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

Abstract

The effects of donor−acceptor (D/A) electronic coupling, HDA, 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)2n+ 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, hDA, of the D/A charge-transfer MM‘CT absorption band and the geometrical distance between donor and acceptor, rDA. Different series of complexes exhibit different correlations between E1/2 and hDA. 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 E1/2 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 E1/2(Ru(NH3)53+,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)26+. 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 E1/2(Ru(NH3)53+,2+), the effect of a one unit increase in charge of the central metal is to increase E1/2 by approximately 65 ± 15 mV, solvent perturbations of E1/2 and the electron-transfer reorganizational energy, λr, are approximately equal in magnitude, solvational corrections can be treated linearly, and the solvational contributions to E1/2 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 state of the Ru(NH3)5 moiety. There is evidence for considerable mixing between these transitions. The mixed valence (Ru(NH3)52+/Ru(NH3)53+), bisruthenates exhibit a unique Ru(NH3)5/M MM‘CT component in addition to the expected Ru(NH3)52+ → Ru(NH3)53+ CT; this relatively weak absorption tracks the dominant Ru(NH3)5/central metal MM‘CT absorption, and it is attributable to the different effects of local Mc(CN-)Ru(NH3)5 electronic coupling in the mixed valence complex. Values of E1/2(obsd), corrected for solvational effects implied by the experimental observations, correlate with hDA, corrected for symmetry effects, E1/2(corr) = E1/2(ref) ± (4.2 × 10-4) hDA/rDA, only if the “solvational correction” for Fe(II)- and Ru(II)-centered complexes is about 70% larger than suggested by the experimental observations. This may imply greater charge delocalization onto (or from) the bridging ligand for these two metal centers. For either interpretation, the correlation between E1/2(obsd) and hDA implies that ≤ 0.62rDA. This relatively small value of can be interpreted in terms of charge delocalization onto (or from) the bridging ligand, and it can be qualitatively described in terms of perturbational mixing of the ground and excited electron-transfer states with higher energy CT states. This mixing is described in terms of a multicenter (Mc−C−N−Rut) vibronic coupling model which was previously (Inorg. Chem. 1996, 34, 3463) used to account for the anamolous shifts of the CN- stretch in CN--bridged D/A complexes.