TETRACOORDINATED PLANAR CARBON IN PENTAATOMIC MOLECULES

Three pentaatomic molecules CSi2Al2, CSi2Ga2, and CGe2Al2 were studied at the B3LYP/6-311+G and MP2/6-311+G* levels of theory (with tests a lso run at multiconfigurational levels) to determine whether the centr al carbon atom exists in a planar geometry. We found that cis-CSi2Al2 and trans-CSi2Al2 planar structures have one imaginary frequency and t hat distortion along this mode leads to slightly pyramidal local minim a. In contrast, cis- and trans-CSi2Ga2 and cis- and tmns-CGe2Al2 are t rue minima in their planar geometries, but their corresponding, tetrah edral structures lie 25-28 kcal/mol higher in energy and are first-ord er saddle points on the respective energy surfaces. A molecular orbita l analysis is presented to explain the preference of the planar anti-v an't Hoff/Lebel structures over the corresponding tetrahedral structur es. This analysis suggests that the presence of 18 valence electrons ( which leads to three C-ligand a bonds, one C-Ligand jr bond, and one l igand-ligand bond) is crucial for planar geometries to be stable and p referred over tetrahedral structures.