Spin densities and covalency in transition-metal complexes: A comparison of discrete variational Xα calculations with polarised neutron diffraction data

Abstract

Spin-unrestricted discrete variational Xα self-consistent charge (DVXα SCC) calculations for the cubic transition-metal complexes [CoCl 4] 2−, [CoBr 4] 2−, [FeCl 4] −, [CrF 6] 3− and [Cr(CN) 6] 3− are reported. The calculated covalencies exhibit the same trends predicted by empirical nephelauxetic parameters. However, agreement between the experimental ligand-field splittings and theoretical values determined via Koopman's theorem is poorer, particularly for the tetrahedral species. The computed spin densities are compared in detail with experimental results derived from polarised neutron diffraction (PND) studies and with unrestricted Hartree-Fock (UHF) calculations. The qualitative agreement with the PND data is good and for [CoCl 4] 2− and [CrF 6] 3− the DVXα SCC and UHF models give almost identical spin densities. However, quantitative agreement with experiment is not obtained because the calculations ignore the real molecular site symmetry, the crystalline environment and configuration interaction (CI). These features appear to influence significantly the experimental spin and charge densities and, except for CI, can be readily included in DVXα SCC calculations. The present work suggests that the DVXα SCC scheme gives a sufficiently accurate description of electron distributions to provide a useful tool for further examination of these effects.