SYNTHESIS AND PROTON-COUPLED REDOX PROPERTIES OF MONONUCLEAR OR ASYMMETRIC DINUCLEAR COMPLEXES OF RUTHENIUM, RHODIUM AND OR OSMIUM CONTAINING 2,2'-BIS(2-PYRIDYL)-6,6'-BIBENZIMIDAZOLE/

New mononuclear and heterodinuclear complexes. [ML(2)(H(2)L(1))](2+) a nd [L(2)'M(H(2)L(1))M'L(2)''](4+) [M, M' = Ru, Rh and/or Os; L' or L'' = 2.2'-bipyridine (bipy), 1,10-phenanthroline or 4,4'-dimethytbipyrid ine], containing the dinucleating ligand 2,2'-bis(2-pyridyl)-6,6'-bibe nzimidazole (H(2)L(1)) have been prepared. The metal-to-ligand charge- transfer bands are almost unaltered when changing from the mono- to di nuclear complexes, indicating that the bridging H(2)L(1) ligand has sl ightly lower pi orbital energy than that of bipy. The bridging H(2)L( 1) ligand acts as a sigma/pi-donor ligand. Both the absorption spectra and the oxidation potentials of the: complexes are strongly dependent on the solution pH, which determines the NH deprotonation of the co-o rdinated ligand H(2)L(1). The mononuclear complexes of Ru and Os act n ot only as basic acids but also as diacidic bases while the heterodinu clear complexes essentially act as dibasic acids. The proton-coupled r edox reaction was demonstrated by plots of E, vs. pH (Pourbaix diagram s). The pK(a) values of the complexes reflect on both the type of meta ls and their oxidation states, M(II) and M(III). The introduction of a symmetry in the dinuclear complexes containing H(2)L(1) can provide no t only a potential difference between the two metal sites but also a p referential protonation (or deprotonation) site.