Electronic coupling between metal ions in cyanide-bridged ground state and excited state mixed valence complexes

An evaluation of metal/metal electronic mixing in the ground and metal-to-metal charge transfer excited states of cyanide-bridged bi-, tri- and tetra-metallic complexes is presented. The ground state spectroscopic and electrochemical properties observed for complexes whose M(II) to M(III) oxidation potentials span almost 2 eV are surveyed and the variations of their electronic mixings are compared to the patterns expected based on standard perturbation theory models for electronic mixing between donor and acceptor metal ions in bridged complexes. Based on the difference between the electrochemical potentials for oxidation of a D/A pairs containing at least one ruthenium complex and that of an analogous Rh(III) complexes, the extent of electronic mixing, or electronic delocalization is 30-70% larger in these cyanide-bridged D/A complexes than expected based absorption spectra using the standard Hush model. The electronic mixing of the remote or next-nearest-neighbor (NNN) metal centers of tri- and tetra-metallic complexes is largely mediated by a bridging (L)(4)M(CN)(2) moiety; for (L)(4) am(m)ine or polypyridine ligands. The extent of this mixing is very strongly dependent on the energy difference between the nearest-neighbor (NN) and the NNN electron transfer excited states. When the donor and acceptor are both polypyridine complexes of ruthenium, the NN and NNN electron transfer excited states can be nearly degenerate. When the diabatic NN and NNN excited states have the same vertical energy (i.e., with respect to the ground state nuclear coordinates), the resulting adiabatic excited states have the same electronic composition and this corresponds to the maximum possible NNN mixing. Many models that treat NNN mixing, "superexchange" coupling models, are obtained for the limit of very weak mixing between well separated electronic states and they are not useful for the limit in which the electron transfer excited states are degenerate or nearly degenerate. The lowest energy electronic excited states of [(L)(4)Cr-III{CNRuII(NH3)(5)}(2)](m+) complexes are also mixed valence donor/acceptor systems, but the electronic configurations of the two D/A pairs involve a metal centered doublet Cr-III excited state a metal to metal charge transfer excited state with a low spin triplet Cr-II species linked by cyanide to Ru-III. There are fewer means for probing the mixed valence properties of these excited states; however, their excited state mixed valence properties are complicated by the electronic coupling to other near in energy electronic excited states. Some of the configurational mixing between electronic states appears to help stabilize the triplet Cr-II electronic configuration. (C) 2012 Elsevier B.V. All rights reserved.