valdosta state university chapter 22 transition elements valdosta state university
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Chapter 22Transition Elements
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Transition Elements – d- and f-block
• Used in construction and manufacturing (iron), coins (nickel, copper, zinc), ornamental (gold, silver, platinum).• Densest elements (osmium d=22.49 g/cm3, iridium d=22.41g/cm3).• Highest melting point (tungsten, mp=3410oC) and lowest melting point (mercury, mp=-38.9oC).
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Metal Chemistry
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• Radioactive elements with atomic number less than 83 (technetium 43; promethium 61).• All elements are solids, but mercury.• Have metallic sheen, conduct electricity and heat.• Are oxidized and form ionic compounds.• Some are essential to living organisms: Cobalt (vitamin B12), iron (hemoglobin and myoglobin), molybdenium and iron (nitrogenase).• Compounds are highly colored and used as pigments: Fe4[Fe(CN)6)3 14 H2O (prussian blue), TiO2 (white).• Ions give color to gemstons: Iron(II) ions give yellow color in citrine and chromium(III) ions produce the red color of a ruby.
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Electron Configurations
• General: [noble gas core] nsa (n-1) db
• Valance electrons for transition elements reside in the ns and (n-1) d subshells.
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Reactions
Fe + O2
Fe + Cl2 Fe + HCl
• All metals undergo oxidation with oxygen, halogens, aqueous acids.• First the outermost electron is removed, followed by one or more d electrons.• Some generate cations with unpaired electrons = paramagnetism.• Are colored.• For first transition series common oxidation numbers are +2 and +3.
Fe2O3
Fe3+
[Ar]3d5
FeCl3Fe3+
[Ar]3d5
FeCl2 + H2
Fe2+
[Ar]3d6
Fe: [Ar]3d64s2
Fe + HCl
Fe + Cl2
Fe + O2
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Trends: Oxidation number
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Most commonMost common
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Trends: Atom Radius
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Trends: Density
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Trends: Melting Point
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Metallurgy: Element Sources
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Pyrometallurgy
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• Involves high temperature, such as Fe
• C and CO used as reducing agents in a blast furnaceFe2O3 + 3 C ---> 2 Fe + 3 CO
Fe2O3 + 3 CO ---> 2 Fe + 3 CO2
• Lime added to remove impurities, chiefly SiO2
SiO2 + CaO ---> CaSiO3
• Product is impure cast iron or pig iron
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Hydrometallurgy
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• Use aqueous solutions (flotation). Some use bacteria.• Add CuCl2(aq) to ore such as CuFeS2 (chalcopyrite)
CuFeS2(s) + 3 CuCl2(aq) --> 4 CuCl(s) + FeCl2(aq) + 2 S(s)
• Dissolve CuCl with xs NaClCuCl(s) + Cl-(aq) --> [CuCl2]-
• Cu(I) disproportionates to Cu metal2 [CuCl2]- --> Cu(s) + CuCl2 (aq) + 2 Cl-
Azurite, 2CuCO3•Cu(OH)2 Native copper
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Coordination Compounds
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– combination of two or more atoms, ions, or molecules where a bond is formed by sharing a pair of electrons originally associated with only one of the compounds.
Pt
Cl
Cl
Cl
CH2
CH2
-
H••
H
H
N
H O H••
••
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Coordination Chemistry
Co(HCo(H22O)O)662+2+
Pt(NHPt(NH33))22ClCl22
Cu(NHCu(NH33))442+2+
““Cisplatin” - a cancer Cisplatin” - a cancer chemotherapy agentchemotherapy agent
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Coordination Chemistry
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An iron-porphyrin, the basic unit of hemoglobinAn iron-porphyrin, the basic unit of hemoglobin
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Myoglobin / Hemoglobin
p.1084
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Coordination Chemistry
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Vitamin B12Vitamin B12A naturally occurring A naturally occurring cobalt-based compoundcobalt-based compound
Co atomCo atomCo atomCo atom
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Coordination Chemistry
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• Biological nitrogen fixation contributes about half of total nitrogen input to global agriculture, remainder from Haber process.• To produce the H2 for the Haber process consumes about 1% of the world’s total energy.• A similar process requiring only atmospheric T and P is carried out by N-fixing bacteria, many of which live in symbiotic association with legumes.• N-fixing bacteria use the enzyme nitrogenase — transforms N2 into NH3.• Nitrogenase consists of 2 metalloproteins: one with Fe and the other with Fe and Mo.
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Nickel ion:coordination compounds
Coordination Chemistry
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Nomenclature
• [Ni(NH3)6]2+
• A Ni2+ ion surrounded by 6, neutral NH3 ligands
• Gives coordination complex ion with 2+ charge.
Ligand: monodentateCoordinate to the metal via a single Lewis base atom.
