Revisiting the main group cyclopentadienyl metal complexes in terms of the through-space coupling concept

Abstract

The structures and properties of metal complexes are traditionally treated in terms of hybridization and electronic ligand effects. What is notoriously neglected, however, is the fact that in such an aggregate the ligands approach so closely to one another — on the order of van der Waals (vdW) distances — that intramolecular packing effects come into play. Actually, non-bonded interactions between any atoms bonded to some central atom are increasingly recognized as an important factor in determining bond distances, bond angles and the like. The class of the main group cyclopentadienyl (Cp) metal complexes appears to be a case study in this respect, because of the large extension of the Cp group and its ring structure on the one hand, and on the other, the minimal d-orbital involvement, dissimilar to the transition-metal analogues. It is shown that the diverse array of structural arrangements, such as linear, ring-slipped, bent, and polymeric chain structures, as well as their reactivities, are brought under the umbrella of one treatment with the aid of the through-space coupling (TSC) concept. This is the molecular orbital representation of vdW repulsive–attractive forces. As a central feature, the individual ligands are at first combined to a united system of TSC orbitals and then allowed to interact with the metal AOs. The energy splitting of the TSC orbitals and the electron density shift from one of them to a vacant metal orbital determine the repulsive and attractive interligand forces and hence fine-tune the geometry of the complex. Along these lines a physical explanation for the interplay between vdW attraction and repulsion becomes available. More specifically we are dealing here with complexes of the type Cp′ n ML m ( n=1–3, m=0–3) via united {Cp′ n L m } molecular orbitals. Bending and slipping of the Cp′ ligands are rooted in vdW attraction and repulsion, respectively, with geometry and hapticity of the Cp′-metal bonding adjusted so as to optimize TSC.