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 [Fe 2 O(phen) 4 Cl 2 ] 4+ (phen = 1, 10–phenanthroline) aquates to [Fe 2 O(phen) 4 (H 2 O) 2 ] 4+ in aqueous phenanthrolinium buffer solution (pH 3.25–4.50). The reaction of [Fe 2 O 2 (phen) 4 (H 2 O) 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 [Fe 2 O 2 (phen) 4 (H 2 O) 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 Mg 2+ and CH 3 COO - ions respectively. A plausible mechanism consistent with the results obtained is proposed. Keywords: kinetics, mechanism, oxo-bridged, phenanthrolinium buffer, thiourea