Structures of X2F4, from carbon to lead. Unsaturation through fluorine bridges in Group 14

The various isomers of X 2 F 4 with X=C, Si, Ge, Sn, and Pb are investigated through ab initio calculations using effective core potentials and taking into account part of the relativistic effects for tin and leads atoms. Geometries are determined at the SCF level and their relative energies are refined through CI calculations. The planar π-bonded structure F 2 X=XF 2 is found to be a true minimum on the C 2 F 4 potential energy surface, a saddle point of the Si 2 F 4 , Ge 2 F 4 , and Sn 2 F 4 surfaces, and a true minimum on the first singlet excited Pb 2 F 4 surface. The F 3 X-XF isomer is found to be a true minimum in all cases but lead