The complex [H(EtOH)(2)][{OsCl(eta(4)-COD)}(2)(mu-H)(mu-Cl)(2)] (1) has been prepared in high yield by treatment of OsCl3 center dot 3H(2)O (54% Os) with 1,5-cyclooctadiene in ethanol under reflux. Under air, it is unstable and undergoes oxidation by action of O-2 to afford the neutral derivative {OsCl(eta(4)-COD)}(2)(mu-H)(mu-Cl)(2) (2). The terminal chlorine ligands of the anion of 1 are activated toward nucleophilic substitution. Thus, reaction of the salt [NBu4][{OsCl(eta(4)-COD)}(2)-( mu-H)(mu-Cl)(2)] (1a) with NaCp in toluene gives [NBu4][{Os(eta(1)-C5H5)(eta(4)-COD)}(mu-H)(mu-Cl)(2){OsCl(eta(4)-COD)}] (3) as a result of the replacement of one of the terminal chlorine atoms by the cyclopentadienyl ligand. The CH2 group of the latter can be deprotonated by the bridging methoxy ligand of the iridium dimer [Ir(mu-OMe)(eta(4)-COD)](2). The reaction leads to the trinuclear derivative [NBu4][{(eta(4)-COD)Ir(eta(5)-C5H4-eta(1))Os(eta(4)-COD)}(mu-H)(mu-Cl)(2 ){OsCl(eta(4)-COD)}] (4) containing a bridging C5H4 ligand that is eta(1)-coordinated to an osmium atom of the dimeric unit and eta(5)-coordinated to the Ir(eta(4)-COD) moiety. Salt 1a also reacts with LiC=CPh. In this case, the reaction produces the substitution of both terminal chlorine ligands to afford the bis(alkynyl) derivative [NBu4][{Os(C equivalent to CPh)(eta(4)-COD)}(2)(mu-H)(mu-Cl)(2)] (5). Complexes 1, 2, 3, and 4 have been characterized by X-ray diffraction analysis. Although the separations between the osmium atoms are short, between 2.6696(4) and 2.8633(5) angstrom, theoretical calculations indicate that only in 2 is there direct metal-metal interaction, as the bond order is 0.5.