Reactions of beta-cis-(diaqua)(triethylenetetramine)cobalt(III) with sulphur (IV) in aqueous medium: A thermal and photochemical study

The initial fast reaction of SO2 with beta-[Co(trien) (OH)OH2)](2+) yields the O-bonded sulphito complex, beta-[Co(trien) (OH2) (OSO2-O)](+) for which k(f) = (4.76 +/- 0.25) x 10(7) dm(3) mol(-1) s(-1), Delta H* = 42.1 +/- 5. 1 kJ mol(-1), Delta S* = 43 +/- 17 J K-1 mol(-1) at 25 degrees C and I = 1. 0 mol dm(-3). The O-sulphito complex undergoes H+-catalysed aquation with k = (1.1 +/- 0.2) x 10(6) dm(3) mol(-1) s(-1) (25 degrees C, I = 1.0 mol dm( -3)). These data are consistent with the rate limiting addition of SO2 to C o-III-OH and breakage of S-O bond in the formation and aquation of the O-su lphito complex respectively. The O-sulphito complex undergoes intramolecula r electron transfer and ligand linkage isomerisation, Co-OSO2+ --> Co-SO3for which k(red)(30 degrees C) = (0.28 +/- 0.03) x 10(-4) s(-1), Delta H* = 76 +/- 12 kJ mol(-1) , Delta S* = -81 +/- 39 J K-1 mol(-1) and k(iso)(30 d egrees C) = (0.44 +/- 0.12) x 10(-4) s(-1), Delta H* = 57 +/- 7 kJ mol(-1), Delta S* = -138 +/- 24 J K-1 mol(-1) respectively. The beta-cis-[Co(trien) (OH2)(SO3-S)(+) has been isolated in the solid state and characterised. It does not show exceptional lability of the aqua ligand for substitution and is substantially stable to intramolecular electron transfer between Co-III and S-IV under thermal condition. The conventional flash photolysis of this S-bonded sulphito complex has been shown to generate the corresponding O-s ulphito complex as a transient while its steady state photolysis (254 nm) r esulted in photo reduction without significant build up of any intermediate .