The importance of agostic-type interactions for the binding energies of Ni+to saturated and α,β-unsaturated alkanes, silanes and germanes

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Abstract
The gas-phase interaction of H3C–CH2–XH3 and H2CC(H)XH3 (X = C, Si, Ge) with Ni+ has been investigated through the use of high-level density functional theory methods. The structures of the corresponding Ni+ complexes were optimized at the B3LYP/6-311G(d,p) level of theory. Final energies were obtained in single-point B3LYP/6-311+G(2df,2p) calculations. In all cases, the most stable complexes are stabilized through agostic-type interactions between the metal cation and the hydrogen atoms of the XH3 group. Only for propene is the conventional π-complex the global minimum of the potential energy surface. These agostic-type linkages can be viewed as three-center bonds resulting from electron-donor interactions between σ bonding orbitals of the neutral and the empty s orbital of the metal and back-donation from pairs of valence electrons of the metal into the corresponding σ* antibonding orbitals of the neutral. As a consequence, these bonds are particularly stable for Si- and Ge-containing compounds, because of the high electron-donor ability of the XH3 group when the heteroatom is Si or Ge. Vinylsilane and vinylgermane lead to non-conventional complexes in which the metal bridges the Cα atom of the CC double bond and one of the hydrogen atoms of the XH3 group. In contrast with the behavior predicted when the reference acid is Cu+, Si- and Ge-derivatives, both saturated and unsaturated, bind Ni+ more strongly than propane and propene, respectively. Ni+ binding energies are systematically greater than Cu+ binding energies and the bond activation effects observed upon Ni+ attachment are sizably larger than those found upon Cu+ association.