BONDING IN CONJUGATED HALOGEN COMPOUNDS

The effect of inclusion of d pi orbitals in halogen conjugation is considered. The resulting p-d hybridization yields a considerable increase, similar to 2 e.v., in bonding energy (greater than that found in Cl-2 through p pi-d pi hybridization). For conjugated C-Cl bonds the valence state of the chlorine atom is estimated at p pi(1,90)d pi.(10). The greatly increased C-Cl bond order indicates considerable triple bond character. The factors determining the conditions for d pi-p pi hybridization are analyzed, and V.B. structures are proposed. In particular, Y+ = R = X-, where Y is a strong electron donating group, seems to be important. It is suggested that dipole moments, resonance energies and intensities in the ultraviolet spectra of halogen compounds (in particular chloro-, bromo- and iodobenzenes) need reinterpretation in the light of the above.