Osmium v

Osmium(viii).—From studies of the extraction of OSO4 from alkaline aqueous solutions by carbon tetrachloride, it has been suggested that the anion [0s04(0H)] may exist in these media. Similar evidence was found for the presence of [OsOjN] in aqueous ammonia solutions of 0s04. Examination of the i.r. spectra of a series of metal complexes of the ligand 8-amino-7-hydroxy-4-methylcoumarin, including [OsO LjjClj, indicate a correlation between certain vibrational bands, e.g. v(M—N), and the stability of the compounds.  

An Os —SnClj charge-transfer complex of the type (25) has been proposed as an intermediate in the 0s04-catalysed reduction of Fe by [SnCl3].  

Osmiiim(V).—Following a preliminary report, details have appeared for the synthesis of the arylimido complexes [OsCl3(PPh3)2(NC6H4R)] (R = H, Cl, or OMe). The paramagnetism of these pink, air-stable solids is consistent with an osmium(v) centre. 

OsmiumfVI).—The preparation and properties of the new terminal nitrido-complexes R[Os(N)X4] (X = Cl or Br R = Ph4As or Bu N) have been described. A preliminary investigation of the slow hydrolysis of [OsP ] has been carried out, from which the initial product appears to be [OsOp4].  

Structure based on central tetrahedron of b four Os atoms, with remaining two Os atoms bridging. Formal oxidation states of Os atoms vary from — 1 to +1. Each Os has three terminal CO ligands Seven Os atoms in a capped octahedral b arrangement 

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Osmium(VI) hydrazido complexes can be generated by electrochemical oxidation of the corresponding osmium(V) hydrazido complexes (Section 5.6.5.3.1). The complex trans-[0s (tpy)(Cl)2(NN(CH2)40)] " (83) is able to oxidize benzyl alcohol to benzaldehyde. It also oxidizes PPhs to PPh30, and R2S to give R2SO the source of O atoms is presumably H2O in the solvent. 

Although a number of stable osmium(IV) phosphoraniminato complexes are known, the corresponding osmium(V) complexes are unstable. The net reaction for one-electron oxidation of [Os (NPPh3)(tpy)(Cl)2] to Os " is shown in Equation (54). The reaction presumably occurs by initial disproportionation of Os to Os " and Os " followed by PPh3 transfer. Similarly,... 

Osmium(V) sulfilimido complexes can be generated by electrochemical or chemical oxidation of the corresponding osmium(IV) complexes (see Section 5.6.6.4.5). 

Osmium(V) porphyrin complexes, [Osv(P)(OR)2] +, have recently been generated by electrochemical oxidation of the corresponding [Osiv(P)(OR)2] and were characterized by UV-VIS spectroscopy (112). The [Osv(salen)(OR)2]+ complexes have been generated and characterized in a similar manner (152). 

Since, like terpyridyl (p. 542), these ligands are good n acceptors by virtue of their considerable ring conjugation, it is the osmium(II) (d6) state which is the commonest and generally the most stable thus [Os(LL)3]3+ species are good one-electron oxidants. Nevertheless there are also examples of osmium(IV) and even a few of osmium(V) and osmium(VI) (the osmyl osmium(VI) complexes are considered on p. 581). 

The osmium(V) species 0s02(terpy)(0H) has also been detected electrochemically at pH 13, from reduction of [0s02(terpy)(0H)]+ the latter is made from trans-K2[0s02(0H)4] and terpy.194 Binuclear complexes. For [0s20(terpy)2(bipy)2]4+ and [Os20(terpy)2(bipy)2]2+, see p. 594.192... 

Electrochemical oxidation of Os2N(S2CNR2)5 (R = Me, Et) showed that a reversible one-electron oxidation occurs to [Os2N(S2CNR2)5]+, an osmium(IV)-(V) species and a further irreversible oxidation, possibly to an osmium(V) dimer.350... 

As chemically summarised in Fig. 2, the compound behaves as a powerful oxidising agent and as a derivative of platinum(v). Its hydrolysis by water vapour to oxygen, platinum dioxide, and hydrogen hexafluoroplatinate(iv) resembles that of osmium(v), iridium(v), and ruthenium(v) fluoro-salts, which are similarly reduced to the acids of their 4+ ions e.g.,... 

Kelly, C. Szalda, D. J. Creutz, C. Schwarz, H. A. Sutin, N. Electron transfer barriers for ground- and excited-state redox couples trans-dioxo(l,4,8,ll-tetramethyl-l,4,8,l 1- tetraazacyclo-tetradecane) osmium(VI)/osmium(V). Inorg. Chim. Acta 1996, 243, 39-45. 

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