Thermolysis of CoRu(CO)(7)(mu -PPh2) (1) in refluxing 1,2-dichloroethane in the presence of the diphosphine ligands 2,3-bis(diphenylphospkino)maleic anhydride (bma) and 4,5-bis(diphenylphosphino)-4-cyclopenten1,3-dione (bpcd) furnishes the new mixed-metal complexes CoRu(CO)(4)(mu -P-P)(mu -PPh2) [where P-P = bma (3a), bpcd (3b)] along with trace amounts of the known complex CoRu(CO)(6)(PPh3)(mu -PPh2) (4). The requisite pentacarbonyl intermediates CoRu(CO)(5)(mu -P-P)(mu -PPh2) [where P-P = bma (2a), bpcd (2b)] have been prepared by separate routes (mild thermolysis and Me3NO activation) and studied for their conversion to CoRu(CO)(4)(mu -P-P)(mu -PPh2). The penta- and tetracarbonyl complexes have been isolated and fully characterized in solution by IR and NMR spectroscopy. The kinetics for the conversion of 2a --> 3a and of 2b --> 3b were measured by IR spectroscopy in chlorobenzene solvent. On the basis of the first-order rate constants, CO inhibition, and the activation parameters (2a --> 3a, DeltaH(double dagger) = 29.2 +/- 1.4 kcal mol(-1) and DeltaS(double dagger) = 8.2 +/- 3.8 eu; 2b - 3b, DeltaH(double dagger) = 27.7 +/- 0.6 kcal mol(-1) and DeltaS(double dagger) = 1.4 +/- 1.6 eu), a mechanism involving dissociative CO loss as the rate-limiting step is proposed. The solid-state structure of CoRu(CO)(4)(mu -bma)(mu -PPh2) (3a), as determined by X-ray crystallography, reveals that the two PPh2 groups are bound to the ruthenium center while the maleic anhydride pi bond is coordinated to the cobalt atom.