Chemical behaviour of the mixed-metal carbonyl clusters [Fe2Rh(CO)10]– and [Fe2Rh2(CO)12]2–. Crystal structures of [Fe2Rh(CO)9(PPh3)]– and [Fe2Rh2(µ-CO)3(CO)9{µ3-Au(PPh3)}]–

Abstract

Addition of triphenylphosphine to the cluster [Fe₂Rh(CO)₁₀]^– yielded the derivative [Fe₂Rh(CO)₉(PPh₃)]^–, which is stable enough to allow a single-crystal X-ray analysis on the [PPh₄]⁺ salt. The cluster contains a triangular metallic framework, the vertices of which are occupied by two Fe(CO)₄ groups and one Rh(CO)(PPh₃) moiety; the unsaturation of the cluster (46 valence electrons) is brought about by this 16-electron square-planar fragment. The cluster [Fe₂Rh₂(CO)₁₂]^2– was obtained by treating [Fe₂(CO)₉] with [Rh(CO)₄]^–(molar ratio 1:2) in refluxing acetone, and was characterized by infrared spectroscopy and elemental analyses. The adduct [Fe₂Rh₂(CO)₁₂{Au(PPh₃)}]^– was prepared by stoichiometric addition of [Au(PPh₃)Cl] in acetone and the crystal structure of the [PPh₄]⁺ salt was elucidated by X-ray analysis. The cluster contains a trigonal bipyramidal arrangement of metal atoms, with one Au(PPh₃) and one Fe(CO)₃ group occupying the apical positions. The vertices of the FeRh₂ equatorial plane are bound to two terminal carbonyl ligands, and the edges are spanned by three bridging carbonyls. The ³¹P NMR spectral data for [Fe₂Rh(CO)₉(PPh₃)]^– and [Fe₂Rh₂(CO)₁₂{Au(PPh₃)}]^– are consistent with the solid-state structures, but do not exclude the presence of fluxional processes in solution.