On the ground state Tg ⊗ εg (Tg = 2T2g, 3T1g) Jahn-Teller-coupling in hexacyano complexes of 3d-transition metals

The linear and quadratic T-g circle times epsilon(g) Jahn-Teller effect in the T-g (T-g - T-2(2g), T-3(1g)) ground states of low-spin octahedral cyano complexes of 3d-transition metals (M = Ti-III, V-III, Mn-III, Fe-III, Cr-II, Mn-II) has been studied. Vibronic coupling parameters have been derived using density functional theory to calculate energies of Slater determinants, which result from various electron distributions within the t(2g)(n) configuration due to the metal based t(2g)(3d) molecular orbitals (n = 1, 2, 3, 4). Tetragonal elongations are found in the case of Ti-III, Mn-III and Cr-II, and compressions for V-III, Fe-III and Mn-II, and these establish the metal-ligand pi-back donation as the dominant effect and driving force for the geometry distortions and energy stabilizations towards non-degenerate B-2(2g)(t(2g)(1), t(2g)(5)) and (3)A(2g)(t(2g)(2), t(2g)(4)) ground states. The strength of the Jahn-Teller coupling is very weak and found not to follow a monotonic trend across the transition metal series but is shown to be weakest for Ti-III, V-III and Fe-III, slightly larger for Mn-III, increases further to Mn-III and is found to be the strongest but is still weak for Cr-II. (c) 2007 Elsevier BN. All rights reserved.