Central Role of Phenanthroline Mono-N-oxide in the Decomposition Reactions of Tris(1,10-phenanthroline)iron(II) and -iron(III) Complexes

1,10-Phenanthroline mono-N-oxide (phenO) is a product of the decomposition of tris(1,10-phenanthroline)iron(III), Fe(phen)₃³⁺, and has a slight autocatalytic effect on the overall reaction. The mechanism is proposed to involve Fe(phen)₃⁴⁺ as a minor intermediate. The addition of phenO significantly influences the kinetic features of the decomposition of Fe(phen)₃³⁺ and the oxidation of Fe(phen)₃²⁺ by HSO₅⁻. The autocatalytic decomposition explains the difficulties in the preparation of Fe(phen)₃³⁺ and may contribute to exotic kinetic phenomena studied using Fe(phen)₃³⁺/Fe(phen)₃³⁺ as a supposedly innocent indicator.