Cyano complexes, osmium

Cyano complexes of osmium, amongst other transition metals, have been reviewed.19 There is a brief section on osmium cyano complexes in ref. 16, p. 976. 

A few substituted osmium(II) cyano complexes are known (for [Os(NO)(CN)s]2 see p. 546). Reaction of (NH4)2[OsC16] and 2,2 -bipyridyl followed by treatment with NaCN gives Os(C-N)2(bipy)2 2H20 in which the CN groups may be cis. Protonation is said to occur in perchloric acid to give 0s(CN)2(bipy)2 2HC104 ... 

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. 

NSiQHn, tert-Butylamine, iV-(trimethyl-silyl)-, 25 8 N2, Dinitrogen iron complexes, 24 208, 210 osmium complex, 24 270 N2BC4H, Borane, cyano-compd. with trimethylamine (1 1),... 

The questirm of periselectivity addresses the issue whether the AH proceeds via a concerted (3+2) cycloaddition pathway or via a (2+2) mechanism (Scheme 11), which was studied for ethylene interacting with the cyano-substituted imido osmium complex. 

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