Spectro-Electrochemistry and Electrochemical Spectroscopy

Abstract

The electron-transfer reactions available to transition metal complexes are intimately linked to their molecular and electronic structures. Electrochemistry provides a direct way of defining these redox processes, thereby mapping the metal and ligand frontier orbitals in a way which complements electronic spectroscopy. For co-ordination complexes, the general possibility of reconciling electrochemical (i.e. equilibrated) and optical charge-transfer data is receiving renewed attention. In the case of hexahalo complexes [MX6]z- of dn ions, the halide to metal charge-transfer envelopes are complicated but generally well understood. The charge-transfer energies for the accessible [MvCl6]1- and [MIVCl6]2- complexes have now been recorded and mapped against the corresponding dn → dn+1 (V/IV or IV/III) couple. A striking linear correlation emerges between hνmax (lmct) and E0 in each series, with a gradient of 1.35 eV/V in both cases. The implications of this orderly linear relationship are discussed and an argument presented that such E0/hν) correlations have a respectable physical basis after all. The present analysis can be extended fruitfully to trends in progressively substituted complexes MX6-nLn (L = π-acceptor ligand) where reconciliation of the electrochemical and spectroscopic data yields a conceptual representation of the relative frontier orbital energies of M, X, and L as a function of stoichiometry.