Reactions of oxygen-bonded and sulphur-bonded (sulphito) pentaminecobalt(III) complexes in solution

Abstract

The reactions of <em>cis</em>-[Co(en)₂(B)OH]²⁺ (B = NH₃, imidazole) and <em>trans-</em>[Co(en)₂(NH₃)OH]^2<em>+</em> with aqueous sulphur dioxide instantaneously generate the corresponding oxygen-bonded sulphito species, N₅CoOSO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0>, as intermediates which undergo slow intramolecular isomerisation to the stable S-bonded sulphito species as the major product. Strong <em>trans </em>activation of S-bonded sulphite results in fast substitution of NH₃ in <em>trans-</em>[Co(en)₂(NH₃)SO₃]<em>⁺ </em>by SO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0> resulting in <em>trans-</em>[Co(en)₂(SO₃)₂]^- in which both the SO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0> are S-bonded to Co (III). On the other hand the species, <em>cis</em>-[Co(en)₂(NH₃/imH)SO₃]⁺ are stable towards further replacement of imH or NH₃ by SO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0> in the time scale of measurement. Simultaneously intramolecular reduction of O-bonded sulphito species, a process which is relatively more facile for the <em>cis</em>-complexes, also occurs to the extent of ~30%. The kinetics of intramolecular isomerisation and reduction of these <em>in situ </em>generated oxygen-bonded sulphito complexes have been investigated over the temperature range of 30-45°C (<em>I </em>= 0.3 mol dm^-3).The rates and activation parameters of these reactions are reported and compared with previously published related data. It is worth noting that while increasing chelation of Co (III) centre by multidentate amine ligand enhances rate of isomerisation of N₅CoO-SO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0> to S-bonded sulphito species, reverse trend in reactivity is observed for intramolecular reduction of O-bonded sulphito complexes; <em>k</em>_red follows the sequence: (NH₃)₅CoOSO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0>> <em>cis</em>-[Co(en)₂ (B)OSO₂]⁺ (B = NH₃, imH) > <em>trans-</em>[Co(en)₂NH₃OSO<img src='http://www.niscair.res.in/jinfo/three.gif' border=0>] > > (tetren)CoOSO, (tetren =tetraethylenepentamine), the last mentioned complex being totally inert to reduction.