Crystal structures of di-iron µ-η2-(C,C)-ketene complexes, [{Fe(η5-C5H5)(CO)2}2(µ-CH2CO)] and [{Fe(η5-C5Me5)(CO)2}2(µ-CH2CO)]. Synergistic metal-to-ligand back donation as revealed by comparison with [{Fe(η5-C5H5)(CO)2}2(µ-CH2CH2CO)]

Abstract

The solid-state structures of di-iron µ-ketene complexes [{Fe(η⁵-C₅H₅)(CO)₂}₂(µ-CH₂CO)](1) and [{Fe(η⁵-C₅Me₅)(CO)₂}₂(µ-CH₂CO)](2) and of a reference complex, [{Fe(η⁵-C⁵H⁵)(CO)₂}₂(µ-CH₂CH₂CO)](3), have been determined by single-crystal X-ray diffraction. In accord with previous spectroscopic studies, the geometry of the bridging ketene ligands reflects three resonance structures. In particular, the contribution of the π-complex form is verified based on the observations that the CH₂–CO bond is shortened and the CH₂CO bond is elongated compared with the situation in complex (3) which lacks the π-complex form. The geometry around the metal centres is determined by the need to minimize steric repulsions among ancillary ligands, and does not reflect orbital interactions.