Comparative absorption, electroabsorption and electrochemical studies of intervalence electron transfer and electronic coupling in cyanide-bridged bimetallic systems: ancillary ligand effects

Electroabsorption or Stark spectroscopy has been used to evaluate the syste ms (NC)5M(II)-CN-Ru-III(NH3)(5)(1-) and (NC)(5)M-II-CN-Ru-III(NH3)(4)py(1-) , where M-II = Fe-II or Ru-II. When a pyridine ligand is present in the axi al position on the Ru-III acceptor, the effective optical electron transfer distance - as measured by the change in dipole moment, \Delta mu \- is inc reased by more than 35% relative to the ammine substituted counterpart. Com parison of the charge transfer distances to the crystal structure of Na[(CN )(5)Fe-CN-Ru(NH3)(4)py] . 6H(2)O reveals that the Stark derived distances a re similar to 50% to similar to 90% of the geometric separation of the meta l centers. The differences result in an upward revision in the Hush delocal ization parameter, c(b)(2), and of the electronic coupling matrix element, Hob, relative to those parameters obtained exclusively from electronic abso rption measurements. The revised parameters are compared to those, which ar e obtained via electrochemical techniques and found to be in only fair agre ement. We conclude that the absorption/ electroabsorption analysis likely y ields a more reliable set of mixing and coupling parameters. (C) 2000 Elsev ier Science B.V. All rights reserved.