Bonding in Transition Metals and Coordination Complexes

Bonding in Transition Metals and Coordination Complexes

Chemistry of the Transition metals

Properties

Atomic Radius : lanthanide contraction – unusual contraction of lanthanide ions.

Binding energy: higher – more unpaired electrons i.e.) m.p. -- higher in the middle of the row : W ( 3410oC), Hg (-39oC)

Oxidation states: higher oxidation state– more covalent bond character lower oxidation state – more ionic bond character

Mn(OH)2, Mn(OH)3, H2MnO3, H2MnO4, HMnO4

basic acidic

Chemistry of the Transition metals

Coordination complex ; coordination chemistry 배위화학

CuSO4 : greenish white

Coordination complex : Cu(H2O)42+

CuSO4.4H2O : bluev.s.

Mn+ + mL M Lmn+

coordinationLigand

Lewis baseLewis acid

Ag+ + 2NH3 (Ag(NH3)2+

Au+ + 2CN- (Au(CN)2-

Coordination number : total number of metal-to-ligand bondUsually 2 ~ 6

Ligands

Br-

Cl-

N3-

CN-

OH-

NH3

H2O

CO

NO2-

O2-

Bromo

Chloro

Azido

Cyano

Hydroxo

Ammine

Aqua(o)

Carbonyl

Nitro

Oxo

H-

ONO-

SCN-

NCS-

NO+

CO32-

Hydrido

Nitrito

Thiocyanato

Isothiocyanato

Nitrosyl

Carbonato

OxalateO

-O

O

O-

H2N NH2Ethylenediamine (En)

bidentateligand

chelates

Nomenclature

NH4[Cr(NH3)2(NCS)4] Reinecke’s salt

Systematic naming

[Co(NH3)5Cl]Cl2 Purpureocobaltic chloride

[Co(NH3)5Cl]Cl2 Pentaamminechlorocobalt(III) chloride

K4[Fe(CN)6] Potassium Hexacyanoferrate(II)

Making coordination complex

charge of a complex = sum of charges of metals and ligandscharge of a complex + charges of counter ions = 0

coordination number = numbers of donor atoms

Rules of Nomenclature

1. Cation Anion b

2. In the complex : names of ligands come first and then name of metal among ligands : alphabetical order

3. Names of ligands : anion – change the last letter to o neutral – same as the original ones

4. Counting number of ligands : di, tri, tetra, penta, hexa, hepta….. if the ligand contains these names in it, use : bis, tris, tetrakis, pentakis……

5. If the compex is an anion : at the end of the name put ate

6. Oxidation number of metal : in parenthesis with roman letter - (IV)

Pentaamminechlorocobalt(III) chloride Potassium Hexacyanoferrate(II)

Influence of Coordination

1. Color

Pale yellow

[Fe(H2O)6]3+ + SCN- [Fe(H2O)5SCN]2+ + H2O

orange

2. Reduction potential

Ag+ + e- Ag Eo = +0.799V

[Ag(CN)2]+ + e- Ag Eo = -0.31V+ 2CN-

3. Chemical reactivity

Structure of coordination complexes

CoCl3.6NH3

CoCl3.5NH3

CoCl3.4NH3

CoCl3.3NH3

Chemical formular(19thC.) color

orange-yellow

pruple

green

green

[Co(NH3)6]3+Cl-3

Chemical formular (Werner)

[Co(NH3)5Cl]2+Cl-2

[Co(NH3)4Cl2]+Cl-

[Co(NH3)3Cl3]

structure

octahedral

[Co(NH3)6]3+Cl-3

[Co(NH3)5Cl]2+Cl-2

[Co(NH3)4Cl2]+Cl-

Cl +2

[Co(NH3)3Cl3]

[Co(En)2Cl2]+Cl-

cis transGeometrical isomers

Chiral structures

[Co(NH3)2(H2O)2Cl2]+

[Pt(En)3]4+

Structure of coordination complexes

Linear[Ag(NH3)2]+

[Zn(NH3)4]2+

Atomic orbitalof metal

coordinationnumber

2

structure

Tetrahedral4

[Pt(NH3)4]2+ Square Planar4

[Co(NH3)6]3+ Octahedral6

d10

d9

d8

d6

Super chelating ligand

EDTA ( ethylenediaminetetraacetate)

