Agostic Interactions in Early Transition‐Metal Complexes: Roles of Hyperconjugation, Dispersion, and Steric Effect

Abstract

The agostic interaction is a ubiquitous phenomenon in catalytic processes and transition‐metal complexes, and hyperconjugation has been well recognized as its origin. Yet, recent studies showed that either short‐range London dispersion or structural constraints could be the driving force, although proper evaluation of the role of hyperconjugation therein is needed. Herein, a simple variant of valence bond theory was employed to study a few exemplary Ti complexes with α‐ or β‐agostic interactions and interpret the agostic effect in terms of the steric effect, hyperconjugation, and dispersion. For the complexes [MeTiCl3(dmpe)] and [MeTiCl3(dhpe)] with α‐agostic interactions, hyperconjugation plays the dominant role with comparable magnitudes in both systems, but dispersion is solely responsible for the stronger agostic interaction in the former compared with the latter. For the complexes [EtTiCl3(dmpe)] and [EtTiCl3(dhpe)] with β‐agostic interactions, however, hyperconjugation and dispersion play comparable roles, and the weaker steric repulsion leads to a stronger agostic effect in the former than in the latter. Thus, the present study clarifies the variable and sensitive roles of steric, hyperconjugative, and dispersion interactions in the agostic interaction.

Agostic Hydrogens in 1‐Norbornyl Metal Cyclopentadienyl Structures

“…[15] This discrepancy may result from hyperconjugation, dispersion, and/or steric effects. [22] The other agostic ν(C-H) frequency in Ti-T of 2961 cm -1 is 194 cm -1 larger than the smaller agostic ν(C-H) frequency.…”

“…Agostic interactions between C-H bonds and transition metals have recently attracted much attention. [15][16][17][18][19][20][21][22] Such interactions play an important role in C-H bond activation. [23][24][25][26] The stereochemistry of the 1-norbornyl group provides the possibility of one or two agostic C-H-M bonds in addition to a single M-C σ-bond to a metal atom.…”

From Dialkyltitanium Species to Titanacyclopropanes: An Ab Initio Study

“…Therefore, it would not be a C–H···Ti bond but rather an interaction between the C–C bond and the metal. The fact that the [TiCl 3 Et] complex has no agostic bond shows that this phenomenon is not solely responsible for the formation of a stable agostic bond, and that other geometric and electronic considerations should also be taken into account. , For instance, the orbital delocalization responsible for the agostic binding is more significant in [TiCl 3 Et (dmpe)], an octahedral complex, than in tetrahedral [TiCl 3 Et].…”

Direct metal–carbon bonding in symmetric bis(C–H) agostic nickel(i) complexes

“…8 For example, if the CH moiety acts as a donor towards an electron-deficient metal centre (acting as a σ donor), the interaction is defined as a σCH interaction. [9][10][11][12] Such a bond is often formally described as Md←σCH donation, reflecting the fact that metal 𝑑 orbitals act as acceptors. 10 Charge transfer leads to weakening of the σCH bond, which is a requisite first step in a wide range of σ-bond activation methodologies, including ambiphilic ligand-metal activation 13 /cyclometallation deprotonation 14 (AMLA/CMD), oxidative addition (OA), 15 σ bond metathesis (SBM), 16 and 2+1 addition.…”