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 RC
5H 4· (R = H, D, Me, Et, Pri, or But). The C 5H 5· radical is a planar π-radical with average D 54symmetry, 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 Aand ψ Smolecular orbitals of the π-system by electron release, destabilising the ψ SMO, and the e.s.r. spectrum reflects the spin density distribution in the configuration ψ AZ ψ S1. Deuterium has a small but detectable perturbing effect : the ψA MO is destabilised by ca. 100 J mol-1, and thermal mixing of the two energy levels results in the configuration ψ S1.515 ψ A1.485. This is compatible with the model of a vibrational perturbation of the resonance integral β, rather than of the Coulomb integral α.