Unstable intermediates. Part 166. Cadmium and mercury centres in irradiated potassium tetrathiocyanato-cadmate(II) and -mercurate(II)

Abstract

After exposure to ⁶⁰Co γ-rays at 77 K, the e.s.r. spectrum for K₂[Hg(SCN)₄] shows well resolved, nearly isotropic, high-field lines assigned to magnetic centres containing ¹⁹⁹Hg and ²⁰¹Hg, with A(¹⁹⁹Hg)= 8 900 G and g= 1.97. A triplet splitting of ca. 6 G on these features and on the central component is assigned to hyperfine coupling to one ¹⁴N nucleus. This has been confirmed by measurements at Q-band frequencies. In dilute methanolic solution. an isotropic centre having A(¹⁹⁹Hg)= 7 820 G is transformed into an anisotropic centre with A_∥(¹⁹⁹Hg)= 7 691 G and A_⊥(¹⁹⁹Hg)= 7 555 G on annealing. These results are interpreted in terms of the capture of electrons by [Hg(SCN)₄]^2– ions in methanol to give [Hg(SCN)₄]^3– having ca. 60% spin density in the 6s mercury orbital and 40% on the ligands. On annealing this becomes distorted, possibly by N-protonation, or loss of one ligand. For the pure solid it is suanested that the electron remains relatively mobile at 77 K until it is trapped at a defect in which one of the ligands is co-ordinated via nitrogen. Similar, but less well defined, results have been obtained for K₂-[Cd(SCN)₄] and its solutions in methanol.