Mixed valency in ruthenium complexes—Coordinative aspects

The metal–metal interaction in ligand-bridged diruthenium(II,III) complex intermediates is commonly characterized by using the redox potential difference, usually converted to the comproportionation constant Kc, and by analyzing intervalence charge transfer (IVCT) absorption features and deriving electronic coupling parameters. Although these two kinds of information may yield very different estimates of the extent of metal–metal interaction in any given system, the absolute values have often been used interchangeably to quantify the strength of electronic “coupling”. Using examples from the literature with substantially different coordination arrangements in terms of denticity and donor/acceptor character of the π conjugated bridging ligand we can show that close correlations between electrochemically and spectroscopically determined coupling are only valid within structurally related classes of compounds with qualitatively similar interaction at the metal–ligand interface. Most strikingly, situations with high Kc values but weak, low energy IVCT absorptions have been identified for complexes with bis- and tris-bidentate acceptor bridges whereas intermediates with small comproportionation constants but intense, narrow IVCT bands are typical with bridging bis-tridentate acceptors.