Photodetachment of FSi(OMe) 4− and FGe(OMe) 4− anions: an experimental and theoretical study of gas-phase hypervalent Si and Ge species

Abstract

The structure and thermochemistry of the hypervalent FSi(OMe) 4 −, FGe(OMe) 4 − and FGe(H)(OMe) 3 − anions have been investigated by a combination of photodetachment spectroscopy and ab initio calculations. All three of these anions undergo photodetachment at wavelengths between 266 and 285 nm but not at 355 nm. The data suggest that the observed photodetachment process for these ions is a one-photon process leading to dissociation of a MeO group for the first two anions, and presumably dissociation of a H atom in the last case. Theoretical calculations are consistent with these observations and predict a fluoride binding energy of 47.5 kcal mol −1 and a methoxide binding energy of 52 kcal mol −1 in FSi(OMe) 4 −. The most stable structure for FSi(OMe) 4 − is predicted to be a trigonal bipyramid with the fluorine occupying an axial position. This is similar to what has been found for the corresponding Ge(OMe) 4 species. These results, the gas-phase chemical behavior of these anions, and the calculated charge distribution are consistent with anions in which a high degree of covalent bonding exists between the electronegative ligand and the central Si or Ge atom.