CYANO-BRIDGED COMPLEXES OF BIS(2,2'-BIPYRIDINE)(PYRIDINE)RUTHENIUM(II)

New complexes with the Ru(bpy)2(py)2+ moiety (bpy = 2,2'-bipyridine, py = pyridine) connected through a cyano group to Ru(NH3)53+ and Fe(CN)52- as electron acceptors-have been prepared and their spectroscopic, electrochemical and photophysical properties investigated. Cyano-bridging is disclosed by changes in the shape and position of the cyanide stretching vibration, nu(C=N), in the IR spectrum of the dinuclear ruthenium species, as compared with the mononuclear parent complex. Blue shifts in the lowest energy d(pi) --> pi* (Ru --> bpy) metal-to-ligand charge transfer (MLCT) transition occur when going from [Ru(bpy)2(py)(CN)]+ (A) to [(bpy)2(py)Ru(II)-CN-Ru(III)(NH3)5]4+ (B) and to [(bpy)2(py)Ru(II)-CN-Fe(III)(CN)5]- (C), thus pointing to the existence of nitrile-bound pentaammineruthenium(III) and pentacyanoferrate(II) capping groups in the mixed-valence species B and C. Besides, new intense and broad absorptions at 697 (in HCl 0.01 M) and 700 nm (in H2O/Me2CO, 1:1 v/v) appear in B and C, respectively, and can be assigned to metal-to-metal charge transfer (MMCT) or ''intervalence'' transitions. The luminescence of A is completely quenched in B, even at 77 K, a fact which can be explained on the basis of efficient excited-state electron transfer to form the electronic mixed-valence isomer of B. The strong asymmetric nature of B, as deduced from cyclic voltammetry data (the difference in redox potentials between both ruthenium sites amounts to 1.30 V), together with a strong electronic coupling [H(AB) = 2200 cm-1, calculated from the ''intervalence'' absorption data] indicate that the back electron transfer (or charge recombination) from the MMCT excited state of B probably lies in the ''inverted'' region.