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)-t etracyanide complexes were prepared as sodium and potassium salts, res pectively. 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 s olution and in organic solvents. Upon changing the solvent, we detecte d strong solvatochromic shifts, the energies of the MLCT bands going t o lower values when the Lewis acidity of the solvent (measured by the Gutmann's acceptor number) was decreased. Redox potential values for t he Ru(III,II), and Os(III,II) couples were also solvent-dependent, dec reasing with the decrease in acceptor number. The basicities of coordi nated cyanides toward the proton were measured through spectrophotomet ric titrations; the pK(a) values so obtained, together with the IR mea surements in the C-N stretching region (in solid samples and in organi c solvents), and the potentials for ligand (L-L) reduction (in dimethy lformamide 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 s ame ligand environment, the Os(II) metal center behaves as a stronger pi-donor compared to ruthenium and iron. The pi-acceptor ability of th e chelating ligand is enhanced in weakly acceptor solvents, associated with the changes in specific cyanide-solvent interactions. The comple xes emit poorly in aqueous solutions at room temperature, with low qua ntum yields, as predicted by the potentials of the M(III,II) couples. The excited states are very strong reductants, as well as very weak ox idants.