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|Title:||Kinetics and mechanism of the reaction of sulphur(IV) with trans-bis(aqua)(N,N' -salicy lidiniminato-ortho-phenylenediamine cobaltate (III) : The trans effect of S-bonded sulphite|
|Authors:||Dash, Anadi C|
Bramha, Gouri S
|Abstract:||The reaction of trans-(aqua)(hydroxo)(N,N'-salicylidiniminato-ortho-phenylenediamine) cobaltate(III) with SO2 results in the formation of S-bonded monosulphito complex, trans-[Co(salophen) (OH2)(SO3)]- as the primary product; the CoIII-S bond formation is concerted with CoIII-O bond breaking. At 25°C, I = 0,5 mol dm-3 (NaNO3), the rate and activation parameters for this process are: kSO2 = (4.8) × 108 dm3 mol-1 s-1, ΔH± = 23.6±3.2 kJ mol-1 , ΔS± = - 0.14±16.7 J K-1 mol-1. The aqua-sulphito complex further undergoes substitution of the aqua ligand by imidazole and SO32-; the formation of the imidazole complex, trans-[Co(salophen)(imH)(SO3)ris, however, reversible. The rate data indicate that the trans effect of the S-bonded sulphite in trans-[Co(salophen)(OH2)(SO3)]- is only marginally stronger for the aqua ligand substitution by imidazole and SO32- than in the case of its salen analogue reported earlier. The kinetic inertness of trans[Co(salophen)(OH2)(SO3)]- towards H+ catalysed aquation in comparison to its salen analogue (kHsalen/kHsalophen= 104 at 25°C) demonstrates the effects of rigidity and extended electron delocalization by the salophen moiety. The rate constants of base hydrolysis of trans[Co(salophen)(OH)(SO3)]2- at [OH-]T = 0.01-0.5 , I= 0.5 mol dm-3 (25°C) obeys kobs = ko + kOH [OH-] with k0 = (0.89±0.04 ) × 10-3 s-1 and kOH =(6.4±0.6) × 10-3 dm3 mol-1 s-1. A finite value of ko indicates that the trans-labilising action of the coordinated OH group is stronger than that of the bound SO32- .It is suggested that the second order rate constant kOH, is associated with a mechanism which involves the formation of a reactive C-conjugate base from the aldimine function , H-C=N-R.|
|Appears in Collections:||IJC-A Vol.42A(01) [January 2003]|
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