Treatment of unsaturated [Os 3 (CO) 8 {μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }(μ-H)] (2) with t BuNC at room temperature gives [Os 3 (CO) 8 (CNBu t )){μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }(μ-H)] (3) which on thermolysis in refluxing toluene furnishes [Os 3 (CO) 7 (CNBu t ){μ 3 -Ph 2 PCHP(Ph)C 6 H 4 }(μ-H) 2 ] (4). Reaction of the labile complex [Os 3 (CO) 9 (μ-dppm)(NCMe)] (5) with t BuNC at room temperature affords the substitution product [Os 3 (CO) 9 (μ-dppm)(CNBu t )] (6). Thermolysis of 6 in refluxing toluene gives 4. On the other hand, the reaction of unsaturated [Os 3 (CO) 9 {μ 3 -η 2 -C 7 H 3 (2-Me)NS}(μ-H)] (7) with t BuNC yields the addition product [Os 3 (CO) 9 (CNBu t ){μ-η 2 -C 7 H 3 (2-Me)NS}(μ-H)] (8) which on decarbonylation in refluxing toluene gives unsaturated [Os 3 (CO) 8 (CNBu t ){μ 3 -η 2 -C 7 H 3 (2-Me)NS}(μ-H)] (9). Compound 9 reacts with PPh 3 at room temperature to give the adduct [Os 3 (CO) 8 (PPh 3 )(CNBu t ){μ-η 2 -C 7 H 3 (2-Me)NS(μ-H)] (10). Compound 8 exists as two isomers in solution whereas 10 occurs in four isomeric forms. The molecular structures of 3, 6, 8, and 10 have been determined by X-ray diffraction studies.