The Jahn-Teller effect in octahedral T Ground states - an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids

We have investigated MIIIX63- polyhedra [MIII: T, V, Co; X: Cl, F] with π-antibonding T ground states by optical spectroscopy and by DFT calculations, which supply reliable structural and energetic informations, as long as the ligands can be considered to be approximately terminal as in the chosen A2′AMIIIX6 [A′, A: K+, Rb+, Cs+] solids of elpasolite structure. While in the case of TiIII and CoIII distinct splittings of the excited Eg state are observed, indicating that a T ⊗ ε-type Jahn-Teller distortion of the respective polyhedra is present, the d-d spectra are of cubic appearence in the case of VIII. Here, it is most likely the t2g2-t2g1eg1 configuration interaction via the two 3A2g states in D4h, which suppresses the T ⊗ ε vibronic coupling. Assuming a purely tetragonal Jahn-Teller interaction the linear coupling constant is consistently found to be of the magnitude Vε ≈ 0.8 (1) [F−] and 0.25 (5) [Cl−] eV Å−1, leading to only very small Jahn-Teller splittings of the T ground states, with dynamic polyhedron distortions at 298 K in the case of TiIII and CoIII. In comparison, the vibronic coupling in the excited Eg states is much more pronounced (A1 ≈ 2.0 (2) [F−]; 1.05 (5) [Cl−] eV Å−1). The Jahn-Teller splittings are considerably enhanced, if terminal and bridging ligands are simultaneously present in the polyhedron, as in solids AMIIIF4. The different covalent binding properties of the two types of ligands can be shown to be responsible for these effects.