The triosmium cluster 1,2-Os 3 (CO) 10 (MeCN) 2 reacts rapidly with the diphosphine ligand 2,3-bis(diphenylphosphino)-N-p-tolylmaleimide (bmi) at room temperature to give bmi-bridged cluster 1,2-Os 3 (CO) 10 (bmi) (2b) as the major product, along with the chelating isomer 1,1-Os 3 (CO) 10 (bmi) (2c) and the hydride-bridged cluster HOs 3 (CO) 9 [μ-(PPh 2 )CC{PPh(C 6 H 4 )}C(O)N(tolyl-p)C(O)] (3) as minor by-products. All three cluster compounds have been isolated and fully characterized in solution by IR and NMR spectroscopies ( 1 H and 31 P), and X-ray crystallography in the case of 2c. Cluster 2b is unstable and readily isomerizes to 2c in quantitative yield on mild heating. The kinetics for the conversion of 2b→2c have been measured over the temperature range of 318–348K in toluene solution, and based on the observed activation parameters a nondissociative isomerization process that proceeds via a transient μ 2 -bridged phosphine moiety is presented. Near-UV photolysis of cluster 2c at room temperature affords HOs 3 (CO) 9 [μ-(PPh 2 )CC{PPh(C 6 H 4 )}C(O)N(tolyl-p)C(O)] (3) with a quantum yield of 0.017. The reactivity of clusters 2b, 2c, and 3 is discussed with respect to related diphosphine-substituted Os 3 (CO) 10 (P–P) clusters prepared by our groups.