CRYSTAL AND MOLECULAR-STRUCTURE, SPECTROSCOPIC PROPERTIES, AND ELECTROPHILIC REACTIVITY OF SODIUM PENTACYANONITROSYLOSMATE(II) DIHYDRATE

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 preci pitation techniques. The compound is isostructural with the iron and r uthenium analogue species. The crystals are orthorhombic (Pnnm) with a = 6.312 (1) Angstrom, b = 12.090 (2) Angstrom, c = 15.828 (3) Angstro m, and Z = 4. Relevant lengths and angles within the distorted anion o ctahedra are compared for the three (Fe, Ru, Os) compounds. IR and UV- visible spectral data are also comparatively discussed. The results ar e indicative of a very strong sigma-, as well as back-bonding pi-inter action from Os toward nitrosyl and cyanides. The complex is electrophi lically 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 nucleo philic attack of OH- into [Os(CN)(5)NO](2-) in the elementary step. Ki netic and equilibrium results are compared with those obtained for iro n and ruthenium pentacyanonitrosyl analogues, as well as for other nit rosyl complexes containing ruthenium and osmium; thus, the different f actors influencing nucleophilic rates and affinities are discussed, na mely charge, radius and polarizability of the reactants, as well as th e energy of the pi(NO) level. The reactions of [Os(CN)(5)NO](2-) with other nucleophiles proceed through initial adduct formation and furth er decomposition.