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Substitution of MeCN ligands in the activated cluster Os3(CO)10(MeCN)2 by the unsaturated diphosphine ligands (Z)-Ph2PCH=CHPPh2 (cDPPEn) or 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) proceeds rapidly at room temperature to furnish the ligand-bridged cluster 1,2-Os3(CO)10(P-P) (P-P represents cDPPEn or bpcd). Heating 1,2-Os3(CO)10(P-P) leads to the formation of the thermodynamically more stable chelating isomer 1,1-Os3(CO)10(P-P). Each compound of Os3(CO)10(P-P) has been characterized by x-ray diffraction, IR, 31P NMR and 1H NMR. Ligand isomerization kinetics have been investigated by UV-VIS and 31P NMR (for cDPPEn) or 1H NMR (for bpcd) spectroscopies. The isomerization mechanism is discussed based on the activation parameters and CO inhibition (for cDPPEn) or ligand trapping experiments (for bpcd). Thermolysis of 1,1-Os3(CO)10(bpcd) in refluxing toluene gives the hydrido cluster HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and the benzyne cluster HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)]. Photolysis of 1,1-Os3(CO)10(bpcd) using near UV light affords HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] as the sole product. HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been characterized in solution by IR and NMR spectroscopies. Furthermore its molecular structure has been determined by X-ray crystallography. Reversible C-H bond formation in HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] is demonstrated by ligand trapping studies to give 1,1-Os3(CO)9L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os3(CO)9(bpcd). The kinetics for reductive coupling in HOs3(CO)9[γ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and DOs3(CO)9[μ-(PPh2-d10)C=C{P(Ph-d5)(C6D4)}C(O)CH2C(O)] in the presence of PPh3 give rise to a kH/kD value of 0.88, whose magnitude supports the existence of a preequilibrium involving the hydride(deuteride) cluster and a transient arene-bound Os3 species that precedes the rate-limiting formation of 1,1-Os3(CO)9(bpcd). Strong proof for the proposed hydride(deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os3(CO)10(bpcd-d4ortho), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Equilibrium and kinetic isotope effects in the orthometallation step has been determined by 1H NMR in photochemical studies. Kinetics for the transformation from HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] to HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been studied by UV-VIS spectroscopy for which the mechanism is discussed.