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

摘要

The anation of (alpha,beta)S-(tetren)CoOH~(2+) by SO_3~(2-) in alkaline medium (pH>12) produced the S-bonded sulphito complex exclusively. The reaction is reversible and k_(obs) = (k_1+k_2 [OH~-])[SO_3~(2-)]_T + k_(-1) [OH~-] is obeyed at [SO_3~(2-)]= 0.005-0.06, [OH~-]_T = 0.02-0.30 moi dm~(-3) (30-45°C,I= 1.0 moldm~(-3)). At 35°C, k_1=(3.6±0.6)x 10~(-3) dm~3 mor~(-1) s~(-1), triangle openH~(not =)=55± 10kJ mol~(-1), triangle openS~(not =) = -112±34JK~(-1)mor~(-1), k_2 =(4.3±0.6) x 10~(-2)dm~6mor~(-2) s(-1), triangle openH~(not =)=44± 11 kJ mol~(-), triangle openS~(not =) = - 130 ± 36 JK~(-1) mo~(-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)OH~(2+)=Co(tetren-H)OH_2~(2+) ). Substantially low values of the activation parameters (triangle openH~(not =) and triangle openS~(not =)) particularly the activation entropy is consistent with associative interchange mechanism (I_a) which is further supported by the rate retardation by anionic micelles of SDS. For the pseudo-phase ion-exchange equilibrium (tetren) in the reaction CoOH_W~(2+) + 2Na_M~+=±(tetren} CoOH_m~(2+) + 2Na_W~+, the calculated value of K_(ex)' is 219 ± 60 at 35°C.