Strong affinity to certain metal ionsSolubilize metal ions

[Ni(H2O)6]2+ + 6NH3 [Ni(NH3)6]2+ + 6H2O

Kf = 4 x 108

[Ni(H2O)6]2+ + 3en [Ni(en)3]2+ + 6H2O

Kf = 2 x 1018

Entropy factor : bigger S

Transition Metals

Partially filled d orbitals

Octet rule in transition metal chemistry : 18 electron rule

Coordination complexStructural variety

Low-lying unoccupied orbitals color

Unpaired electrons Magnetic property

Many oxidation states Catalysts, new reactions

Ligands Donates electron pairs

coordination Changes color, reactivity, reduction potential

number of electrons in 4s + 3d + 2 x number of ligands = 18

18-electron rule for transition metal complexes

Octet rule : Lewis structure

consider a transition metal : Cr

Chromium: [Ar] (4s)2(3d)4 6 valence electrons

Chromium need 18 electrons in its most outer shell. 18-electron rule

Therefore the complex of Cr with CO will look like

i.e. CO provides 2 x 6 = 12 electrons Cr provides 6 electrons

Total 18 electrons

Using the 18-electron rule

Given that H2Fe(CO)x exists, what does x equal?

Iron: [Ar] (4s)2(3d)6 8 valence electrons 8

n = 4

Total : 10 + 2n = 18 electrons

hydrogen: 1s1 1 valence electrons x 2 = 2

CO: 2 valence electrons x n = 2n

H2Fe(CO)4

Understanding of metal-ligand binding mode

1. Color : only for partially filled d orbitals i.e. d0, d10 : colorless

facts

2. magnetism: paramagetic v.s. diamagnetic unpaired electrons

[Co(NH3)6]3+ diamagnetic – no unpaired electrons

[CoF6]3- paramagnetic – 4 unpaired electrons

[CrF6]3- [Cr(H2O)6]3+ [Cr(NH3)6]3+

[Cr(CN)6]3-

green violet yellow yellow

3. tetrahedral or square planar

[NiCl4]2- [Ni(CN)4]2-

tetrahedral square planar

Crystal Field Theory

Color, magnetic properties, and choice of tetrahedral, square planar & octahedralHow to explain

Crystal field theory : ionic description of the metal-ligand bonds

Consider only the energy changes of d orbitals of metal during coordination

Consider only electrostatic interaction with ligands : charge-charge, charge-dipole

Begin with octahedral geometry

Low spin complex : when o is large

High spin complex : when o is small

magnetism

d1 ~ d5 : always paramagnetic

d7 ~ d9 : always paramagnetic

d10 : always diamagnetic

d6 : depending on the ligands

Square planar & tetrahedral complexes

Tetrahedral Reversal of octahedral !

M

Square planar & tetrahedral complexes

Tetrahedral

Reversal of octahedral !

Square planar & tetrahedral complexes

Square planar Removal of axial ligands from octahedral

Square planar Removal of axial ligands from octahedral

I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN-

Spectrochemical Series

Color of complexes and magnetic properties are determined by o

o can be determined by ligands

Weak field Ligands Strong field Ligands

small olarge o

High spin complex Low spin complex

[CoF6]3- [Co(CN)6]3-v.s.

Weak point of crystal field theory

1. Coordination is not fully ionic.

2. Spectrochemical series is all empirical. Ligand field theory

Ligand Field Theory

Consider ionic interaction onlyCrystal field theory :

Modification : addition of covalent aspect of coordination.

How? : construction of molecular orbitals.

Using 4s, 4p, 3d orbitals of metals & coordinating orbitals of ligands

Ligand field theory :

For an octahedral complex

1. Orbital overlap is 0 for dxy, dyz, dzx

become nonboinding orbitals

2. Varying overlapping ligand orbitals

for Cl- : p NH3 : sp3

3. MO’s can be formed from 6 ligand orbitals and 6 metal orbitals (4s, 4p, 3d)

For [CoF6]3-

I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN-

Now, we can explain Spectrochemical Series

Weak field Ligands Strong field Ligands

small olarge o

Interaction between dxy of metal and py of halide :

ionic ---- increases energy level of t2g

Makes smaller o for I- and less smaller one for F-

back-bonding

bonding of ligand can overlap with dxy orbital

Lowers the energy level of t2g

Makes larger o for CO, CN-

For [CoF6]3-

Organometallic compounds and Catalysis

Catalytic converter : Pt catalyst

CO + O2 CO2

Pt ( cat.)

Haber process

N2 + 3H2 NH3

Fe ( cat.)

Olefin metathesis reaction

+Ru ( cat.)X

Y

Z

W

+

ZX

WY“Grubbs catalyst”

2005 Nobel Prize in Chemistry

Coordination complexes and Life

We need transition elements for life : V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo…….

Porphine structure Important for oxygen transfer, detoxification,

photosynthesis, nitrogen fixation

숙제18 장 : 6, 12, 22, 26, 34, 42, 46

제출일 : 10 월 19 일