Kinetics and mechanism of the reactions of hexaaqua rhodium (III) with sulphur (IV) in aqueous medium

An O-bonded sulphito complex, Rh(OH2)5(OSO2H)2+, is reversibly formed in the stoppedflow time scale when Rh(OH2)6 3+ and SO2/HSO3 – buffer (1 < pH < 3) are allowed to react. ForRh(OH2)5OH2+ + SO2 _ Rh(OH2)5(OSO2H)2+ (k1/k–1), k1 = (2,2 plus-minus0,2)cross03 dm3 mol–1 s–1, k–1 = 058 plus-minus0.16 s–1 (25C,1 equel 0, mol dm–3). The protonated O-sulphito complex is a moderate acid (Kd = 3cross10–4 mol dm–3, 25C,1 equel0.5 mol dm–3). This complex undergoes (O, O) chelation by the bound bisulphite with k = 14 cross10–3 s–1 (31C) to Rh(OH2)4(O2SO)+ and the chelated sulphito complex takes up another HSO3 – in a fast equilibrium step to yield Rh(OH2)3(O2SO)(OSO2H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH2)3(O2SO)(OSO2)– Rh(OH2)3(O2SO)(SO3)– (kiso = 3 cross10–4 s–1, 31C). A dinuclear (m-O, O) sulphite-bridged complex, Na4[Rh2(m-OH)2(OH)2(m-OS(O)O)(O2SO)(SO3) (OH2)]5H2O with (O, O) chelated and S-bonded sulphites has been isolated and characterized. This complex is sparingly soluble in water and most organic solvents and very stable to acid-catalysed decomposition.