Effective Charge Transfer Distances in Cyanide-Bridged Mixed-Valence Transition Metal Complexes

Abstract

Stark (electroabsorption) spectroscopy can provide quantitative information on the change in dipole moment and polarizability for an electronic transition. In the case of mixed-valence transitions, the change in dipole moment associated with the intervalence charge-transfer band can be used to establish the effective charge-transfer distance required for estimating the intermetallic electronic coupling using Hush theory (Oh; Boxer J. Am. Chem. Soc. 1990, 112, 8161). Stark spectra are reported for a series of cyanide-bridged complexes with −RuIII(NH3)5 as the acceptor and different metal/ligand combinations as the donor. The origins of differences between the effective charge-transfer distance based on the intermetallic distance and the dipole moment differences measured by Stark spectroscopy are discussed. The possible difference in the effective charge-transfer distances for spin−orbit states in [(NC)5OsII-CN-RuIII(NH3)5]-, recently described by Karki and Hupp (J. Am. Chem. Soc. 1997, 110, 4070), is reevaluated, and it is shown that these distances are the same within experimental error for these spin−orbit states. These data provide informative examples of methods used to evaluate Stark data and the power of using higher-order Stark spectroscopy (Lao et al. J. Phys. Chem. 1995, 99, 496) to measure the change in dipole moment for broad and relatively weak transitions often encountered in mixed-valence systems.