Tetranuclear and dinuclear phosphine- and arsine-substituted rhodium carbonyls; infrared spectral evidence for the formation of dirhodium octacarbonyl

Abstract

The reactions of Rh₄(CO)₁₂ with phosphine and arsine ligands have been investigated. Treatment of Rh₄(CO)₁₂ with one or two moles of ligand under mild conditions yields the compounds Rh₄(CO)₁₁L and Rh₄(CO)₁₀L₂ respectively, where L = PPh₃, P(p-CH₃C₆H₄)₃, P(p-FC₆H₄)₃, and AsPh₃, 2L = Ph₂PCH₂CH₂PPh₂, in which the basic structure of the parent cluster carbonyl is thought to be retained. Further reactions of the disubstituted derivatives with carbon monoxide under pressure, preferably in the presence of excess phosphine, yield the unstable dinuclear species Rh₂(CO)₆L₂, where L = PPh₃, P(p-CH₃C₆H₄)₃, and P(p-FC₆H₄)₃, which readily revert to the tetranuclear compounds. The significance of this dimer-tetramer equilibrium is discussed in relation to the difficulty experienced in characterising Rh₂(CO)₈. Infrared spectral evidence for the reversible formation of Rh₂(CO)₈ from the reaction of Rh₄(CO)₁₂ with high pressures of carbon monoxide at low temperatures is also described.