Phosphine ligand attack at both the methylidyne cap and the CpNi center in HCCo2NiCp(CO)6 by 2,3-bis(diphenylphosphino)maleic anhydride (bma):: P-C bond cleavage reactivity, kinetics, and X-ray structures of the zwitterionic clusters Co2NiCp(CO)4(μ-CO)[μ2,η2,η1-C(H)PPh2C=C(PPh2)C(O)OC(O)] and Co2NiCp(CO)4[μ2,η2,σ-C(H)PPh2C=CC(O)OC(O)](μ2-PPh2)

Thermolysis of the hydrogen-capped cluster HCCo2NiCp(CO)(6) (1) with the redox-active diphosphine 2,3-bis(diphenylphosphino)maleic anhydride (bma) in DCE at ca. 80 degreesC affords the zwitterionic cluster Co2NiCp(CO)(4)(mu-CO)[mu(2),eta(2),eta(1)-C(H)PPh2C=C(PPh2)C(O)OC(O)] (2) as the first observable product. Cluster 2 is not stable and transforms into the phosphido-bridged cluster Co2NiCp(CO)(4)[mu(2),eta(2), sigma-C(H)PPh2C=CC(O)OC(O)](mu(2)-PPh2) (3). Both clusters 2 and 3 have been isolated and characterized in solution by IR and P-31 NMR spectroscopies, and the solid-state structures of both 2 and 3 have been determined by X-ray crystallography. Cluster 2 exhibits an open Co2Ni core having one Co-Co and Co-Ni bond. The bma ligand is attached to the hydrocarbyl and CpNi moieties via the PPh2 groups, and one of the Cocenters is tethered to the pi bond of the bma ligand. The opened Co2Ni core of cluster 3 is structurally similar to 2 and consists of a mu(2)-PPh2 ligand that spans the Co-Co bond and a Ni-C a bond resulting from the formal coupling of the maleic anhydride residue with the CpNi moiety. The reaction involving the conversion of 2 to 3 was examined by UV-vis spectroscopy and found to obey first-order kinetics. A rate-limiting step involving dissociative CO loss is supported by the rate retardation in the presence of added CO and the activation parameters (DeltaHdouble dagger = 31.4 +/- 1.2 kcal/mol and DeltaSdouble dagger = 21 +/- 4 eu). Our studies provide the first structural evidence for the attack of a phosphine ligand on the mu(3)-hydrocarbyl ligand in this genre of tetrahedral cluster and suggest that the nature of the capping CR group and the electrophilicity of the CpNi center play a role in determining the course of ligand substitution. Reactivity comparisons with the related bma-substituted clusters RCCo3(CO)(7)(bma) are discussed.