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 [OsH2 (C6 H4 pyPh)(PiPr3 )2 ]+ , 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(C6 H4 pyOPh)(η2 -H2 )(PiPr3 )2 ]+ . In contrast to the latter, the former shows a marked tendency to undergo reductive elimination of the heterocycle.