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

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 C_α 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.