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Nomenclature++
Inner coordination sphereInner coordination sphere
ClCl--
Co3+ + 2 Cl- + 2 neutral ethylenediamine molecules
Cis-dichlorobis(ethylenediamine)cobalt(II) chloride
Ligand: polydentateLigand: polydentatealso chelating ligandsalso chelating ligandsCoordinate with more Coordinate with more than one donor atom.than one donor atom.(Bidentate)(Bidentate)
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Bidentate Ligands
Acetylacetone (acac)Acetylacetone (acac)
Ethylenediamine (en)Ethylenediamine (en)
Bipyridine (bipy)Bipyridine (bipy)
Oxalate (ox)Oxalate (ox)
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Bidentate Ligands
Acetylacetonate Acetylacetonate ComplexesComplexes
Commonly called the “acac” ligand. Forms complexes with all transition elements.
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Multidentate LigandsEDTAEDTA4-4- - ethylenediaminetetraacetate ion - ethylenediaminetetraacetate ion
Multidentate ligands are sometimes called CHELATING ligands
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Co2+ complex of EDTA4-
Multidentate Ligands
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Give the formula of a coordination compound
A Co3+ ion bound to one Cl- ion, one ammonia molecule, and two ethylenediamine (en) molecules.
1. Determine the net charge (sum the charges of the various components).
2. Place the formula in brackets and the net charge attached.
[Co(H2NCH2CH2NH2)2(NH3)Cl]2+
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Determine the metal’s oxidation number and coordination numberPt(NH3)2(C2O4)
Oxalate: (C2O4)2-
Ammonia: NH3
Pt must be 2+ (oxidation number = +2)
Coordination number = 4 (two from oxalate and each ammonia filling one).
[Co(NH3)5Cl]SO4
Chloride: Cl-
Sulfate: SO42-
Overall complex must be 2+
Co must be 3+ (oxidation number = +3)
Coordination number = 6 (sulfate is not coordinated to the metal).
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Nomenclature
1. Positive ions named first2. Ligand names arranged alphabetically3. Prefixes -- di, tri, tetra for simple ligands
bis, tris, tetrakis for complex ligands4. If M is in cation, name of metal is used5. If M is in anion, then use suffix -ate
CuCl42- = tetrachlorocuprate6. Oxidation no. of metal ion indicated in roman
numerals.
Cis-dichlorobis(ethylenediamine)cobalt(III) chlorideCis-dichlorobis(ethylenediamine)cobalt(III) chloride
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Nomenclature
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Co(HCo(H22O)O)662+2+
Pt(NHPt(NH33))22ClCl22
Cu(NHCu(NH33))442+2+
Hexaaquacobalt(II)
Tetraamminecopper(II)
diamminedichloroplatinum(II)
HH22O as a ligand is O as a ligand is aquaaqua
NHNH33 as a ligand is as a ligand is ammineammine
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Nomenclature
IrCl(CO)(PPhIrCl(CO)(PPh33))22
Vaska’s compoundVaska’s compound
Carbonylchlorobis(triphenylphosphine)iridium(I)
[Ni(NH[Ni(NH22CC22HH44NHNH22))33]]2+2+
Tris(ethylenediamine)nickel(II)
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Geometry of Coordination Compounds
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Defined by the arrangement of donor atoms of ligands around the central metal ion.
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Isomerim of Coordination Compounds
• Two forms of isomerism– Constitutional– Stereoisomerism
• Constitutional– Same empirical formula but different atom-to-atom
connections
• Stereoisomerism– Same atom-to-atom connections but different
arrangement in space.
Geometric and Optical
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Constitutional Isomers
Aldehydes & ketonesAldehydes & ketones CH3-CH2-CH
O
C
O
CH3H3C
3C, 1O, 6H
- Coordination isomerism: it is possible to exchange a ligand and the uncoordinated counterion.Example: [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br
(violet) (red)- Linkage isomerism: it is possible to attach a ligand to the metal through different atoms.Usually: SCN- and NO2
-
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Constitutional Isomers
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CoH3N
H3N NO2
NH3
NH3
NH3
2+
CoH3N
H3N ONO
NH3
NH3
NH3
2+
sunlightsunlight
Such a transformation could be used as an energy Such a transformation could be used as an energy storage device.storage device.
Pentaamminenitritocobalt(III) Pentaamminenitrocobalt(III)
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Stereoisomerism
Note: there are VERY few tetrahedral Note: there are VERY few tetrahedral complexes. Would not have geometric isomers.complexes. Would not have geometric isomers.
ciscis transtrans
• One form is commonly called geometric isomerism or cis-trans isomerism. Occurs often with square planar complexes.
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Geometric Isomers
Cis and trans-dichlorobis(ethylenediamine)cobalt(II) Cis and trans-dichlorobis(ethylenediamine)cobalt(II) chloridechloride
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Geometric Isomers
fac isomer mer isomer
For octahedral complexes (MX3Y3):
fac isomer has three identical ligands lying at the corners of a triangular face of octahedron (fac=facial).
mer isomer ligands follow a meridian (mer=meridional).
