Kinetics of the reduction of μ–oxo–tetrakis(1, 10–phenanthroline) diiron (iii) complex by thiourea in aqueous phenanthrolinium buffer

Abstract

The homovalent oxo-bridged binuclear iron(III) complex ion [Fe₂O(phen)₄Cl₂]⁴⁺ (phen = 1, 10–phenanthroline) aquates to [Fe₂O(phen)₄(H₂O)₂]⁴⁺ in aqueous phenanthrolinium buffer solution (pH 3.25–4.50). The reaction of [Fe₂O₂(phen)₄(H₂O)₂]⁴⁺ with thiourea in this buffer solution to give [Fe(phen)₃]²⁺, sulphite ion and urea was studied spectrophotometrically at 510 nm and found to be first order in [Fe₂O₂(phen)₄(H₂O)₂]⁴⁺ and of mixed zero and first order in thiourea, i.e. reaction order with respect to thiourea varies from one to zero on increasing the thiourea concentration from 1.0–7.0 × 10^-2 mol dm^-3. The reaction is characterized by adduct formation. Changes in pH from 3.25 to 4.50 and ionic strength from 0.10 moldm^-3 to 0.60 moldm^-3 of the reaction solution were found to have no effect on the reaction rate. Catalysis and retardation were observed with the addition of Mg²⁺ and CH₃COO^- ions respectively. A plausible mechanism consistent with the results obtained is proposed.

Keywords: kinetics, mechanism, oxo-bridged, phenanthrolinium buffer, thiourea