Aspects of intervalence charge transfer in cyanide-bridged systems: Modulated electric field assessment of distances, polarizability changes, and anticipated first hyperpolarizability characteristics

Electroabsorption studies of the systems (CN)₅M^II-CN-M′^III(NH₃) ₅^1- (M = Fe, Ru, Os; M′ = Ru, Os) reveal that the metal-to-metal or intervalence charge-transfer transitions are associated with a change in dipole moment, |Δμ|, ranging from 11 to 17 D. This change corresponds to a charge-transfer distance of ca. one-half to two-thirds of the geometric separation between the donor and acceptor metals. This result has consequences for electron transfer parameters, where electronic coupling energies are now upwardly revised to as high as 3000 cm^-1. The result is also pertinent in the context of nonlinear optics, where Δμ can be utilized in a two level model to estimate wavelength-dependent molecular first hyperpolarizabilities. The change in polarizability (Δα) accompanying the optical intervalence transitions varies from -10 to +310 ų. Comparing the experimental results to a simple two state model suggests that the two state picture is seriously deficient for these systems and that a multitude of available additional states must be included to achieve a quantitative description of the polarizable intervalence excited states of these systems.