Correlations between spectroscopic, electrochemical, and kinetic properties of cyano-bridged binuclear complexes. Analyses of temperature, pressure, and solvent effects

Several asymmetric cyano-bridged mixed-valence binuclear complexes were studied using electrochemical (square wave and cyclic voltammetry), spectroscopic (UV-vis-NIR absorption and H-1 NMR), and kinetic (stopped-flow) techniques, with special attention to the correlations between thermodynamic and optical charge-transfer characteristics determined through the variation of temperature and pressure. The CN-bridged isomers for representative EDTA-ligated complexes, viz,, [(EDTA)Ru-III(mu-CN)M(CN)(x)](5-) (M = Fe-II, x = 5; M = Mo-IV, x = 7), were synthesized and characterized and shown to exhibit essentially different electrothermodynamic and optical charge-transfer parameters compared to "normal" isomers. The binuclear complex-formation kinetics, spectroscopic and preliminary H-1 NMR data indicate that at least two EDTA ligand conformer states are formed in the course of the dimerization process studied. Using the difference DO-D stretching overtone spectroscopy of heavy water acting as solvent, it was shown that the change in hydrogen-bonding order of specifically solvating water molecules present in the first solvation shells around the redox centers may be largely responsible fur the observed dramatic changes in electrothermodynamical and optical characteristics upon the variation of the nature of the ligands and their conformer (EDTA) or bridging-isomer (mu-CN) states.