Ligand substitution in the mixed-metal cluster PhCCo2Ni(CO)6Cp by 2,3-bis(diphenylphosphino)maleic anhydride (bma): An intimate picture involving the stepwise conversion of the trinuclear cluster PhCCo2Ni(CO)4(η2-bma)Cp to the mononuclear compound CpNi[PP

Heating the mixed-metal cluster PhCCo2Ni(CO)6Cp with the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) in 1,2-dichloroethane proceeds by CO loss and formation of the cobalt-bridged cluster PhCCo2Ni(CO)4(η2-bma)Cp (2). The bma ligand is fluxional in solution and is in equilibrium with the cobalt-chelated isomer, as demonstrated by VT IR and 31P NMR measurements. The van’t Hoff parameters (ΔH = 1.49 ± 0.02 kcal/mol; ΔS = 12.0 ± 0.1 eu) for the chelate-to-bridge equilibrium have been evaluated from IR band-shape analyses of the in-phase anhydride carbonyl stretching band over the temperature range 173–116 K. Cluster 2 readily loses CO to furnish the cluster PhCCo2Ni(CO)3(μ,η2-bma)Cp (3), where the 6e− donor bma ligand chelates one of the cobalt centers via the phosphine groups and is tethered to the second cobalt center by the maleic anhydride π bond. Continued heating of cluster 3 is followed by the formation of 50e− cluster Co2Ni(CO)4Cp[μ2,η2,η1-C(Ph)Cdouble bond; length as m-dashC(PPh2)C(O)OC(O)](μ2-PPh2) (4), which in turn gives the mononuclear complex CpNi[PPh2CPhCdouble bond; length as m-dashC(PPh2)C(O)OC(O)] (5) as the end-product of the thermolysis reaction. Each of these new compounds has been isolated and their thermolysis reactivity independently examined, allowing for the unequivocal sequence associated with the decomposition of PhCCo2Ni(CO)4(η2-bma)Cp to be established. Compounds 2–5 have been fully characterized in solution by IR and NMR(1H, 13C, 31P) spectroscopies, and the solid-state structures of all four products have been determined by X-ray crystallography. The solution spectroscopic data of the new products are compared with the X-ray diffraction structures and the structural highlights of each compound are discussed. The coordination of the maleic anhydride π bond in PhCCo2Ni(CO)3(μ,η2-bma)Cp (3) provides crucial insight into one of the necessary requirements for P–C bond cleavage of the bma ligand at a tetrahedral cluster. The reactivity of the heterometallic cluster PhCCo2Ni(CO)4(η2-bma)Cp is contrasted with its homometallic analogue PhCCo3(CO)7(η2-bma).