Thermolysis of the hydrogen-capped cluster HCCo 2 NiCp(CO) 6 ( 1 ) with the redox-active diphosphine 2,3-bis(diphenylphosphino)maleic anhydride (bma) in DCE at ca. 80 °C affords the zwitterionic cluster ( 2 ) as the first observable product. Cluster 2 is not stable and transforms into the phosphido-bridged cluster ( 3 ). Both clusters 2 and 3 have been isolated and characterized in solution by IR and 31 P NMR spectroscopies, and the solid-state structures of both 2 and 3 have been determined by X-ray crystallography. Cluster 2 exhibits an open Co 2 Ni core having one Co−Co and Co−Ni bond. The bma ligand is attached to the hydrocarbyl and CpNi moieties via the PPh 2 groups, and one of the Co centers is tethered to the π bond of the bma ligand. The opened Co 2 Ni core of cluster 3 is structurally similar to 2 and consists of a μ 2 -PPh 2 ligand that spans the Co−Co bond and a Ni−C σ 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 (Δ H ⧧ = 31.4 ± 1.2 kcal/mol and Δ S ⧧ = 21 ± 4 eu). Our studies provide the first structural evidence for the attack of a phosphine ligand on the μ 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 RCCo 3 (CO) 7 (bma) are discussed.
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Bott et al. (2003) studied this question.
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