The cyclopentadienyl complex Os(η 5 -C 5 H 5 )-Cl(P i Pr 3 ) 2 (1) reacts with diphenylphosphine to give the mixed-ligand compound Os(η 5 -C 5 H 5 )Cl(PHPh 2 )(P i Pr 3 ) (2). Treatment of 2 with TlPF 6 in humid acetone and in methanol affords the cationic dihydride complexes 4)), respectively. Similarly, the treatment of 2 with TlPF 6 in (CD 3 ) 2 CO containing D 2 O and in CD 3 OD gives the deuterated complexes [OsHD(η 5 -C 5 H 5 ){P(OR)Ph 2 }(P i -Pr 3 )]PF 6 (R ) D (3-d 2 ), CD 3 (4-d 4 )). The reaction of 3 with NaOCH 3 produces its deprotonation and the formation of the dihydride-phosphinito derivative OsH 2 -(η 5 -C 5 H 5 ){P(O)Ph 2 }(P i Pr 3 ) (5). Under the same conditions, the deprotonation of 4 yields OsH(η 5 -C 5 H 5 )-{P(OMe)Ph 2 }(P i Pr 3 ) ( 6) as a result of the extraction of one of the two hydrides. The treatment of 3-d 2 with NaOMe selectively affords OsHD(η 5 -C 5 H 5 ){P(O)Ph 2 }(P i -Pr 3 ) (5-d).