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

Abstract

An O-bonded sulphito complex, Rh(OH 2) 5 (OSO 2 H) 2+ , is reversibly formed in the stoppedflow time scale when Rh(OH 2) 6 3+ and SO 2 /HSO 3 buffer (1 < pH < 3) are allowed to react. For Rh(OH 2) 5 OH 2+ + SO 2 Rh(OH 2) 5 (OSO 2 H) 2+ (k 1 /k-1), k 1 = (2⋅2 ± 0⋅2) × 10 3 dm 3 mol-1 s-1 , k-1 = 0⋅58 ± 0⋅16 s-1 (25°C, I = 0⋅5 mol dm-3). The protonated O-sulphito complex is a moderate acid (K d = 3 × 10-4 mol dm-3 , 25°C, I = 0⋅5 mol dm-3). This complex undergoes (O, O) chelation by the bound bisulphite with k = 1⋅4 × 10-3 s-1 (31°C) to Rh(OH 2) 4 (O 2 SO) + and the chelated sulphito complex takes up another HSO 3 in a fast equilibrium step to yield Rh(OH 2) 3 (O 2 SO)(OSO 2 H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH 2) 3 (O 2 SO)(OSO 2)-→ Rh(OH 2) 3 (O 2 SO)(SO 3)-(k iso = 3 × 10-4 s-1 , 31°C). A dinuclear (µ-O, O) sulphite-bridged complex, Na 4 [Rh 2 (µ-OH) 2 (OH) 2 (µ-OS(O)O)(O 2 SO)(SO 3) (OH 2)]5H 2 O 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.

Kinetics of oxidation of d(+)melibiose and cellobiose by N-bromoacetamide using a rhodium(III) chloride catalyst

“…NBA has been used as an oxidant for uncatalysed [8][9][10][11][12] and catalysed [13][14][15][16][17][18] reactions involving transition metals including Ru(III), Ir(III) and Pd(II) as homogeneous catalysts. The coordination chemistry of Rh(III) [19] has drawn considerable attention due to its chemical reactivity, anti-tumour activity, electronic structure and catalytic functions of its complexes with potential industrial applications. NBS [20] oxidises maltose and D-galactose in the presence of Rh(III) chloride in nano-concentrations as homogeneous catalyst in acidic medium.…”