

The topology of the groundstate potential energy surface of M(CN)_{6} with orbitally degenerate ^{2}T_{2g} (M = Ti^{III} (t_{2g}^{1}), Fe^{III} and Mn^{II} (both lowspin t_{2g}^{5})) and ^{3}T_{1g} ground states (M = V^{III} (t_{2g}^{2}), Mn^{III} and Cr^{II} (both lowspin t_{2g}^{4})) has been studied with linear and quadratic Jahnâˆ’Teller coupling models in the fivedimensional space of the Îµ_{g} and Ï„_{2g} octahedral vibrations (T_{g}âŠ—(Îµ_{g}+Ï„_{2g}) Jahnâˆ’Teller coupling problem (T_{g} = ^{2}T_{2g}, ^{3}T_{1g})). A procedure is proposed to give access to all vibronic coupling parameters from geometry optimization with density functional theory (DFT) and the energies of a restricted number of Slater determinants, derived from electron replacements within the t_{2g}^{1,5} or t_{2g}^{2,4} groundstate electronic configurations. The results show that coupling to the Ï„_{2g} bending mode is dominant and leads to a stabilization of D_{3}_{d} structures (absolute minima on the groundstate potential energy surface) for all complexes considered, except for [Ti(CN)_{6}]^{3}, where the minimum is of D_{4}_{h} symmetry. The Jahnâˆ’Teller stabilization energies for the D_{3}_{d} minima are found to increase in the order of increasing CNâˆ’M Ï€ backdonation (Ti^{III} < V^{III} < Mn^{III} < Fe^{III} < Mn^{II} < Cr^{II}). With the angular overlap model and bonding parameters derived from angular distortions, which correspond to the stable D_{3}_{d} minima, the effect of configuration interaction and spinâˆ’orbit coupling on the groundstate potential energy surface is explored. This approach is used to correlate Jahnâˆ’Teller distortion parameters with structures from Xray diffraction data. Jahnâˆ’Teller coupling to trigonal modes is also used to reinterpret the anisotropy of magnetic susceptibilities and g tensors of [Fe(CN)_{6}]^{3}, and the ^{3}T_{1g} groundstate splitting of [Mn(CN)_{6}]^{3}, deduced from nearIR spectra. The implications of the pseudo Jahnâˆ’Teller coupling due to t_{2g}âˆ’e_{g} orbital mixing via the trigonal modes (Ï„_{2g}) and the effect of the dynamic Jahnâˆ’Teller coupling on the magnetic susceptibilities and g tensors of [Fe(CN)_{6}]^{3} are also addressed. 


Copper(II) complexes of the pentadentate bispidine ligands exist in two isomeric forms (see structure) with bondlength differences up to 0.5 Ã…. The stabilization of either isomer may be achieved by a variation of the substituent at N7. 