Ligand recoil and solid state exchange reactions in hexahalo complexes of Re(IV)

Abstract

The chemical effects of ( n, γ)- and ( n, 2 n)-reactions on ligand halogen atoms have been studied in crystalline hexahalorhenates K 2[Re X 6] ( X = F, Cl, Br, I). The radiohalogen activity was shown to appear in only two chemical forms, the labelled parent complex and the free halide. The ∗ X −-yields are independent of the type of nuclear process (recoil energy and range), but decrease with the Re X bond energy from the hexafluoro- to the hexaiodorhenate. Intermolecular competition experiments in K 2[ReCl 6]/K 2[ReBr 6] (1:1) mixed crystals show that the recoil bromine almost exclusively appears in the two monosubstituted parent complexes and as free bromide. The results again indicate that the hot spot model in the sense of a molten zone cannot be applicable. Furthermore, it becomes obvious, that the solid state reactions of the recoil halogen take place in a quite undisturbed zone, involving simple exchange and substitution processes. Model experiments on solid state exchange reactions have been carried out in radiohalide-doped hexahalorhenates to obtain information on the mechanism of annealing in the neutron-irradiated complexes. In both cases thermal treatment leads to the entry of recoil or dopant halogens into the ligand sphere. Both processes can be described by a first-order kinetics showing a striking similarity in reaction rates and activation energies. The results are explained on the basis of a nucleophilic substitution mechanism involving X −-ions and ligand vacancies. As a possible transport mechanism a ligand vacancy migration is proposed.