Discrete variational Xα calculations of the spectra of chlorocuprate(II) complexes: a detailed comparison with experiment and the cellular ligand field model

Abstract

Discrete variational Xα self-consistent-charge (DVXα s.c.c.) calculations of the d–d and charge-transfer (c.t.) spectra of five chlorocuprate(II) complexes have been performed. The molecules studied are planar and distorted tetrahedral CuCl₄^2–, square-pyramidal CuCl₅^3–, and two tetragonal-octahedral CuCl₆^4– species. Calculated orbital-energy differences using near-minimal basis sets optimised for each molecule are in excellent agreement with experiment for the d–d bands. Transition-state calculations for the c.t. absorptions predict higher energies than observed but the energy differences between successive bands are well reproduced also. Analysis of the DVXα s.c.c. charge distributions gives a description of the metal–ligand bonding closely parallel to cellular ligand field (c.l.f.) treatments. Further comparison of DVXα s.c.c. and c.l.f. data indicates that the former gives not only a good account of ‘d–s mixing’ in these systems but also quantitative confirmation of the empirical c.l.f. ‘sum rule.’ The present study suggests that the DVXα scheme provides independent validation of the underlying assumptions of the c.l.f. model in particular and ligand field theory in general.