Electronic Spectra of Coordination Compounds

Transcription

1 Electronic Spectra of Coordination Compounds Microstates and free-ion terms for electron configurations Identify the lowest-energy term

2 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 Determine the maximum possible value of M L 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 M L : =3 4 F

3 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 Between d orbitals are forbidde Between d and p orbitals are allowed 2. Transitions between states of different spin multiplicities are forbidden: spin selection rule 4 A 2 and 4 T 1 : spin-allowed 4 A 2 and 2 T 2 : spin-forbidden

4 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 coupling relax the first selection rule:d-d transition 2. Tetrahedral complexes often absorb more strongly than O h complexes. Metal-ligand sigma bonds can be described as involving a combination of sp 3 and sd 3 hybridization of the metal orbitals: relax the first selection rule 3. spin-orbit coupling provides a mechanism of relaxing the second selection rule

5 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): d 2 ; 3 F, 3 P, 1 G, 1 D, 1 S. 2. Strong ligand field. t 2 t 2g e e 2 2g g g

6 Electronic Spectra of Coordination Compounds: correlation diagrams

7 Electronic Spectra of Coordination Compounds: correlation diagrams The free-ion terms will be split into states corresponding to the irreducible representation.

8 Electronic Spectra of Coordination Compounds: correlation diagrams

9 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 strongfield irreducible representation. The spin multiplicity of the ground state.

10 Electronic Spectra of Coordination Compounds: correlation diagrams

11 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.

12 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams

13 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams High spin vs low spin High spin Low spin Ground state and spin multiplicity changed

14 Electronic Spectra of Coordination Compounds: Tanabe-Sugano diagrams

15 Jahn-Teller Distortions and Spectra d 1 d 9 complexes: might expect each to exhibit one absorption band: excitation from the t 2g to the e g levels. e g e g t 2g t 2g Two closely overlapping absorption bands.

16 Jahn-Teller Distortions and Spectra To lower the symmetry of the molecule and to reduce the degeneracy. Distortion from O h to D 4h : results in stabilization of the molecule. The most common distortion observed is elongation along z axis.

17 Jahn-Teller Distortions and Spectra : Symmetry labels for configurations Electron configurations have symmetry labels that match their degeneracies. T E A or B

18 Jahn-Teller Distortions and Spectra : Symmetry labels for configurations 2 D term for d 9 Lower energy Higher energy the opposite of the order of energies of the orbitals 2 E g 2 T 2g Distortions can be splitting of bands. Too weak

19 Tanabe-Sugano Diagrams: Determining o from Spectra;d 1, d 4, d 6, d 9

20 Tanabe-Sugano Diagrams: Determining o from Spectra

21 Tanabe-Sugano Diagrams: Determining o from Spectra;d 3, d 8 The lowest energy

22 Tanabe-Sugano Diagrams: Determining o from Spectra;d 2, d 7 (high spin)

23 Tetrahedral Complexes The lack of a center of symmetry: makes transitions between d orbitals more allowed; much more intense absorption bands. Hole formalism: d 1 O h configuration is analogous to the d 9 T d configuration: the hole in d 9 results in the same symmetry as the single electron in d 1. We can use the correlation diagram for d 10-n configuration in O h geometry e g t 2 hole octahedral t 2g e tetrahedral

24 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) Formal reduction of the metal: Co(III) to Co(II) LMCT

25 Charge-Transfer Spectra IrBr 6 2- (d 5 ): two band IrBr 6 3- (d 6 ): one band Why? Formal reduction of the metal: Co(III) to Co(II) LMCT

26 Charge-Transfer Spectra MLCT π-acceptor ligand (π* orbitals): CO, CN -, SCN -, bipyridine.. Oxidation of the metal d-d transitions may be completely overwhelmed and essentially impossible to observe. Formal oxidation of the metal: Fe(III) to Fe(IV) MLCT