Kinetic studies of some substituted hexarhodium carbonyl clusters

Abstract

Reactions of the halides X⁻ (X⁻ = chloride, bromide or iodide) with the substituted cluster Rh₆(CO)₁₅(PPh₃) in oxygen-free chloroform lead to [Rh₅(CO)₁₄(PPh₃)]⁻, Rh(CO)₂(PPh₃)₂X and [Rh(CO)₂X₂]⁻ in the molar ratios 2 ∶ 1 ∶ ∼13. Oxidation by the solvent is assumed to lead to most of the Rh(I) product, and the stoichiometry for reactions with I⁻ can be defined as 4Rh₆(CO)₁₅(PPh₃) + 27I⁻ + 12CHCl₃ → 2[Rh₅(CO)₁₄(PPh₃)]⁻ + Rh(CO)₂(PPh₃)₂I + 13[Rh(CO)₂I₂]⁻ + 6C₂H₂Cl₄ + 4CO + 12Cl⁻. This can be rationalized quite simply with the aid of a few generally justifiable assumptions. Rate constants for reactions with bromide increase to a limiting value with increasing [Br⁻] in a way that shows that breaking of one Rh–Rh bond, with an unusual closo to nido structural change, is rate determining. This opening of the cluster might be spontaneous or solvent induced. To complete the reaction, the bromide has to compete with the reverse nido to closo change. The same closo to nido change is also a major rate determining step for reactions with P(OPh)₃ in oxygen-free solutions, and for reactions with bromide in oxygenated solutions in the presence of trifluoroacetic and some other acids. The limiting rates increase slightly with increasing basicity of the ligands P(p-XC₆H₄)₃ along the series X = F₃C, Cl, F, H and MeO. Activation parameters for these reactions are reported.