Tetracoordinated Planar Carbon in Pentaatomic Molecules

Three pentaatomic molecules CSi 2 Al 2, CSi 2 Ga 2, and CGe 2 Al 2 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 -CSi 2 Al 2 and trans -CSi 2 Al 2 planar structures have one imaginary frequency and that distortion along this mode leads to slightly pyramidal local minima. In contrast, cis - and trans -CSi 2 Ga 2 and cis - and trans -CGe 2 Al 2 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 σ bonds, one C−ligand π bond, and one ligand−ligand bond) is crucial for planar geometries to be stable and preferred over tetrahedral structures.