Extra-long C[sbnd]C single bonds via negative hyperconjugation in perfluoropinacolate complexes

Abstract

A computational study on a series of pinacolate ligands with varying degrees of fluorination (from four to zero CF3 groups) has been carried out to understand the exceptionally long central CC bonds in crystal structures with the perfluoropinacolate ligand, (pinF). The systems were studied with both DFT (PBE0) and ab-initio (MP2 NEVPT2) models to elucidate the features of the electronic structure responsible for the elongated central bond routinely observed in {A(pinF)} species. Two main influences are responsible: (i) negative hyperconjugation exists between the alkoxide O atom lone pairs and the central CC σ* bond which also have resonance forms that formally break the central CC bond and (ii) the lack of hydrogen bonding between any CH bonds and the ligand O atoms. Steric influences do not play a significant role, but a central atom A with a point charge of at least plus two bound to pinF is required to reproduce the crystallographically determined CC distances.