Selective diphosphine ligand chelation and pi bond coordination in CoRu(CO)(7)(mu-PPh2): Kinetics and X-ray structure of CoRu(CO)(4)(mu-bma)(mu-PPh2)

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 a nhydride (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) [wh ere P-P = bma (3a), bpcd (3b)] along with trace amounts of the known comple x CoRu(CO)(6)(PPh3)(mu -PPh2) (4). The requisite pentacarbonyl intermediate s CoRu(CO)(5)(mu -P-P)(mu -PPh2) [where P-P = bma (2a), bpcd (2b)] have bee n prepared by separate routes (mild thermolysis and Me3NO activation) and s tudied for their conversion to CoRu(CO)(4)(mu -P-P)(mu -PPh2). The penta- a nd tetracarbonyl complexes have been isolated and fully characterized in so lution by IR and NMR spectroscopy. The kinetics for the conversion of 2a -- > 3a and of 2b --> 3b were measured by IR spectroscopy in chlorobenzene sol vent. On the basis of the first-order rate constants, CO inhibition, and th e activation parameters (2a --> 3a, DeltaH(double dagger) = 29.2 +/- 1.4 kc al mol(-1) and DeltaS(double dagger) = 8.2 +/- 3.8 eu; 2b - 3b, DeltaH(doub le 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 st ep 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 gr oups are bound to the ruthenium center while the maleic anhydride pi bond i s coordinated to the cobalt atom.