Nature of α,β-CCC agostic bonding in metallacyclobutanes

Abstract

The α,β-CCC agostic bonding in metallacyclobutanes is examined on the basis of structural, bonding, energetics, and electron density features. The structural features such as C α–C β, M–C α, and M–C β distances and C αC βC α and MC αC β 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 α,β-CCC agostic interaction ( E agostic). E agostic 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 ρ value at the C α–C β BCPs, larger ellipticity of the C α–C β BCPs, and diminished covalent character of C α–C β bonds as seen in their Laplacian of the electron density (∇ 2 ρ) at the BCPs, when compared with the non-agostic systems, fully supported the agostic bonding interactions. The α,β-CCC bonding is considered as the first example of a π type orbital formed between a metal atom and a carbon atom, where there is no σ bond connectivity and this type of bonding is anticipated with all transition metals provided that the metal center is highly electron deficient.