The variations in the thermal stabilities of the trichlorides, tribromides, and tri-iodides of the metals of the first transition series at 25°c

Abstract

The problem of the variations in the feasibilities of the following decompositions at 25° MX₃(s)→ MX₂(s)+½X₂(usual form) in going across the transition series from M = Sc to M = Zn, for X = Cl, Br, and I, is examined by stating it precisely, restating it precisely in a form that is more susceptible to theoretical study, and then studying it theoretically. It is found, by use of current theories, that the overall increases in feasibility across the series may be attributed to the increase in nuclear charge, that the discontinuities Cr < Mn > Fe < Co may be attributed to the reversion of the gain in “exchange” energy in going from M³⁺ to M²⁺ to zero after the d shell has become half full, and that the fluctuations in the ligand-field stabilisation energies of the halides have only a mitigating effect.