Experimental and Theoretical Studies of SiFn(CO)2+Cations withn= 2 and 3: A Search for Pentacoordinate Cationic Silicon

Abstract

The addition of carbon monoxide to the fluorosilicon cations SiF+, SiF2•+, and SiF3+ has been investigated in helium buffer gas at (294 ± 3) K and (0.35 ± 0.01) Torr using a selected-ion flow tube (SIFT) apparatus. The monofluorosilicon cation was found to be unreactive toward CO, whereas both the difluoro- and trifluorosilicon cations consecutively added two CO molecules. Molecular orbital calculations, using density functional theory (DFT) performed at the B-LYP/6-31G(d,p) level, showed that the lowest-energy isomer on the SiF3(CO)2+ potential energy surface has a trigonal bipyramidal structure in which a pentacoordinate Si atom is surrounded by two axial CO ligands (bonded through C) and three equatorial F substituents. The ion at the global minimum on the SiF2(CO)2•+ potential energy surface has a structure between that of a distorted tetrahedron and a trigonal bipyramid in which both CO molecules are axial and the two fluorine atoms and the unpaired electron are equatorial. Other low-lying isomers ha...