A theoretical perspective of the agostic effect in early transition metal compounds

Abstract

Agostic interaction refers to the bonding between a coordinatively unsaturated metal atom and its ligand, and is characterized by the drawing of the ligand towards the metal. The most important type of agostic interaction in organometallic chemistry is the C-H⋯Metal coordination that has the capability of activating the inert C–H bond in transition metal complexes. While the existence of such CH⋯M coordination seems unambiguous and well recognized, there are a few quite different theoretical interpretations on the agostic interaction, leading its origin remaining elusive. Therefore, an improved understanding of the nature for agostic interactions is in need with improved theoretical approaches. So far, significant efforts have been made for the purpose of characterizing and elucidating the agostic interaction by means of a wide range of computational approaches, including the natural orbital bond (NBO) method, the quantum theory of atoms in molecules (QTAIM) approach, various energy decomposition analysis (EDA) schemes and the block-localized wavefunction (BLW) method. To this end, in this review, we discussed the development of the agostic interaction concept in history and outlined the computational tools for characterizing the agostic interaction. More importantly, we summarized the popular theoretical perspective on the origin of the agostic interaction in early-transition metal compounds, especially from the view of valence bond theory, and demonstrated that the nature of the agostic effect is complicated and varies in different compounds.