On the Existence of α-Agostic Bonds:  Bonding Analyses of Titanium Alkyl Complexes

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.