Reactions of oxygen-bonded and sulphur-bonded (sulphito)(amine)cobalt(III) complexes in solution:Part II1 - kinetics and mechanism of reactions of cis(diaquo) - bis(ethylenediamine) cobalt(III) and cis(diaquo)bis- (trimethylenediamine)cobalt(III) cations with aqueous sulphur dioxide

Abstract

Kinetics of title reactions have been investigated at 35-50°C and <em>I </em>= 0.3 mol dm ^-3. In the presence of excess sulphite and at <em>p</em>H = 4.5-6.7, the diaquo complexes are completely transformed to the O-bonded disul-phito complexes, <em>c</em><em>is</em>-[Co(AA)₂(OSO₂)₂]^- (AA = en, tmd) which undergo two kinetically identifiable reactions: (i) intramolecular reduction to Co(II); and (ii) isomerisation to O- and S-bonded sulphito complexes, <em>Cis</em>-[Co(AA)₂(OSO₂)(SO)]^-. This mixed sulphito complexes also undergo intramolecular reduction to Co(II) at rates much slower than those of their bis-O-sulphito congeners. The observed biphasic kinetics have been resolved. The rates and activation parameters for these three kinetic processes are reported. Rate comparison with similar other systems establishes the reactivity sequence for intramolecular reduction process as: <em>trans</em>-[CO(en)₂(OSO₂H)(OSO₂)] »<em> cis</em>-[Co(en)₂(OH₂)(OSO₂)]⁺ > <em>cis</em>-[Co(AA)₂(OSO₂)₂]^-(AA = en, tmd) > <em>cis</em>-[Co(AA)₂(OSO₂)(SO₃)] > <em>cis</em>-[Co(en)₂(OSO₂)⁺ (B = NH₃, imidazole). The activation enthalpy and entropy data for the partial intramolecular isomerisation of <em>cis-</em>[Co(AA)₂(OSO₂)₂]^- to <em>ci</em><em>s</em><em>- </em>[Co(AA)₂(OSO₂)(SO₃)]^‑ are significantly lower than those for the intramolecular reduction of <em>bis</em><em>-</em>O-sulphito or (O -, S - ) sulphito species.