The reaction of [Ir-2(CO)(3)(dppm)(2)] (dppm = Ph2PCH2PPh2) with methyl triflate yields the methylene-bridged hydride [Ir2H(CO)(3)(mu-CH2)(dppm)(2)] [CF3SO3] (2), in which the hydride and methylene hydrogens are rapidly scrambling at ambient temperature. Under CO this species yields the methyl and acyl products [Ir-2(R)(CO)(4)(dppm)(2)] [CF3SO3] (R = CH3, C(O)CH3). Removal of one carbonyl from 2 yields the fluxional methyl complex [Ir-2(CH3)(CO)(2)(dppm)(2)] [CF3SO3] (3) in which the methyl group readily migrates from metal to metal. Addition of CO, PR3, (CNBu)-Bu-t or SO2 to 3 results in C-H bond cleavage of the methyl group yielding the methylene-bridged, hydride species, [Ir2H(CO)(2)L(mu-CH2)(dppm)(2)] [CF3SO3] (L = CO, PR3, (CNBu)-Bu-t) or [Ir2H(CO)(2)(mu-CH2)(mu-SO2)(dppm)(2)] [CF3SO3] (11). Both the carbonyl and SO2 adducts are fluxional, having the hydrogens of the hydride ligand and the methylene group exchanging rapidly at ambient temperature. The activation parameters for this reversible C-H bond-making and -breaking step have been determined (Delta H double dagger = 10.3 kcal/mol? Delta S double dagger = -11.2 cal/mol K (CO); Delta H double dagger = 6.1 kcal/mol, Delta S double dagger: = -6.2 cal/mol K (SO2)). X-ray structure determinations of compound 3, [Ir2H(CO)(2)(PMe3)(mu-CH2)(dppm)(2)][CF3SO3] (6), and compound 11 have been determined to confirm the proposed structures. Density functional theory calculations have been carried out on cations related to 3 and 11 by substitution of the phenyl substituents on the dppm ligands by hydrogens, and on key isomers of these, to gain an understanding of the factors promoting C-H bond cleavage in this system. A proposal is presented rationalizing the facile C-H bond cleavage in 3 upon addition of the substrate molecules, in which the roles of the adjacent metals are described.