Recently, Dr. Xuhui Lin from the school of life science and engineering, published a paper entitled "A theoretical perspective of the agostic effect in early transition metal compounds“in “Coordination Chemistry Reviews". The paper reviewed the development and history of agostic bond in transition metal compounds, especially the theoretical works on its nature.
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Theagostic bond is a three-center two-electron covalent bond formed by a transition metal center and the C-H group on its ligand. It is one of the most important discoveries in organometallic chemistry in the twentieth century. The discovery ofagostic bond has attracted the interest of many chemists because it plays a very important and even decisive role in organic chemical reactions and catalysis, such as the activation of C-H bonds, β elimination reactions, para metal reactions, polymerization reactions and Some stereo reactions and so on. Based on this, a large number of experiments and theoretical work have conducted sufficient research on it. However, the nature of the bond-formingagostic bond has always been controversial.
Theoretically revealing the nature of the graspingagostic bond has very important theoretical value for understanding the nature of the chemical bond participated by the transition metal and its catalytic activity, and has important guiding significance for the experiment. This article comprehensively reviews and summarizes a variety of theoretical calculation methods, such as the theory of atoms in molecules (QTAIM), the natural bond orbit method (NBO) and the energy decomposition analysis method (EDA) to study the nature of theagostic bond. Since most of the above calculation methods are based on molecular orbital theory, they have certain deficiencies in revealing the nature of chemical bonds. Here, the block-localized wave function method (BLW) which is an variant ofab initiovalence bond theorty, was performed to give an improved understanding on the nature of the agostic bond. The present study showed that the agostic interaction is equally governed by the charge transfer (or hyperconjugation) from the C-H bond to the metal center and the dispersion effect. In contrast, steric effect is usually repulsive in nature for most agostic cases, thus preventing the agostic interaction. Moreover, there is no obvious correlation between the magnitude of structural deformation and the corresponding agostic stabilization energy, particularly in large complexes with β-agostic interactions.
Figure.(a) Types of agostic interactions and their involvements in (b) β-Hydrogen elimination，(c) Ziegler–Natta polymerization, (d) C-H bond activation and (e) cyclometallation.
Dr. Lin is the first author, and Prof. Yirong Mo from the University of North Carolina at Greensburg is the corresponding author. This work was also supported by Prof. Wei Wu from Xiamen University. The calculations were performed in part with the Xiamen Valence Bond (XMVB) program developed by Prof. Wu's group.
Original link: https://doi.org/10.1016/j.ccr.2020.213401
