Solvatochromism of dinuclear complexes: an alternative explanation

Abstract

Abstract : Solvatochromism refers to changes in electronic absorption spectra with solvent. Although most, if not all, complexes are solvatochromic to some extent, the term is usually applied to species which show shifts in energy of at least a few hundred wavenumbers with variation in solvent. The very large solvatochromism of the metal to ligand charge transfer transitions in various, formally non-polar, ligand-bridged dinuclear metal carbonyl complexes is discussed. The similarity of this behaviour to that of related mononuclear species and the good correlations obtained with the 'polar' part of McRae's equation are used to demonstrate that dipole-dipole interactions are the main cause of the solvatochromism. This contradicts previous explanations which have attributed the solvatochromism to changes in dispersion forces. It is concluded that, in the simplest approximation, the molecules may be regarded as two polar halves, each of which interacts with the solvent. Detailed interpretation in terms of McRae's equation is not possible without knowing whether the metal to ligand charge transfer excited state is localized on one metal center or delocalized over both. The data available do not allow us to distinguish between these possibilities.