The first neutral pentaatomic tetracoordinate planar carbon molecules, CAl3
Si and CAl3Ge, as well as their anions, CAl3Si- and CAl3Ge-, were experimen
tally observed in the gas-phase and characterized by ab initio calculations
and anion photoelectron spectroscopy. A four-center bond involving ligand-
ligand interactions was found to be critical in stabilizing the planar stru
ctures of the 17-valence-electron CAl3Si and CAl3Ge and 18-valence-electron
CAl3Si- and CAl3Ge-. These species violate the conventional expectation of
tetrahedral structure for tetracoordinate carbon atoms and thus extend our
concept of the range of chemical bonds that carbon can form.