Agostic vs π-interactions in complexes of ethynylsilanes and ethynylgermanes with Cu+ in the gas phase

The gas-phase interaction of HCequivalent toC-XH3 (X = C, Si, and Ge) with Cu+ has been investigated through the use of high-level density functional theory methods. The structures of the corresponding Cu+ complexes were optimized at the B3LYP/6-311G(d,p) level of theory, while the final energies were obtained in single-point B3LYP/6-311+G(2df,2p) calculations. In all cases, it is found that complexes presenting nonconventional (agostic) interactions with the XH3 terminal group lie slightly higher in energy than conventional pi-complexes. An analysis of their charge density clearly indicates that these interactions have a nonnegligible covalent character and that they are favored by the hydride character of the hydrogen atoms of the XH3 substituent. Accordingly, substitution of some of these hydrogens by electron releasing groups enhances the stability of the corresponding agostic complex which becomes very close in stability to pi complexes. These agostic interactions lead to a significant weakening of the X-H linkage involved, reflected in a very large red shifting of the X-H stretching frequency.