Electrochemical and spectroscopic manifestations of donor-acceptor coupling in cyanide bridged transition metal complexes: contrasts between RuCNRu, CoCNRu and RhCNRu systems

Abstract

An issue of contrasting donor-acceptor electronic coupling in cyano-bridged transition metal complexes containing ruthenium, rhodium or cobalt acceptors has been addressed by means of spectroscopic and electrochemical comparisons of several series of M(CNRu(NH 3) 5) 2 complexes where M is Ru II(bpy) 2, Co III(MCL), Rh III(bpy) 2 or Rh III(MCL) and MCL is a tetraazamacrocyclic ligand. All the complexes exhibit metal-to-metal (M II→Ru III(NH 3) 5 or Ru II(NH 3) 5→M III) charge transfer (MMCT) absorptions. These are highest in energy for M=Rh(III) and lowest for M=Ru(II). The electrochemical half-wave potentials of the Ru(NH 3) 3+,2+ 5 couples are strongly correlated with the energies of the MMCT transitions and span a 300 mV range. The M=Co III(MCL), Rh III(bpy) 2 and Rh III(MCL) complexes, in which Ru II(NH 3) 5→M IIIMMCT transitions are observed, have E 1 2 values greater than 0.22 V versus SCE, while the M=Ru II(bpy) 2 complexes have values of E 1 2 which are less than 0.0 V. The results suggest that common perturbational treatments of the MMCT transitions in RuCNRu complexes are inadequate owing to the exceptionally strong electronic coupling, and that the perturbational MMCT band shape analysis underestimates the electronic coupling. This work indicates that the donor-acceptor electronic coupling is greater in RuCNRu complexes than in RuCNCo complexes, qualitatively in accord with reports of contrasting MMCT excited state back electron transfer behavior.