Abstract A number of anionic adducts of H2Os3(CO)10 have been prepared. Potassium hydride reacts with H2Os3(CO)10 in a 1 : 1 molar ratio in THF to yield K[H3Os3(CO)10]. When a 2 : 1 molar ratio of KH to H2Os3(CO)10 is employed, one mole of H2 is given off and K2[H2Os3(CO)10] is formed. Effectively two electrons have added to H2Os3(CO)10. In the presence of an electron carrier, (C6H5)2CO, potassium reacts with H2Os3(CO)10 to give K2[H2Os3(CO)10] which is spectroscopically identical to K2[H2Os3(CO)10] obtained from the reaction of 2 KH with 1 H2Os3(CO)10. Halide ions as [(C6H5)3P]2N+ salts readily add to H2Os3(CO)10, forming [H2Os3(CO)10X]−, where X− = Cl−, Br−, and I−. Metallocarbonylates react with H2Os3(CO)10 in a analogous to the reaction of halide ions with this cluster. The following mixed metal ions habe been prepared: [H2Os3(CO)10Mn(CO)5]−, [H2Os3(CO)10Fe(CO)4]2−, [H2Os3(CO)10Mo(η5-C5H5(CO) 3]−, [H2Os3(CO)10V(η5-C5H5)(CO)3]2−, and [H2Os3(CO)10Fe(η-C5H5)(CO)2]−.