Reactions of the [K](+) salts of the [nido-7,9-C2B9H12](-) anion (2) and its C-phenylated derivative [7-Ph-nido-7,9-C2B9H11](-) (4) with [OsCl2(PPh3)(3)] (3) proceed in benzene at ambient temperature with the formation of 16-electron chlorohydrido-Os(IV) exo-nido complexes, [exo-nido-10,11-{(Ph3P)(2)OsHCl}-10,11-(mu-H)(2)-7-R-7,9-C2B9H8] (5: R = H; 6: R = Ph), along with the small amounts of the charge-compensated nido-carboranes [nido-7,9-C2B9H11PPh3] (7) and [7-Ph-nido-7,9-C2B9H10PPh3] (8) as byproducts. However, when carried out under mild heating in ethanol, the reaction of 2 with 3 selectively afforded a 16-electron dihydrido-Os(IV) exo-nido complex [exo-nido-10,11-{(Ph3P)(2)OsH2}-10,11-(mu-H)(2)-7,9-C2B9H9] (9). Structures of both complexes 5 and 9 have been confirmed by single-crystal X-ray diffraction studies, which revealed that nido-carboranes in these species function as a bidentate dicarbollide ligands [7-R-nido-7,9-C2B9H10](2-) linked to the Os(IV) center via two B-H center dot center dot center dot Os bonds involving adjacent B-H vertices in the upper CBCBB belt of the carborane cage. Thus, compounds 5 and 9 represent the first structurally characterized exo-nido-metallacarboranes based on meta-dicarbollide-type ligands. Variable-temperature H-1 and P-31{H-1} NMR experiments indicate that complex 9 is fluxional in solution and shows an unusual exchange between terminal Os-(H)(2) and bridging {B-H}(2)center dot center dot center dot Os hydrogen atoms. Upon heating in d(8)-THF at 65 degrees C, complex 9 converts irreversibly to its closo isomer [2,2-(PPh3)(2)-2,2-H-2-closo-2,1,7-OsC2B9H11] (13), which could thus be obtained as a pure crystalline solid. The structure of 13 has been established on the basis of analytical and multinuclear NMR data and a single-crystal X-ray diffraction study.