Spectroscopic and Electrochemical Probes of Electronic Coupling in Some Cyanide-Bridged Transition Metal Donor/Acceptor Complexes

The effects of donor−acceptor (D/A) electronic coupling, H DA, on the spectroscopic and electrochemical properties of several series of CN--bridged transition metal complexes have been examined. The complexes employed were formed by ruthenation of M(L)(CN)2 n + parent complexes (for n = 0, M = Ru(II) or Fe(II), and L = bpy or phen; for n = 1, M = Cr(III), Rh(III), or Co(III), and L = bpy, phen, or a tetraazamacrocyclic ligand). The observed half-wave potentials of the resulting CN--bridged D/A complexes spanned a 300−350 mV range in contrast to the range of about 80 mV expected on the basis of the oscillator strength, h DA, of the D/A charge-transfer MM‘CT absorption band and the geometrical distance between donor and acceptor, r DA. Different series of complexes exhibit different correlations between E 1/2 and h DA. Several factors have been found to contribute to these differences: (a) symmetry effects; (b) solvational differences that arise when nonbridging ligands are changed; (c) solvational effects arising from differences in overall electrical charges; (d) partial delocalization of electron density along the D/A axis in such a way as to reduce the effective distance between centers of charge, . To take account of the effects of the solvational factors, systematic examination has been made of (a) the metal independent shifts of E 1/2 which occur when nonbridging ligands are changed; (b) the differences in E /12 that occur in closely related Ru(III)/Ru(II) couples which differ in charge; and (c) solvent peturbations of E 1/ 2(Ru(NH3)5 3+,2+) and solvatochromic shifts of the central metal-to-ligand charge transfer (MLCT) and MM‘CT absorbancies of (bpy)2(CN)Ru(CNRu(NH3)5)3+ and (bpy)2Ru(CNRu(NH3)5)2 6+. The experimental observations indicate that changes in the nonbridging ligand of the central metal can result in a range of about 90 mV variation in E 1/2(Ru(NH3)5 3+,2+), the effect of a one unit increase in charge of the central metal is to increase E 1/2 by approximately 65 ± 15 mV, solvent perturbations of E 1/2 and the electron-transfer reorganizational energy, λr, are approximately equal in magnitude, solvational corrections can be treated linearly, and the solvational contributions to E 1/2 that arise from charge delocalization are less than about 10 mV in these complexes. The complexes have a very rich charge-tansfer spectroscopy, and in some complexes as many as seven different CT transitions can be identified which depend on the oxidation stat