Elongated Dihydrogen Versus Compressed Dihydride in Osmium Complexes

Small modifications on the co-ligands of complexes containing two coordinated hydrogen atoms can determine the elongated dihydrogen versus compressed dihydride nature of these species and therefore their chemical behavior. 2,6-diphenylpyridine favors the formation of the osmium(IV) cation [OsH (C H pyPh)(PiPr ) ] , whereas 2-phenoxy-6-phenylpyridine, which contains an oxygen atom between the heterocycle and one of the phenyl groups, stabilizes the osmium(II) elongated dihydrogen species [Os(C H pyOPh)(η -H )(PiPr ) ] . In contrast to the latter, the former shows a marked tendency to undergo reductive elimination of the heterocycle.