An electron spin resonance study of the equilibrium between tetrahalogeno- and pentahalogeno-nitridotechnetate(VI) ions in solution

Abstract

The e.s.r. spectra of [AsPh₄][TcNCl₄], Cs₂[TcNCl₅], [AsPh₄][TcNBr₄], and Cs₂[TcNBr₅] have been studied in non-aqueous and concentrated aqueous acid solutions. None of the spectra shows evidence for the co-ordination of a fifth halide ligand in the trans position, even under circumstances such as a 2 000-fold excess of halide ion, which would be expected to favour the formation of the pentahalogenonitridotechnetate ion. The predominant species in solution is the tetrahalogenonitrido-technetate ion, where the trans position may be vacant or occupied by a solvent molecule in the case of the non-aqeuous solvents and by a water molecule in the case of HCl and HBr solutions. This conclusion may be contrasted with the behaviour of a number of tetra- and penta-halogeno-oxometal complexes, where the equilibrium [MOX₄]^n–+ X^–⇌[MOX₅]^{(n+ 1)–} is clearly established.