Osmium complexes pyrazines

In this section we consider the osmium complexes of the three potentially bifunctional ligands pyrazine (pyz), pyrimidine (pym) and pyridazine (pyd) (Table 8). All form terminal complexes with... 

There are also bis(dinitrogen) complexes of osmium(II). The first to be made was cis-[Os(N2)2(NH3)4]2+ by a diazotization procedure using [Os(N2)(NH3)5]2" and nitrite in acid solution.270 Vibrational spectroscopic data using both 2H and 15N substitution were obtained for the salts,271 which are useful precursors for other osmium complexes involving pyrazine (see p. 536). A similar preparative procedure involving [Os(N2)(NH3)4L]2+ gives m-[Os(N2)2(NH3)3L]2+ (L = isonicotinamide).98... 

Pyrazine, bridged supramolecular complexes, 46 206-213, 240-243 p-Pyrazine complexes, osmium, 37 307 Pyrazolates, ligand with rhodium complex, 44 278 Pyrazoles... 

These constitute an important recent feature of osmium chemistry, but for convenience (and chemical sense) they are considered under the sections dealing with bridging ligands. For such complexes with bridging dinitrogen, see p. 556, with bridging pyrazine, see p. 536, and bridging nitride, see p. 563. 

Salts of [OsL(NH3)5]3+ (L = pyrazine, pyrimidine, pyridazine) can be made by displacement of nitrogen from [Os(N2)(NH3)5]2+9° or from the triflato complex [0s(0S02CF3)(NH3)J]2+ and the ligands.62 Cyclic voltammetry showed reduction to the osmium(II) species, 90 and the surface-enhanced Raman spectra (SERS) of [Os(pyz)(HNH3)5]3+, [Os(pyz)(NH3)5]2+ and of their ammine-d5 derivatives were measured, as were the IR spectra.91... 

Irradiation of an aqueous solution of [Os(NH3)5(N2)] by electrons or y-rays gives [Os(NH3)4(NH2XN2)] by action of OH radicals. Complexes involving heterocyclic ligands [Os(NH3)4(N2)L] (L = pyrazine, JV-methylpyrazium, isonicotinamide, and isonicotinic acid) have been synthesized by displacement of N2 from [Os(NH3)4(N2)2] under Ar. Aerial oxidation of these complexes in the presence of HX (L = pyr or isonicotinic acid) leads to the reversible formation of [Os(NH3)4X(L)]X2 (X = Cl or Br) pK, measurements indicate marked back bonding from osmium to the heterocycle. 

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