Jahn-Teller distortion in bis(terpyridine)nickel(III) – Elongation or compression?

Abstract

DFT methods were used to evaluate the type of Jahn-Teller distortion in the low spin d⁷ bis(terpyridine)nickel (III) doublet. The strain of the tridentate terpyridine ligand in bis(terpyridine)nickel(III) leads to longer Ni(III)-N terminal bond lengths than the Ni(III)-N central bonds. In addition, in all DFT optimized bis(terpyridine)nickel (III) molecules, the one set of Ni(III)-N terminal bonds are longer than the other set of Ni(III)-N terminal bonds. Three pairs of Ni(III)-N bonds are present in bis(terpyridine)nickel(III), namely 2 longer Ni(III)-N_terminal1 terminal bonds, 2 medium length Ni(III)-N_terminal2 terminal bonds and 2 shorter Ni(III)-N_central central bonds, similar to orthorhombic Jahn-Teller distortion in octahedral complexes. Evaluation of bond length differences, contraction of bonds, spin density plots and the character of the singly occupied molecular orbital (SOMO) and singly unoccupied molecular orbital (SUMO), led to a set of criteria to consider in evaluating the type of Jahn-Teller distortion in the low spin d⁷ bis(terpyridine)nickel(III) doublet. Jahn-Teller distortion can lead to elongation (z-out) or compression (z-in) of a pair of opposite Ni-N bonds. One key criteria found, is, if the difference in bond length between Ni(III)-Nterminal1 and Ni(III)-N_terminal2 is smaller than the difference in bond length of Ni(III)- N_terminal2 and Ni(III)-N_central, then it is a Jahn-Teller compression geometry (and vice versa for a Jahn-Teller elongation geometry).