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| - Phosphine Ligand Attack at Both the Methylidyne Capand the CpNi Center in HCCo2NiCp(CO)6 by2,3-Bis(diphenylphosphino)maleic Anhydride (bma): P−C Bond Cleavage Reactivity, Kinetics, and X-rayStructures of the Zwitterionic ClustersCo2NiCp(CO)4(μ-CO)[μ2,η2,η1-C(H)PPh2CC(PPh2)C(O)OC(O)]andCo2NiCp(CO)4[μ2,η2,σ-C(H)PPh2CCC(O)OC(O)](μ2-PPh2)
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| - The reaction between the mixed-metal cluster HCCo2NiCp(CO)6 and the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) in DCE solvent proceeds in a stepwise fashion to give initially the compound (2), followed by the formation of the phosphido-bridged cluster (3). The X-ray structures for clusters 2 and 3 have been determined, and phosphine ligand attack on the hydrocarbyl capping ligand and the CpNi center in 2 as opposed to the expected Co(CO)3 centers has been verified.
- Thermolysis of the hydrogen-capped cluster HCCo2NiCp(CO)6 (1) with the redox-activediphosphine 2,3-bis(diphenylphosphino)maleic anhydride (bma) in DCE at ca. 80 °C affordsthe zwitterionic cluster (2) asthe first observable product. Cluster 2 is not stable and transforms into the phosphido-bridged cluster (3). Both clusters2 and 3 have been isolated and characterized in solution by IR and 31P 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 ligandis attached to the hydrocarbyl and CpNi moieties via the PPh2 groups, and one of the Cocenters is tethered to the π bond of the bma ligand. The opened Co2Ni core of cluster 3 isstructurally similar to 2 and consists of a μ2-PPh2 ligand that spans the Co−Co bond and aNi−C σ bond resulting from the formal coupling of the maleic anhydride residue with theCpNi moiety. The reaction involving the conversion of 2 to 3 was examined by UV−visspectroscopy and found to obey first-order kinetics. A rate-limiting step involving dissociativeCO loss is supported by the rate retardation in the presence of added CO and the activationparameters (ΔH⧧ = 31.4 ± 1.2 kcal/mol and ΔS⧧ = 21 ± 4 eu). Our studies provide the firststructural evidence for the attack of a phosphine ligand on the μ3-hydrocarbyl ligand in thisgenre of tetrahedral cluster and suggest that the nature of the capping CR group and theelectrophilicity 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) arediscussed.
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