Die isomerieverhältnisse bei metall-komplexen des typs [C 5H 5M(CO) 2] 2

Abstract

A detailed spectroscopic study of [C 5H 5Ru(CO) 2] 2 and [C 5H 5Fe(CO) 2] 2 in the carbonyl stretching frequency region at temperatures between −100° and +100° has shown that, in solution, both complex compounds exist as an equilibrium mixture of two structurally isomeric molecular forms. At low temperatures the proposed form with two bridging carbonyl groups appears to be more stable whereas, at higher temperatures, the isomeric form with a pure metal—metal bond and four terminal carbonyl groups is favoured. In the iron complex the “low temperature” form still predominates at room temperature while with the ruthenium complex both isomers are present in approximately equal proportions. The homologous osmium compound is present only as a form without carbonyl bridges both in solution and in the solid state. In solution both isomers seem to appear exclusively in a cis-configuration or at least a form without a centre of symmetry, while in the crystal the trans-configuration is apparently preferred. This remarkable tendency of the free molecules to adopt only the cis-configuration may be ascribed to some participation of π- and δ-contributions in the metal—metal bond. Similar studies on derivatives of the type [C 5H 5M(CO) 2] 2X(M = Fe, Ru; X = Hg, H or halogen) have led to closely similar spectroscopic and structural results in that an analagous non-bridged form is present in these materials.