The first neutral pentaatomic tetracoordinate planar carbon molecules, CAl3Si and CAl3Ge, as well as their anions, CAl3Si-and CAl3Ge-, were experimentally 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 structures 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.