CHARGE-TRANSFER COMPLEXES OF TETRACYANOETHYLENE WITH ALKYL AND ARYL DERIVATIVES OF THE HALOGENS

Spectral characteristics (lambda(CT),Delta nu(1/2), and epsilon) and a ssociation constants (in dichloromethane) of charge-transfer (CT) comp lexes of tetracyanoethylene (T) with donors containing C-X linkages (X = F, Cl, Br, and I) are presented. The donor orbitals from which CT t ransitions originate are identified by correlation of lambda(CT) and D elta nu(1/2) values of CT bands of the complexes with the ionization b ands of the photoelectron spectra of the donor molecules. CT energies of the complexes are influenced by intramolecular conjugative and indu ctive effects between halogen atoms and alkyl and aryl groups. Haloalk anes form weak complexes with T through interaction of the p(x) and p( y) orbitals of the halogen with the pi LUMO of T. Monoiodoalkane-T co mplexes exhibit two spectral bands arising from the transfer of nonbon ded electrons from the 5p orbitals of iodine which are split into (2)E (1/2) and (2)E(32) states through spin-orbit coupling. lambda(1) and l ambda(2) values of monoiodoalkanes increase progressively with increas ing bulk of the alkyl group. Halobenzenes form complexes with T throug h interaction of the uppermost pi orbitals of the benzene ring with th e pi LUMO of T. Spectra of fluoro-, chloro-, and bromobenzenes exhibi t two bands arising from the uppermost pi orbitals of the benzene ring . lambda(max) and K values tend to increase with decreasing electroneg ativity of the halogen, Spectra of iodobenzenes exhibit an additional band which arises from the in-plane nonbonded orbital of iodine. Halog en atoms attached to a benzene ring tend (a) to withdraw electrons ind uctively from the pi orbitals through the sigma-bond framework of benz ene and (b) to donate electrons into the pi orbitals through conjugati on with the out-of-plane p(y) orbital of the halogen.