Electronic absorption and MCD spectra for BiX63-, X = Cl-, Br-, and I-, inacetonitrile solution: Metal-centered vs ligand-to-metal charge-transfer assignments

Electronic absorption and 8.0 T magnetic circular dichroism (MCD) spectra a t room temperature have been carefully measured for BiX63- complexes in ace tonitrile solutions containing excess halide ligand. In each case the spect ra exhibit a prominent low-energy band that is accompanied by a strong posi tive A term and is assigned to a metal-centered (MC) 6s to 6p transition to a formally triplet state which correlates with the P-3(1) atomic state (A band). At higher energy intense bands are observed with accompanying MCD sp ectra that resemble somewhat weaker negative A terms, though the MCD is rat her broad and consists of overlapping features. These bands are ascribed to ligand-to-metal charge-transfer (LMCT) transitions from nonbonding or weak ly pi bonding halide np orbitals to the metal 6p orbital. Previous assignme nt of these higher energy bands to the MC 6s to 6p transition to the predom inantly singlet state which correlates with the P-1(1) atomic state (C band ) is shown to be inconsistent with the observed MCD.