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Stereoisomers
• Enantiomers: stereoisomers that have a non-superimposable mirror image.
• Diastereoisomers: stereoisomers that do not have a non-superimposable mirror image (cis-trans isomers).
• Asymmetric: lacking in symmetry—will have a non-superimposable mirror image.
• Chiral: an asymmetric molecule.
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Enantiomers
[Co(NH2C2H4NH2)3]2+
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Stereoisomers
CoN
N NH3
NH3
Cl
OH2
2+
CoN
N Cl
OH2
NH3
NH3
2+
CoN
N NH3
Cl
NH3
OH2
2+
CoN
N NH3
OH2
NH3
Cl
2+
These two isomers have a plane of symmetry. Not chiral.
These two are asymmetric. Have non-superimposable mirror images.
[Co(en)(NH3)2(H2O)Cl]2+
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Stereoisomers
These are non-superimposable mirror images:enantiomers
[Co(en)(NH3)2(H2O)Cl]2+
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Bonding in Coordination Compounds
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• Model must explain– Basic bonding between M and ligand– Color and color changes– Magnetic behavior– Structure
• Two models available– Molecular orbital– Electrostatic crystal field theory– Combination of the two ---> ligand field theory
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Bonding
• As ligands L approach the metal ion M+, – L/M+ orbital overlap occurs– L/M+ electron repulsion occurs
• Crystal field theory focuses on the latter, while MO theory takes both into account
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Ligand Field Theory
• Consider what happens as 6 ligands approach an Fe3+ ion: Orbitals split into two groups as the ligands approach.
4s five 3d orbitals
[Ar] All electrons have the same energy in the free ion
Value of ligand field sppliting: ∆o depends on L: e.g., CN- > H2O > Cl-
eg
t2g
0
energy
d(x2-y2) dz2
dxy dxz dyz
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Octahedral Ligand Field
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Tetrahedral and Square Planar Ligand Fields
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Crystal Field Theory
• Tetrahedral ligand field.
• Note that ∆t = 4/9 ∆o and so ∆t is small.
• Therefore, tetrahedral complexes tend to
absorb “red wavelengths” and be colored blue.
d(x2-y2) dz2
dxy dxz dyz
e
t2
energy
t
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Ways to Distribute Electrons
• For 4 to 7 d electrons in octahedral complexes,
there are two ways to distribute the electrons.
– High spin — maximum number of unpaired e-
– Low spin — minimum number of unpaired e-
• Depends size of ∆o and P, the pairing energy.
• P = energy required to create e- pair.
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Magnetic Properties of Fe2+
• High spin• Weak ligand field strength and/or lower Mn+ charge• 0 is smaller than P• [Fe(H2O)6]2+
• Low spin• Stronger ligand field strength and/or higher Mn+ charge• 0 is larger than P• [Fe(CN)6]4-
ParamagneticParamagnetic
DiamagneticDiamagnetic
eg
t2g
energy
d(x2-y2) dz2
dxy dxz dyz
eg
t2g
E small
energy
d(x2-y2) dz2
dxy dxz dyz
eg
t2g
E large
energy
d(x2-y2) dz2
dxy dxz dyz
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High and Low Spin Octahedral Complexes
High or low spin octahedral complexes only possible High or low spin octahedral complexes only possible for dfor d44, d, d55, d, d66, and d, and d77 configurations. configurations.
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Why are complexes colored?
FeFe3+3+ CoCo2+2+ CuCu2+2+NiNi2+2+ ZnZn2+2+
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Why are complexes colored?
– Note that color observed is transmitted light.
Red and blue are absorbed
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Why are complexes colored?
– Note that color observed is transmitted light.
– Color arises from electron transitions between d orbitals (d-to-d transitions).
– Color often not very intense.
• Spectra can be complex– d1, d4, d6, and d9 --> 1 absorption band
– d2, d3, d7, and d8 --> 3 absorption bands
• Spectrochemical series — ligand dependence of light absorbed. The ability to split the d orbitals is determined by spectroscopy.
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Light absorption by octahedral Co3+ complex
eg
t2g
energy
d(x2-y2) dz2
dxy dxz dyz
eg
t2g
energy
d(x2-y2) dz2
dxy dxz dyz
Ground stateGround state Excited stateExcited state
Usually excited complex returns to ground state by losing energy, which is observed as heat.
+ energy (= )
(light absorbed)
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Spectrochemical Series
As ∆ increases, the absorbed light tends to blue, and so the transmitted light tends to red.
• d orbital splitting (value of ∆o) is in the order:
small ∆o large ∆o
I- < Cl- < F- < H2O < NH3 < en < phen < CN- < CO
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Other ways to induce color
• Intervalent transfer bands (IT) between ion of adjacent oxidation number.– Aquamarine and kyanite are examples
– Prussian blue
• Color centers– Amethyst has Fe4+
– When amethyst is heated, it forms citrine as Fe4+ is reduced to Fe3+
Prussian blue contains Fe3+ and Fe2+