Valence Charge Concentrations, Electron Delocalization and β-Agostic Bonding in d0 Metal Alkyl Complexes

In this paper we describe a range of model d0 metal ethyl compounds and related complexes, studied by DFT calculations and high resolution X‐ray diffraction. The concept of ligand‐opposed charge concentrations (LOCCs) for d0 metal complexes is extended to include both cis‐and trans‐ligand‐induced charge concentrations (LICCs) at the metal, which arise as a natural consequence of covalent metal–ligand bond formation in transition metal alkyl complexes. The interplay between locally induced sites of increased Lewis acidity and an ethyl ligand is crucial to the development of a β‐agostic interaction in d0 metal alkyl complexes, which is driven by delocalization of the MC bonding electrons. Topological analysis of theoretical and experimental charge densities reveals LICCs at the metal atom, and indicates delocalization of the MC valence electrons over the alkyl fragment, with depletion of the metal‐directed charge concentration (CC) at the α‐carbon atom, and a characteristic ellipticity profile for the CαCβ bond. These ellipticity profiles and the magnitude of the CC values at Cα and Cβ provide experimentally observable criteria for assessing quantitatively the extent of delocalization, with excellent agreement between experiment and theory. Finally, a concept is proposed which promises systematic control of the extent of CH activation in agostic complexes. The interplay between locally induced sites of Lewis acidity and the β‐CH moiety of an ethyl ligand is crucial to the development of a β‐agostic interaction in d0 metal alkyl groups. This is driven by delocalization of the MC valence electrons over the alkyl fragment; as revealed by experimental and theoretical charge‐density studies. The concept of both cis‐ and trans‐ligand‐induced charge concentrations at the metal centre is introduced as an extension of the concept of ligand‐opposed charge concentrations. This represents a new criterion for assessing quantitatively the extent of delocalization, and offers the possibility to control the extent of M⋅⋅⋅H‐C interaction in agostic complexes. The figure illustrates an experimental envelope map showing the ligand‐induced polarization.