Electronic Spectra of Coordination Compounds We know how to determine Free Ion Terms; d-electron configurations given below Determining Lowest Energy Free Ion Term From Table above: highest multiplicity, largest L if more than one (d3 = 4F) From Metal ion and coordination geometry: Cr3+ Oh hs Sketch d-orbitals with electrons Spin Multiplicity = # unpaired e- + 1 (3 + 1 = 4) Max Value of ML (sum of ml values) = 2 + 1 + 0 = 3 (L = 3 = F) Example: ls Mn3+ Oh

Selection Rules Laporte Selection Rule: transitions between states of same parity (g/u) are forbidden gg and uu forbidden (all dd transitions); gu allowed (dp allowed) Spin Selection Rule: transitions between different spin multiplicities are forbidden 4A24T1 are allowed; 4A22A2 are forbidden Seem to rule out almost all transitions in metal complexes: Why Colored? Mechanisms that “relax” the selection rules 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 Tetrahedral complexes strongly absorb No center of symmetry, so no Laporte forbideness sp3/sd3 hybridization mixes p orbitals (u) with d orbitals (g) to relax Laporte 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

Correlation Diagrams Free Ion (no ligand field) Term Symbols (Previous Lecture) shown on left Strong Ligand Field Term Symbols shown on the right: overcomes LS coupling Possible d2 electron configurations (t2g2 is ground state lowest energy) Actual complexes lie in between these weak and strong field limits Free ion terms can be reduced to constituent irreducible representations

Do = ligand field splitting ch 10 B = Racah parameter = repulsion Each Free Ion Term irreducible representation Correlates with one from the strong-field limit Transitions from Ground State must match same Spin Multiplicity (Bold Lines) Non-Crossing Rule: lines connecting states of same symmetry do not cross 6. Tanabe-Sugano Diagrams Special correlation diagrams useful for interpreting electronic spectra Lowest energy state plotted along horizontal axis 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 ground state n1 n2 n3

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

d4-d7 configurations have High Spin and Low Spin possibilities d4 example S = 2 (mult = 5) S = 1 (mult = 3) Weak Field limit defined as Do /B = 27 Weak Field (High Spin): ground state d4 = 5Eg 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

Jahn-Teller Distortions and Electronic Spectra d1 and d9 electron configurations are Jahn-Teller distorted Cu2+ and Ti3+ spectra show some splitting of single expected absorption Jahn-Teller distortion break degenerate electronic states (strongest if eg like d9) Electron Configurations can be symmetry labelled based on degeneracy Two absorbances in Cu2+ spectrum. B1gA1g too low E to be observed.

e. Ti3+ d1 spectrum also shows splitting, but t2g Jahn-Teller effects are small? 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 account for transition and explain splitting Transition is 5T2g5Eg Splitting due to Jahn-Teller Distortion of Doubly Degenerate eg

Determining Do from Electronic Spectra Not always possible, as overlapping bands and complex math limits utility Often possible to calculate Do and/or B directly from spectrum d1, d4(hs), d6(hs), d9 complexes give only one band = Do

d3, d8 have ground state F terms d3, d8 have ground state F terms. Do is found from lowest energy transition d2, d7(hs)—Complicated by splitting into many states. See text for complex explanation of estimate of Do, but we will not cover these. d5(hs), d4-d7(ls) d5(hs) has no excited states with the correct multiplicity, so all transitions are Spin Forbidden. Very weak absorptions due to Spin-Orbit relaxation only. 2. d4-d7(ls) have many excited states, making analysis difficult

Tetrahedral Complexes More intense absorptions due to no center of symmetry making Laporte Selection Rule moot d-orbitals are split in opposite direction than octahedral Hole Formalism: d1 Oh treated similarly as d9 tetrahedral Use Correlation Diagram for the d10-n Oh configuration to describe dn Td Example for d2 Td case, use d8 Oh diagram Orgel Diagrams (L. E. Orgel, J. Chem. Phys., 1955, 1004.) dn (Oh) and dn±5 have the same diagram dn (Td) and dn±5 have the same diagram dn and dn±5 (Oh) is the reverse of dn and dn±5 (Td) dn (Oh) is the reverse of d10-n (Oh) dn (Td) is the reverse of d10-n (Td) High Spin Only, No Energies Listed, No Multiplicities Listed, add g for Oh

Orgel Diagrams Simple tool (can memorize them) to predict # of bands and Term Symbols Charge Transfer Spectra a. Exchange of an electron from one Ligand to Metal or Metal to Ligand LMCT MLCT

Often in UV, but can be in visible region Much more intense (50,000 M-1cm-1) than dd bands because Laporte Allowed LMCT: MnO4- dark purple color, O ligand orbitals to empty Mn+7 d-orbitals MLCT: Usually with p-acceptor ligands (CO, CN-, SCN-) Both can occur in same complex: Cr(CO)6 Not always possible to determine LMCT vs. MLCT May cover up dd bands Often have to collect spectra at two different concentrations Low concentration to get CT bands on scale High concentration to get dd bands on scale Intraligand Bands Some ligands absorb light themselves (sp* or pp*) Aromatic and/or conjugated ligands (dyes for example) can be highly colored Coordination of metal ion may significantly change those bands