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[PPh2CPhC=C(PPh2)C(O)OC(O)]

Heating the mixed-metal cluster PhCCo2Ni(CO)(6)Cp 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)(eta(2) -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 P-31 NMR measurements. The van't Hoff parameters (Delta H = 1.49 +/- 0.02 kcal/mol; AS = 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)(mu,eta(2) -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 pi bond. Continued heating of cluster 3 is followed by the formation of 50e(-) cluster Co2Ni(CO)(4)Cp[mu(2),eta(2), C(Ph)C = C(PPh2)C(O)OC(O)]( mu-PPh2) (4), which in turn gives the mononuclear complex CpNi[PPh2CPhC = C(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 PhCCoNi(CO)(4)(eta(2) -bma)Cp to be established. Compounds 2-5 have been fully characterized in solution by IR and NMR(H-1, C-13, P-31) Spectroscopics, 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 pi bond in PhCCo2Ni(CO)(3)(mu, eta(2) -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)(eta(2) -bma)Cp is contrasted with its homometallic analogue PhCCo3(CO)(7)(eta(2)-bma). (c) 2005 Elsevier B.V. All rights reserved.