COMPARATIVE BONDING AND PHOTOPHYSICAL PROPERTIES OF 2,2'-BIPYRIDINE AND 2,2'-BIPYRAZINE IN TETRACYANO COMPLEXES CONTAINING RUTHENIUM AND OSMIUM

The (2,2'-bipyrazine)ruthenium(II) - and (2,2'-bipyridine)osmium(II)-tetracyanide complexes were prepared as sodium and potassium salts, respectively. Assignments of the MLCT (d pi(M) --> pi*(L-L)) and IL (pi(L-L) -->, pi*(L-L)) bands were performed analyzing spectra in aqueous solution and in organic solvents. Upon changing the solvent, we detected strong solvatochromic shifts, the energies of the MLCT bands going to lower values when the Lewis acidity of the solvent (measured by the Gutmann's acceptor number) was decreased. Redox potential values for the Ru(III,II), and Os(III,II) couples were also solvent-dependent, decreasing with the decrease in acceptor number. The basicities of coordinated cyanides toward the proton were measured through spectrophotometric titrations; the pK(a) values so obtained, together with the IR measurements in the C-N stretching region (in solid samples and in organic solvents), and the potentials for ligand (L-L) reduction (in dimethylformamide solutions) were used as additional indicators of the sigma-pi bonding interactions. Bipyrazine behaves as a stronger pi-acceptor than bipyridine for a given metal center. On the other hand, for the same ligand environment, the Os(II) metal center behaves as a stronger pi-donor compared to ruthenium and iron. The pi-acceptor ability of the chelating ligand is enhanced in weakly acceptor solvents, associated with the changes in specific cyanide-solvent interactions. The complexes emit poorly in aqueous solutions at room temperature, with low quantum yields, as predicted by the potentials of the M(III,II) couples. The excited states are very strong reductants, as well as very weak oxidants.