The synthesis of (μ-H)(μ-η2-RR′NCO2)Os3(CO)10 (R=R′=CH3 2a; R=R′=CH2CH3,2b; R=CH3, R′=CH2CH3,2c) and their cyclic analogs (μ-H)(μ-η2- -CO2)Os3(CO)10(n=42d,n=5,2e) from carbon dioxide, secondary amine, and Os3(CO)10(CH3CN)2 (1) are reported. A solid-state structure of2c reveals a bonding mode for the carbamato ligand very similar to that observed for related carboxylato complexes. Compound2c crystallizes in the orthorombic space group Pbca witha=9.136 (3),b=15.310 (4) andc=30.361 (5) A;V=4247 A3,Z=8. Least-squares refinement of 2405 observed reflections gave a final agreement factor ofR=0.043 (R w =0.043). The reactivity of the complexes2a–2e was examined. Compound2c or2b give good yields of the cluster derivatives (μ-H)(μ-X)Os3(CO)10 (X=Cl,3; X=OCH3,4; X=N(CH3)2,7) when reacted with HX. Reaction of2a with P(CH3)3 at 68°C gives good yields of the otherwise difficult to obtain 1,1,2-(P(CH3)3)3Os3(CO)9 (5). Evidence is presented that suggests that2a–2e form by oxidative addition of preformed carbamic acids to1.