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Electronic Spectra of Coordination Compounds Microstates and free-ion terms for electron configurations Identify the lowest-energy term
Electronic Spectra of Coordination Compounds
Microstates and free-ion terms for electron configurations
Identify the lowest-energy term
Electronic Spectra of Coordination Compounds
Identify the lowest-energy term
1. Sketch the energy levels, showing the d electrons.
2. Spin multiplicity of lowest-energy state = number of unpaired electrons + 1.
3. Determine the maximum possible value of ML for the configuration as shown. This determines the type of free-ion term.
4. Combine results of steps 2 and 3 to get ground term.
Spin multiplicity = 3+1=4
Max. of ML: 2+1+0 =3
4F
Electronic Spectra of Coordination Compounds: Selection Rules
On the basis of the symmetry and spin multiplicity of ground and excited electronic states
1. Transitions between states of the same parity are forbidden.: Laporte selection rule
2. Transitions between states of different spin multiplicities are forbidden: spin selection rule
4A2 and 4T1: spin-allowed4A2 and 2T2: spin-forbidden
Between d orbitals are forbiddeBetween d and p orbitals are allowed
Electronic Spectra of Coordination Compounds: Selection Rules
Some rules for relaxation of selection rules
1. Vibrations may temporarily change the symmetry(the center of symmetry is temporarily lost: vibronic couplingrelax the first selection rule:d-d transition
2. Tetrahedral complexes often absorb more strongly than Oh complexes. Metal-ligand sigma bonds can be described as involving a combination of sp3 and sd3
hybridization of the metal orbitals: relax the first selection rule
3. spin-orbit coupling provides a mechanism of relaxing the second selection rule
Electronic Spectra of Coordination Compounds: correlation diagrams
To relate the electronic spectra of transition metal complexes to the ligand field splitting: correlation diagrams and Tanabe-Sugano diagrams
1. Free ions (no ligand field): d2; 3F, 3P, 1G, 1D, 1S.2. Strong ligand field.
t2g2 eg
2t2geg
Electronic Spectra of Coordination Compounds: correlation diagrams
Electronic Spectra of Coordination Compounds: correlation diagrams
The free-ion terms will be split into states corresponding to the irreducible representation.
Electronic Spectra of Coordination Compounds: correlation diagrams
Electronic Spectra of Coordination Compounds: correlation diagrams
Irreducible representations may be obtained for the strong-field limit configurations.
Each free-ion irreducible representation is matched with a strong-field irreducible representation.
The spin multiplicity of the ground state.
Electronic Spectra of Coordination Compounds: correlation diagrams
Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams
B = Racah parameter, a measure of the repulsion between terms of the same multiplicity; the energy difference between 3F and 3P is 15B.
E is the energy above the ground state.
Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams
Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams
High spin vs low spin
Ground state and spin multiplicity changedHigh spin Low spin
Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams
Jahn-Teller Distortions and Spectra
d1 d9 complexes: might expect each to exhibit one absorption band: excitation from the t2g to the eg levels.
t2g
eg
t2g
eg
Two closely overlapping absorption bands.
Jahn-Teller Distortions and Spectra
To lower the symmetry of the molecule and to reduce the degeneracy.Distortion from Oh to D4h: results in stabilization of the molecule.
The most common distortion observed is elongation along z axis.
Jahn-Teller Distortions and Spectra: Symmetry labels for configurations
Electron configurations have symmetry labels that match their degeneracies.
T
E
A or B
the opposite of the order of energies of the orbitals
Too weak
Jahn-Teller Distortions and Spectra: Symmetry labels for configurations
2D term for d9
Lower energy Higher energy
2Eg2T2g
Distortions can be splitting of bands.
Tanabe-Sugano Diagrams: Determining ∆o from Spectra;d1, d4, d6, d9
Tanabe-Sugano Diagrams: Determining ∆o from Spectra
Tanabe-Sugano Diagrams: Determining ∆o from Spectra;d3, d8
The lowest energy
Tanabe-Sugano Diagrams: Determining ∆o from Spectra;d2, d7 (high spin)
Tetrahedral Complexes
The lack of a center of symmetry: makes transitions between d orbitals more allowed; much more intense absorption bands.
Hole formalism: d1 Oh configuration is analogous to the d9 Td configuration: the hole in d9 results in the same symmetry as the single electron in d1.
We can use the correlation diagram for d10-n
configuration in Oh geometry
t2g
egt2
eoctahedral tetrahedral
hole
Charge-Transfer Spectra
Charge-transfer absorptions is much more intense than d-d transitions.
Involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa)
LMCT
Formal reduction of the metal: Co(III) to Co(II)
Charge-Transfer Spectra
IrBr62- (d5): two band
IrBr63- (d6): one band
Why?
LMCT
Formal reduction of the metal: Co(III) to Co(II)
Charge-Transfer Spectra
MLCTπ-acceptor ligand (π* orbitals): CO, CN-, SCN-, bipyridine..
Oxidation of the metald-d transitions may be completely overwhelmed and essentially impossible to observe.
MLCT
Formal oxidation of the metal: Fe(III) to Fe(IV)
