Sodium pentacyanonitrosylosmate(II) dihydrate, Na-2[Os(CN)(5)NO].2H(2)O, was synthesized by photolyzing a mixture of hexacyanoosmate(II) and nitrite ion, with further purification through ion-exchange and precipitation techniques. The compound is isostructural with the iron and ruthenium analogue species. The crystals are orthorhombic (Pnnm) with a = 6.312 (1) Angstrom, b = 12.090 (2) Angstrom, c = 15.828 (3) Angstrom, and Z = 4. Relevant lengths and angles within the distorted anion octahedra are compared for the three (Fe, Ru, Os) compounds. IR and UV-visible spectral data are also comparatively discussed. The results are indicative of a very strong sigma-, as well as back-bonding pi-interaction from Os toward nitrosyl and cyanides. The complex is electrophilically reactive toward several bases such as OH-, SH- and N2H4. With OH-, an equilibrium reaction is established, [Os(CN)(5)NO](2-) + 2OH(-) reversible arrow [Os(CN)(5)NO2](4-) + H2O, with K = 42 +/- 1 M(-2) (I = 1 M, 25.0 degrees C). From the kinetic and mechanistic analysis, k(obs) = 1.37 x 10(-4) M(-1) s(-1), is determined by the rate of nucleophilic attack of OH- into [Os(CN)(5)NO](2-) in the elementary step. Kinetic and equilibrium results are compared with those obtained for iron and ruthenium pentacyanonitrosyl analogues, as well as for other nitrosyl complexes containing ruthenium and osmium; thus, the different factors influencing nucleophilic rates and affinities are discussed, namely charge, radius and polarizability of the reactants, as well as the energy of the pi*(NO) level. The reactions of [Os(CN)(5)NO](2-) with other nucleophiles proceed through initial adduct formation and further decomposition.