An infrared spectroscopic study of the tautomeric equilibria in solutions of tricarbonylbis(η-dienyl)isocyanidedi-iron complexes

Abstract

Infrared spectra of solutions of [Fe₂(η-dienyl)₂(CO)₃(CNR)] derivatives (dienyl = C₅H₅, C₅H₄Me, or C₉H₇; R = alkyl or benzyl) show that four isomers are present. They have structures based on cis- and trans-[Fe₂(η-C₅H₅)₂,-(µ-CO)₂(CO)₂]. In two isomers the isocyanide ligand has replaced a µ-CO group, whilst in others it has replaced a terminal (t) ligand. The equilibria shift towards this latter pair when (a) R = Me < Et < Buⁿ < Buⁱ < Bu⁸ < cyclo-C₆H₁₁ < Bu^t, (b) R = CH₂C₆H₄X-p as X is varied along the series Cl < H < Me < OMe, and (c) for dienyl = C₅H₅∼ C₅H₄Me < C₉H₇. The destabilization of the µ-RNC isomers is attributed in (a) partly to increasing steric interactions between R and the Fe(η-dienyl)(CO)_t moieties, and in both (a) and (b) to the decreasing overall electron-withdrawing ability of R. Changes in solvent and temperature also affect the equilibria.