The series of complexes [Os-II(CN)(5)L](n-), with L = pyridine or pyrazine derivatives, were prepared in aqueous solution and, in some cases, as sodium or potassium salts. The main feature in the UV-visible spectra is the appearance of an intense, asymmetric MLCT band, split under spin-orbit coupling. The energies and intensities of the MLCT bands decrease and increase, respectively, with the electron-acceptor ability of L, and strong solvatochromic energy shifts are observed in different organic media. The Os(II) complexes can be oxidized chemically or electrochemically to the Os(III) species; the latter show typical LMCT bands in the visible region, independent of L. The redox potentials for the Os-III,Os-II couples (range 0.6-1.0 V (NHE)), shift positively when L becomes more electron-withdrawing or less basic. Reduction potentials for the bound and free Mepz(+) ligand showed similar values, ca. -0.53 V (NHE), for the three [M(CN)(5)L](n-) complexes, suggesting similar back-bonding abilities of Fe, Ru, and Os toward a given L ligand; this is confirmed by the linear plots with unit slope obtained for the energy of the MLCT bands of the [M(CN)(5)L](n-) complexes (M = Fe, Ru) against the values for the [Os(CN)(5)L](n-) complexes. The IR spectra show intense and weak bands at ca. 2050 and 2100 cm(-1), associated with equatorial and axial cyanide stretchings, respectively. The dissociation rate constant for pyrazine release from the [Os(CN)(5)pz](3-) ion shows a saturation kinetic behavior, typical of dissociative mechanisms found for the iron and ruthenium analog complexes; the specific dissociation rate constant, k(-pz) = 2.0 x 10(-8) s(-1) (25 degrees C, I = 0.5 M), is about 3 and 4 orders of magnitude slower than the values found for [Ru(CN)(5)pz](3-) and [Fe(CN)(5)pz](3-), respectively; this is ascribed mainly to the strong a interaction in the Os-L bond.