Evidence for πCHR→dM bonding in transition metal carbene compounds (LnM![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
CHR) and its decisive role in the α-agostic effect†
Abstract
It has been generally recognized that the α-agostic interaction (M⋯H–C) in transition metal carbene compounds LnMCHR (R = H, Me etc.) can be interpreted with a double metal–carbon bonding model. This bonding model involves the reorganization of the σ component, which can be illustrated in terms of three-center two-electron (3c-2e) M–H–C covalent bond as in transition metal alkyl compounds. Herein, we propose an alternative partial triple metal-carbon bonding model to elucidate the agostic interaction in LnMCHR. Apart from the well-defined σ and π bonds, there exists a seemingly weak but decisive third force, namely the πCHR→dM bonding between an occupied π-like symmetric CHR orbital and a vacant metal d orbital, which is the true origin of the α-agostic effect. This partial triple bonding model is authenticated on both Fischer- and Schrock-type carbenes by an ab initio valence bond (VB) method or the block-localized wavefunction (BLW) method, which has the capability to quantify this notable π bonding and further demonstrate its geometric, energetic and spectral impacts on agostic transition metal carbene compounds. We also show that ancillary ligands can modulate the πCHR→dM bonding through electronic and steric effects.
![Graphical abstract: Evidence for πCHR→dM bonding in transition metal carbene compounds (LnM [[double bond, length as m-dash]] CHR) and its decisive role in the α-agostic effect](/en/Image/Get?imageInfo.ImageType=GA&imageInfo.ImageIdentifier.ManuscriptID=D2CP03870F&imageInfo.ImageIdentifier.Year=2022)
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![[double bond, length as m-dash]](https://www.rsc.org/images/entities/h2_char_e001.gif)
CHR) and its decisive role in the α-agostic effect
