A. Electronic Spectra of Coordination Compounds 1. We
A. Electronic Spectra of Coordination Compounds 1. We know how to determine Free Ion Terms; d-electron configurations given below a. Determining Lowest Energy Free Ion Term 1. From Table above: highest multiplicity, largest L if more than one (d 3 = 4F) 2. Selection Rules a. Laporte Selection Rule: transitions between states of same parity (g/u) are forbidden gg and uu forbidden (all dd transitions); gu allowed (dp allowed) b. Spin Selection Rule: transitions between different spin multiplicities are
forbidden 4A24T1 are allowed; 4A22A2 are forbidden c. 3. Seem to rule out almost all transitions in metal complexes: Why Colored? Mechanisms that relax the selection rules a. Vibronic Coupling: vibrations of bonds cause loss of the center of symmetry Relaxes Laporte Selection Rule and allows 5-50 M-1 cm-1 metal transitions b. Tetrahedral complexes strongly absorb 1. No center of symmetry, so no Laporte forbideness 2. sp3/sd3 hybridization mixes p orbitals (u) with d orbitals (g) to relax Laporte 3. Spin-Orbit Coupling relaxes Spin Selection Rule in some case (<1 M -1cm-1) Most important for 2nd and 3rd row transition metals 4. [V(H2O)6]3+ d2 Example 5.
Correlation Diagrams a. Free Ion (no ligand field) Term Symbols (Previous Lecture) shown on left b. Strong Ligand Field Term Symbols shown on the right: overcomes LS coupling a. Possible d2 electron configurations (t2g2 is ground state lowest energy) b. Actual complexes lie in between these weak and strong field limits c. Free ion terms can be reduced to constituent irreducible representations d. 6. Each Free Ion Term irreducible representation Correlates with one from the strong-field limit e. Transitions from Ground State must match same Spin Multiplicity (Bold Lines) f. Non-Crossing Rule: lines connecting states of same symmetry do not cross
Tanabe-Sugano Diagrams g. Special correlation diagrams useful for interpreting electronic spectra h. Lowest energy state plotted along horizontal axis i. Vertical distance above is correlated to energy of transition Horizontal Axis: Do /B Do = ligand field splitting ch 10 B = Racah parameter = repulsion between terms of same multiplicity Vertical Axis: E/B E = energy of excited state above n ground state 1 n2 n3
7. More on Tanabe-Sugano Diagrams a. Simplified Tanabe-Sugano Diagrams show allowed transitions for Octahedral Geom d2 and d8 opposite order d3 and d7 opposite order d4 and d6 opposite order b. c. d4-d7 configurations have High Spin and Low Spin possibilities 1. d4 example S = 2 (mult = 5) S = 1 (mult = 3) 2.
Weak Field limit defined as Do /B = 27 3. Weak Field (High Spin): ground state d4 = 5Eg 4. Strong Field (Low Spin): ground state d4 = 3T1g Weak Field Aqua Complex Spectra for 1st-Row Transition Metal Ions: compare number of peaks to Tanabe-Sugano Diagrams 8. Jahn-Teller Distortions and Electronic Spectra a. d1 and d9 electron configurations are Jahn-Teller distorted b.
c. Cu2+ and Ti3+ spectra show some splitting of single expected absorption Jahn-Teller distortion break degenerate electronic states (strongest if e g like d9) Two absorbances in Cu2+ spectrum. B1gA1g too low E to be observed. d. Electron Configurations can be symmetry labelled based on degeneracy e. Ti3+ d1 spectrum also shows splitting, but t2g Jahn-Teller effects are small? 1. f. g.
Excited state (with eg Jahn-Teller distortion) is responsible for splitting Distortion from pure Octahedral Geometries are common, so Electronic Spectra are often more complex than the Tanabe-Sugano Diagrams indicate Example: [Fe(H2O)6]2+ has split peak at 1000nm. Use Tanabe Sugano Diagram to Transition is 5T2g5Eg account for transition and explain splitting Splitting due to Jahn-Teller Distortion of Doubly Degenerate eg 9. Determining Do from Electronic Spectra a. b. Not always possible, as overlapping bands and complex math limits utility
Often possible to calculate Do and/or B directly from spectrum c. d1, d4(hs), d6(hs), d9 complexes give only one band = Do d. d3, d8 have ground state F terms. Do is found from lowest energy transition e. d2, d7(hs)Complicated by splitting into many states. See text for complex explanation of estimate of Do, but we will not cover these. f. d5(hs), d4-d7(ls) 1. d5(hs) has no excited states with the
correct multiplicity, so all transitions are Spin Forbidden. Very weak absorptions 10. Tetrahedral Complexes a. More intense absorptions due to no center of symmetry making Laporte Selection Rule moot b. d-orbitals are split in opposite direction than octahedral c. Hole Formalism: d1 Oh treated similarly as d9 tetrahedral 1. Use Correlation Diagram for the d10-n Oh configuration to describe dn Td 2. Example for d2 Td case, use d8 Oh diagram 11. Orgel Diagrams (L. E. Orgel, J. Chem. Phys., 1955, 1004.) a. dn (Oh) and dn5 have the same diagram
b. dn (Td) and dn5 have the same diagram c. dn and dn5 (Oh) is the reverse of dn and dn5 (Td) d. dn (Oh) is the reverse of d10-n (Oh) g. Orgel Diagrams h. Simple tool (can memorize them) to predict # of bands and Term Symbols
LMCT 12. Charge Transfer Spectra a. Exchange of an electron from one Ligand to Metal or Metal to LigandMLCT b. c. d. Often in UV, but can be in visible region Much more intense (50,000 M-1cm-1) than dd bands because Laporte Allowed LMCT: MnO4- dark purple color, O ligand orbitals to empty Mn+7 d-orbitals e. f. MLCT: Usually with p-acceptor ligands (CO, CN-, SCN-) Both can occur in same complex: Cr(CO)6
g. h. i. Not always possible to determine LMCT vs. MLCT May cover up dd bands Often have to collect spectra at two different concentrations 1. Low concentration to get CT bands on scale 2. High concentration to get dd bands on scale 13. Intraligand Bands a. Some ligands absorb light themselves (sp* or pp*)
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