The photolysis of cyclopentadienyl compounds of tin and mercury. Electron spin resonance spectra and electronic configuration of the cyclopentadienyl, deuteriocyclopentadienyl, and alkylcyclopentadienyl radicals

Abstract

Cyclopentadienyl derivatives of tin(IV) and mercury(II) and alkylcyclopentadienyl derivatives of mercury(II) are photolysed in solution to show the e.s.r. spectra of the appropriate radicals RC₅H₄·(R = H, D, Me, Et, Prⁱ, or Bu^t). The C₅H₅· radical is a planar π-radical with average D_5h symmetry, and its spectrum is broadened in the presence of organic bromides, perhaps by a charge-transfer mechanism. The introduction of alkyl groups breaks the degeneracy of the ψ_A and ψ_S molecular orbitals of the π-system by electron release, destabilising the ψ_S MO, and the e.s.r. spectrum reflects the spin density distribution in the configuration ψ_A²ψ_S¹. Deuterium has a small but detectable perturbing effect: the ψ_A MO is destabilised by ca. 100 J mol^–1, and thermal mixing of the two energy levels results in the configuration ψ_S^1.515ψ_A^1.485. This is compatible with the model of a vibrational perturbation of the resonance integral β, rather than of the Coulomb integral α.