Nature of α,β-CCC Agostic Bonding in Metallacyclobutanes

Abstract

The a,b-CCC agostic bonding in metallacyclobutanes is examined on the basis of structural, bonding, energetics, and electron density features. The structural features such as Ca–Cb, M–Ca, and M–Cb distances and CaCbCa and MCaCb angles of the agostic metallacyclobutanes were distinctly different from those of the corresponding non-agostic complexes. Two different orbital interactions characteristic of the agostic complex, causing the deformation of the propane-1,4-diyl unit of the metallacyclobutane were identified. The energy difference between the propane-1,4-diyl unit of metallacyclobutane in an agostic complex to that in a non-agostic complex is proposed as a good measure for the strength of a,b-CCC agostic interaction (Eagostic). Eagostic values of 37.0, 36.2, 13.7, 28.6, and 23.9 kcal/mol were obtained for the Grubb’s first generation Ru, Grubb’s second generation Ru, Ti, W, and Ta metallacyclobutane complexes and these values showed a linear correlation with the electron density at the ring CPs. The QTAIM features of the agostic complexes viz. the smaller q value at the Ca–Cb BCPs, larger ellipticity of the Ca–Cb BCPs, and diminished covalent character of Ca–Cb bonds as seen in their Laplacian of the electron density ($2q) at the BCPs, when compared with the non-agostic systems, fully supported the agostic bonding interactions. The a,b-CCC bonding is considered as the first example of a p type orbital formed between a metal atom and a carbon atom, where there is no r bond connectivity and this type of bonding is anticipated with all transition metals provided that the metal center is highly electron deficient.