Kinetics and mechanism of anation of (α,β)S-(hydroxo) (tetraethylenepentamine)cobalt (III) by sulphite in basic medium, the role of anionic micelles

The anation of (α,β)S-(tetren)CoOH2+ by SO3 2- in alkaline medium (pH > 12) produced the S-bonded sulphito complex exclusively. The reaction is reversible and kobs=(k1+k2[OH-])[SO 32-]T+k-1[OH-] is obeyed at [SO32- ]=0·005-0·006, [OH-]T =0·02-0·30 mol dm-3 (30-45°C, I=1·0 mol dm-3). At 35°C, k1=(3·6±0·6)×10-3 dm3 mol-1 s-1,ΔH ≠=55± 10 kJ mol-1, ΔS ≠ = -112±34 JK-1 mol-1,k2=(4·3±0·6)×10-2dm6 mol-2s-1, ΔH≠ = 44±11 kJ mol-1, ΔS ≠ = -130±36 JK-1 mol-1. The unusually high rate of sulphite substitution particularly in the OH- independent path is attributed to the involvement of a reactive internal conjugate base due to the coordinated OH- assisted NH-deprotonation of the tetren moiety (Co(tetren)OH2+□Co(tetren-H)OH22+). Substantially low values of the activation parameters (ΔH ≠ and ΔS≠) particularly the activation entropy is consistent with associative interchange mechanism (Ia) which is further supported by the rate retardation by anionic micelles of SDS. For the pseudo-phase ion-exchange equilibrium (tetren) in the reaction CoOHW2+ + 2NaM+ □(tetren)CoOHM2+ + 2NaW+ , the calculated value of K'ex is 219±60 at 35°C

Electron transfer between aquapentaamminecobalt(III) and sulphur(IV) in aquo-organic solvent media: delineation of solvent effects

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