Mechanism of the conversion of [Ru(CO)5] into [Ru3(CO)12]

Abstract

The conversion of [Ru(CO)₅] into [Ru₃(CO)₁₂] in cyclohexane at 294–308 K has been shown to involve initial carbon monoxide dissociation to give the intermediate [Ru(CO)₄]; the latter then combines further with [Ru(CO)₅] in a series of fast steps to form ultimately the trimer [Ru₃(CO)₁₂]. The mechanism is thus closely related to the dissociative mechanism demonstrated previously for substitution of one CO of [Ru(CO)₅] by PPh₃, and essentially identical activation parameters are found for both types of reactions [for the ‘trimerization’, ΔH^‡= 109 (± 15) kJ mol^–1, ΔS^‡ 53 (± 15) J K^–1 mol^–1]. The equilibrium constant for the reaction 3[Ru(CO)₅]⇆[Ru₃(CO)₁₂]+ 3CO at 298 K is 3.0 (± 0.5)× 10⁶ mol dm^–3.