Cyclooctatetraene in metal complexes—planar does not mean aromatic

Abstract

The cyclooctatetraene (COT) ring may act as an effective π-type ligand in metal complexes and very often in such complexes it adopts the planar structure, which in fact is not the most favourable conformation when compared with the free COT molecule. Such planarization is usually considered as an effect of COT ring aromatization due to charge transfer from the metal centre into the COT ligand and reorganization of the π-electron structure from a 4n to 4n+2 Huckel system. In this paper it has been shown that the conformational changes of the COT ring, leading to its planarization and partial bond equalization, in the first order are connected with the complexation efficiency and not with the aromatization of COT itself. Such a conclusion was drawn on the basis of statistical analysis of X-ray data collected in the Crystal Structure Database (47 COT rings were taken into consideration, all found in the selected highest quality X-ray measurements) and advanced quantum-chemical calculations, including analysis of the electron density distribution made in the framework of Atoms-in-Molecules Quantum Theory.