On the Nature of the Agostic Bond between Metal Centers and β-Hydrogen Atoms in Alkyl Complexes. An Analysis Based on the Extended Transition State Method and the Natural Orbitals for Chemical Valence Scheme (ETS-NOCV)

We have in the present account analyzed the bonding in beta-agostic alkyl complexes between the carbon chain (R) and the transition metal center. The analysis is based on a recently proposed energy decomposition scheme (ETS-NOCV). We have considered R = Et, n-Pr, i-Pr, n-Bu, attached to the cationic Ni(II)- and Pd(II)-bis-diimine Brookhart complexes (1), the cationic Ti(IV)- or Zr(IV)-metallocenes (2), and the neutral Pd(II) Drent (3) complexes. We find for a given metal that the total M-R dissociation energy -Delta E-total follows the order n-Bu > n-Pr > Et >> i-Pr. For the same R-group (n-Pr), -Delta E-total for second-row metals is larger than for first-row, as 4d forms better overlaps with the alkyl orbitals than 3d. The major stabilizing contribution to -Delta E-total is the a-bond between the alpha-carbon and the metal. It is augmented by smaller contributions from the C-beta-M sigma-interaction as well as file hyperconjugation of charge into the sigma(CC)* and sigma(CH)* alkyl orbitals. We finally have the beta-agostic contribution from the interaction between a hydrogen atom on the beta-carbon and the metal center. The strength of this bond is rather constant for the cationic species 1 and 2. It can be considered as originating from a (largely) Coulombic interaction between the metal center and the electron pair in the C-beta-H bond, where the density of the pair has been polarized by the positive metal charge. The neutral Drent, system (3) exhibits a weaker beta-agostic interaction, as the net charge on the metal center is less positive.