Optical Spectra of Divalent Manganese Salts. I. Energy Levels for Cubic and Lower-Symmetry Complexes

Abstract

An extensive study of the optical absorption spectra of Mn(II) salts is introduced with a re-examination of the nature of crystal-field independent transitions and their dependence upon slight changes in ligand positions. Evidence for an unexpected but significant decrease in metal—ligand bond lengths for the 4A2 state with respect to those for 6A1 equilibrium ground state is obtained both from experiment, in terms of the excitation of totally symmetric displacements, and from approximate calculations. The latter involve both an extended formulation of the nephelauxetic effect and molecular orbital (MO) parameters obtained by a semiempirical LCAO MO scheme. A classification scheme for crystal-field independent transitions, not necessarily in Mn(II) complexes, is introduced, and is based on the type of molecular orbitals, σ, π, or both, appearing in the exchange integrals associated with the transition energies. Low-symmetry effects, especially for the 4E states, are discussed.