REDUCTIVE NITROSYLATION OF TETRAOXOMETALLATES .16. GENERATION AND REACTIVITY OF THE OS(NO)3+ MOIETY - SYNTHESIS, CHARACTERIZATION, AND ELECTROCHEMISTRY OF OXALATO-NITROSYL AND HALOGENO-NITROSYL DERIVATIVES OF OSMIUM - CRYSTAL AND MOLECULAR-STRUCTURE OF BIS(1,10-PHENANTHROLINIUM) PENTABROMONITROSYLOSMATE DIHYDRATE

Osmium tetraoxide smoothly reacts with NH2OH.HCl in the presence of oxalate ion furnishing solid complexes A[Os(NO)(C2O4)2].H2O (A = K, 1a; PPh4, 1b; or AsPh4, 1c). The complex anion possesses a square-pyramidal structure, as is evident from analytical, spectroscopic (IR, UV/VIS), and thermoanalytical data. The complexes react with concentrated HX (X = Cl, Br or l) forming besides [Os(NO)(C2O4)Cl3]2- 3, [Os(NO)Br4(H2O)]- 4a and [Os(NO)Br4]- 4b; [Os(NO)X5]2- (X = Cl, 2a; Br, 2b; or l, 2c) as the main product. The latter (X = Cl or l) reacts with phen (1,10-phenanthroline) producing [Os(NO)X3(phen)], whereas the bromo complex furnishes [Hphen]2[Os(NO)Br5].2H2O, 5c. The crystal and molecular structure of 5c has been determined by Patterson and Fourier methods and refined to R = 0.052 for 1221 unique reflections with l > 3-sigma-(l). The crystals are monoclinic, space group P2,/c, a = 7.371 (1), b = 13.646(1), c = 14.001 (2) angstrom, beta = 99.88(1)-degrees and Z = 2. The structure consists of [Hphen]+ cations, disordered [Os(NO)Br5]2- anions, and two water molecules of crystallisation; the anions have approximate octahedral geometry with a near linear nitrosyl ligand in the axial positions. Cyclic voltammograms of compounds 2a and 2b show a reversible oxidative response in MeCN at E298-degrees + 0.94 and + 0.96 V respectively, while 2c shows an irreversible oxidation peak at + 1.08 V in dimethylformamide, indicating the ease of oxidation of Os(NO)3+ --> Os(NO)4+ (confirmed by ESR study), increasing in the series Cl- < Br- < l-.