Aspects of three-electron two-centre, four-electron three-centre and six-electron five-centre bonding in cycloimmonium ylides

The weights for several types of local valence bond structures which are involved in two-electron two-centre, three-electron two-centre and four-electron three-centre bonding units for the π-electrons of pyridinium dicyanomethylide are calculated via a Poly-Electron Population Analysis of a Hartree-Fock MO wavefunction. It is shown that three-electron two-centre and four-electron three-centre behavior for the ylide bond is better developed towards the substituents of the ylide carbon atom, rather than the pyridinium group; this gives rise to a polarized increased-valence bonding, which is the principal specificity of cycloimmonium ylides. Qualitative consideration is given to six-electron five-centre bonding units for the π-electrons.