Stereochemistry of the iron atom in [Fe(CO)2(η-C5H5)Y] compounds and its application to the tautomerism in bis[dicarbonyl(η-cyclopentadienyl)-iron] and related compounds

Abstract

The stereochemistry of compounds of the type [Fe(CO)₂(cp)Y], where cp =η-cyclopentadienyl and Y covers a range of univalent groups and includes groups which can form bridges to the iron atom, is shown to be remarkably close to a regular octahedron. The octahedral geometry does not depend on whether there are bridges present or whether Y is a ‘simple’ radical. Small deviations are discussed. By assuming that the presence of octahedral geometry in a wide range of structures indicates a certain rigidity in this arrangement, it is shown that the tautomer-interconversion mechanism for [{Fe(CO)₂(cp)}₂] must involve simultaneous making or breaking of two carbonyl-bridge systems. This mechanism has been suggested previously, in order to explain n.m.r. results, but has not previously been explained. A presentation is made of the general interconversion of isomers of [M(cp)M′(cp)-ABCD] where A, B, C, and D are ligands which may bridge in pairs.