The new complex, K-3[Os(CN)(5)NH3](.)2H(2)O, a convenient precursor for the pentacyano-L-osmate(II) series, was prepared and characterized by chemical analysis, cyclic voltammetry, and IR and UV-vis spectroscopies. By controlled aquation in weakly acidic medium, the [Os(CN)(5)H2O](3-) ion was generated. Weak absorptions in the UV region for L = H2O, NH3, and CN- were found at 287, 272, and 240 nm, respectively, and were assigned to d-d transitions, in terms of a model for tetragonally distorted ions also valid for the members of the iron and ruthenium series, The kinetics of the formation and dissociation reactions of the [Os(CN)(5)L](n-) ions, L = pyridine (py), pyrazine (pz), N-methylpyrazinium (mpz(+)), etc., were studied. At 25.0 degreesC, the formation rate constants for the neutral ligands pz and isonicotinamide were ca. 0.13 M-1 s(-1) and slightly increased for L = mpz(+) and decreased for isonicotinate. The enthalpies of activation were ca, 22.0 kcal mol(-1), independently of the entering L, and the activation entropies were all positive, ca, 11-13 cal K-1 mol(-1). The dissociation reactions showed a saturation rate behavior of k(obs) (s(-1)) as a function of the concentration of the scavenger ligand. The specific dissociation rate constant at 25.0 degreesC was 1.06 x 10(-7) s(-1) for L = NH3 and around 10(-9) s(-1) for py, pz, and mpz(+) (extrapolated to 25.0 degreesC from values measured in the range 60-95 degreesC). These small values are associated with high activation enthalpies (range 30-35 kcal mol(-1)) and positive activation entropies (range 10-20 cal K-1 mol(-1)). The evidence for both the formation and dissociation processes shows that dissociative mechanisms are operative, as for the iron and ruthenium analogues.