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

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 RC5H 4· (R = H, D, Me, Et, Prⁱ, or Bu^t). The C5H5· radical is a planar π-radical with average D54 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 ψA and ψS molecular orbitals of the π-system by electron release, destabilising the ψS MO, and the e.s.r. spectrum reflects the spin density distribution in the configuration ψA^Z ψS¹. 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 ψS^1.515 ψA^1.485. This is compatible with the model of a vibrational perturbation of the resonance integral β, rather than of the Coulomb integral α.