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

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 < But, (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.