On the existence of α-agostic bonds: Bonding analyses of titanium alkyl complexes

摘要

The main characteristics of the so-called >agostic bonding> in titanium complexes were computationally investigated in terms of diverse analyses, including the quantum theory of atoms in molecules (QTAIM) and the electron localization function (ELF). Computations on a set of titanium-based molecular models that were presumably able to present α- and β-like bonding showed a clear distinction between α- and β-bonding schemes. In view of the geometric, energetic, and electronic data of these molecules, we concluded that the geometries presenting a Ti⋯H approximation similar to α bonding are not the result of a bonding attraction. On the contrary, their origin arises from a short-range repulsion between the metal core and the lone pairs of the alkylidene group, the latter pivoting in its own plane around carbon and thereby allowing simultaneously a closer approach between the Ti and the C atoms and indirectly resulting in a short Ti⋯H distance. Additionally, we found that the electronic and geometric distortion of the C-H bond present in the agostic bonding are not univocally linked to this kind of bonding; instead, this originates in the close distance of the C-H bond and any metal center, independently of whether it is due to an agostic bond or not. Therefore, the lengthening of the C-H bond and the reduction of its electron density at the bond critical point should not be considered as indication of agostic bonding but as a side effect. © 2006 American Chemical Society.