The dihydrido-olefin complex OsH2(eta(2)-CH2=CHEt)(CO)((PPr3)-Pr-i)(2) (2) reacts with H(2)SiPh(2) to give OsH3(SiHPh(2))(CO)((PPr3)-Pr-i)(2) (3). The molecular structure of 3 has been determined by X-ray diffraction (monoclinic, space group P2(1)/c with a = 16.375(2) Angstrom, b = 11.670(1) Angstrom, c = 18.806(2) Angstrom, beta = 107.67(1)degrees, and Z = 4) together with ab initio calculations on the model compound OsH3(SiH3)(CO)(PH3)(2). The coordination geometry around the osmium center can be rationalized as a heavily distorted pentagonal bipyramid with one hydride ligand and the carbonyl group in the axial positions. The two other hydride ligands lie in the equatorial plane, one between the phosphine ligands and the other between the SiHPh(2) group and one of the phosphine ligands. Complex 3 can also be prepared by reaction of OsH(eta(2)-H2BH2)(CO)((PPr3)-Pr-i)(2) (4) With H(2)SiPh(2). Similarly, the treatment of 3 with HSiPh(3) affords OsH3(SiPh(3))(CO)((PPr3)-Pr-i)(2) (5), while the addition of H(3)SiPh to 4 in methanol yields OsH3{Si(OMe)(2)Ph}(CO)((PPr3)-Pr-i)(2) (6). Complex 2 also reacts with HGeR(3) and HSnR(3) to give OsH3(GeR(3))(CO)((PPr3)-Pr-i)(2) (GeR(3) = GeHPh(2) (7), GePh(3) (8), GeEt(3) (9)) and OsH3(SnR(3))(CO)((PP3)-P-i)(2) (R = Ph (10), (n)Bu (11)), respectively. In solution, compounds 3 and 5-11 are fluxional and display similar H-1 and P-31{H-1} NMR spectra, suggesting that they possess a similar arrangement of ligands around the osmium atom.