Preparation and characterization of a monocyclopentadienyl osmium−allenylcarbene complex

5 pages, 3 schemes, 1 table, 1 figure.-- Supporting Information Available: http: //pubs.acs.org.The dihydride−dihydrogen complex [OsH2(η5-C5H5)(η2-H2)(PiPr3)]BF4 (1) reacts with phenylacetylene to give the allenylcarbene derivative [Os(η5-C5H5){CPh(η2-CHCCHPh)}(PiPr3)]BF4 (2) via the π-phenylacetylene intermediate [Os(η5-C5H5)(η2-PhCCH)(PiPr3)]BF4 (3). The reactions of 2 with NaOMe and LiCCPh afford 3:1 mixtures of two hydride−osmaindene isomers of formula OsH(n5-C5H5)(C(C=CPh)=CHC6H4)(piPr3)(4 and 5) instead of the expected osmabenzyne complex.Financial support from the MEC of Spain (Projects CTQ2005-00656 and Consolider Ingenio 2010 CSD2007-00006)) and Diputación General de Aragón (E35) is acknowledged. Y.A.H. thanks the Spanish MEC for his grant.Peer reviewe