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, p y = 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-bridg ing is disclosed by changes in the shape and position of the cyanide s tretching vibration, nu(C=N), in the IR spectrum of the dinuclear ruth enium species, as compared with the mononuclear parent complex. Blue s hifts 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 nit rile-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-t o-metal charge transfer (MMCT) or ''intervalence'' transitions. The lu minescence of A is completely quenched in B, even at 77 K, a fact whic h can be explained on the basis of efficient excited-state electron tr ansfer to form the electronic mixed-valence isomer of B. The strong as ymmetric nature of B, as deduced from cyclic voltammetry data (the dif ference 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 th e back electron transfer (or charge recombination) from the MMCT excit ed state of B probably lies in the ''inverted'' region.