1. Lecture 30 Electronic Spectra of Coordination Compounds 1) Jahn-Teller effect
Octahedral complexes can be a subject to tetragonal or trigonal distortions leading to less symmetrical but more stable structures.
According to the Jahn-Teller theorem, a non-linear molecule with a not-completely filled degenerate electronic levels can undergo a (vibrational) distortion lowering its symmetry and energy.
In the case of octahedral complexes the tetragonal distortion reduces Oh symmetry of a complex to D4h producing either elongated or compressed tetragonal bipyramid and reduces degeneracy of eg and t2g orbitals.
The most pronounced stabilization due to the tetragonal distortion is expected for the following configurations: d1 (compression), d4, d7, d9 (various types), d2 (elongation).
No stabilization and thus no distortion is expected for d3, d5 (high spin), d6 (low spin) and d8 configurations.

2. 2) Static and dynamic Jahn-Teller effect
The Jahn-Teller effect is more pronounced when the former eg level is not completely filled (d1 > d2), that is for configurations d4 (high spin), d7 (low spin) and d9. In these cases a static Jahn-Teller effect can be observed and the species of D4h symmetry can exist in a solid phase.
Much weaker stabilization corresponds to the case of the asymmetrically filled former t2g level such as in d1 species.
In such cases the tetragonal distortion is reflected mainly in electron absorption. We observe appearance of new bands like in the case of Ti(H2O)63+ (two new bands appears as shoulders).
d1 Ti(H2O)63+

3. 3) Charge transfer bands
Similar to d-d transitions, charge-transfer (CT) transitions also involve the metal d-orbitals. CT bands are observed if the energies of empty and filled ligand- and metal-centered orbitals are similar.
The direction of the electron transfer is determined by the relative energy levels of these orbitals: i) ligand-to-metal charge transfer (LMCT) like in MnO4-, CrO42- etc. or ii) metal-to ligand charge transfer (MLCT) like in [Fe(bpy)3]2+. The simplified diagrams below are the modified versions of what we had in Lecture 26. Bold arrows show possible CT transitions.