Stabilization of planar tetracoordinate carbon

Abstract: The energies of planar vs. tetrahedral geometries of tetracoordinate organic molecules have been surveyed by ab initio molecular orbital calculations. Because of their 7 ~ acceptor and u donor character, electropositive substituents, especially lithium, are particularly effective in stabilizing the planar arrangements selectively. Multiple substitution by such metals and by three-membered rings provides further stabilization, and 1,l-dilithiocyclopropane (XVII) and 3,3-dilithiocyclopropene (XVIII), inter alia, are actually calculated (RHF/STO-3G theory) to be more stable planar than tetrahedral. van’t Hoff and LeBel’s proposal that tetracoordinate carbon prefers tetrahedral geometry2 celebrated its centenary in 1 974.3 No fundamental exception, e.g., an organic compound in which all four substituents lie in a plane, is known. It is not difficult to understand why this should be so. Singlet planar methane is sp2 hybridized with a lone pair of electrons in the remaining p orbital (I).4 Consequently, only six electrons instead of eight are involved in bonding. Numerous theoretical calculations5 at different levels of sophistication have been applied to this problem and are summarized in Table I. The best available ab initio quantum mechanical calculations indicate D4h singlet planar methane should be about 150 kcal/ mol less stable than the tetrahedral form.6 Since the bond dissociation energy of the C-H bond is only 104 kcal/mo17 D4h planar methane would not be expected to exist under any circumstances. Figure 1 shows the molecular orbitals of planar