Pr3+ salts?

4. Comment on points of in