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 also run at multiconfigurational levels) to determine whether the central carbon atom exists in a planar geometry. We found that cis-CSi2Al2 and trans-CSi2Al2 planar structures have one imaginary frequency and that distortion along this mode leads to slightly pyramidal local minima. In contrast, cis- and trans-CSi2Ga2 and cis- and tmns-CGe2Al2 are true minima in their planar geometries, but their corresponding, tetrahedral structures lie 25-28 kcal/mol higher in energy and are first-order saddle points on the respective energy surfaces. A molecular orbital analysis is presented to explain the preference of the planar anti-van't Hoff/Lebel structures over the corresponding tetrahedral structures. 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 ligand-ligand bond) is crucial for planar geometries to be stable and preferred over tetrahedral structures.