Charge-transfer complexes of tetracyanoethylene with alkyl and aryl derivatives of the halogens

Spectral characteristics (lambda(CT),Delta nu(1/2), and epsilon) and association constants (in dichloromethane) of charge-transfer (CT) complexes of tetracyanoethylene (T) with donors containing C-X linkages (X = F, Cl, Br, and I) are presented. The donor orbitals from which CT transitions originate are identified by correlation of lambda(CT) and Delta nu(1/2) values of CT bands of the complexes with the ionization bands of the photoelectron spectra of the donor molecules. CT energies of the complexes are influenced by intramolecular conjugative and inductive effects between halogen atoms and alkyl and aryl groups. Haloalkanes 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 complexes exhibit two spectral bands arising from the transfer of nonbonded 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 lambda(2) values of monoiodoalkanes increase progressively with increasing bulk of the alkyl group. Halobenzenes form complexes with T through interaction of the uppermost pi orbitals of the benzene ring with the pi* LUMO of T. Spectra of fluoro-, chloro-, and bromobenzenes exhibit two bands arising from the uppermost pi orbitals of the benzene ring. lambda(max) and K values tend to increase with decreasing electronegativity of the halogen, Spectra of iodobenzenes exhibit an additional band which arises from the in-plane nonbonded orbital of iodine. Halogen atoms attached to a benzene ring tend (a) to withdraw electrons inductively from the pi orbitals through the sigma-bond framework of benzene and (b) to donate electrons into the pi orbitals through conjugation with the out-of-plane p(y) orbital of the halogen.