The π-donating ability of heteroatoms in α-substituted methyl cations

An ab initio SCF geometry optimization on the simple cations H₂X, with X = F, Cl, NH₂ and PH₂ has been performed at the split valence shell (4-31G) level. The computed optimum conformations correspond in each case to a structure in which all atoms lie in the same plane. Comparison of the computed charge distributions reveals that the third period heteroatom (Cl and P) is a better π electron donor than the corresponding second period analogue (F and N). This result parallels that obtained recently [Can. J. Chem.53, 1144 (1975)] for S and O in H₂OH and H₂SH, but contradicts current notions based on assumed values of the C_π-X_π overlap integrals. It is shown here by explicit calculations of overlap integrals that these assumptions are not always correct. Furthermore, it is shown that arguments based only on overlap are necessarily incomplete since they neglect terms like the energy difference between the interacting orbitals which can play a dominant role. The relative importance of such terms is discussed for these species.