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The aquopentaammineplatinum(IV) complex ion reacts in aqueous sulfite solutions to produce the O-bonded complex ion [Pt(NH3)50S02]2+. Both S02 and HSOf take part in the process, according to the expression rate = (/C(The relatively large rate constant values (/ct and k2 are 6.8 X 106 7and 5.9 X 102 M™1 s"1, respectively, at 25 °C) are consistent with a mechanism of direct S(IV) addition to the Pt-OH moiety without metal-oxygen bond rupture. The product of this reaction (which has been isolated and identified as [Pt(NH3)50S02] SG3-2H20) undergoes relatively slow rearrangement in the presence of excess sulfite (but independent of its concentration) to form a bis(sulfito) complex, which was isolated and identified as rii-[Pt(NH3)4(S03)2]-2H20. The kinetic characteristics of the reaction suggest that there is rapid -3 replacement by water (on heating to reaction temperature) followed by rate-determining O-bonded to S-bonded sulfito isomerization (k = 4.0 X 10-5 s™1 at 25 °C), followed by rapid replacement of the cis-aquo ligand by sulfite to yield the observed bis(sulfito) complex. The cis-bis(sulfito) species is stable to internal redox in neutral solution at room temperature. However, at elevated temperatures in acidic solution, a single-step two-electron transfer takes place to yield a Pt(II) product (which was isolated and identified as [Pt(NH3)3S03]-2H20) according to the rate law rate = &[Pt(NH3)4(S03)(S03H)+]. In this, k has the value 5.6 X 1CP4 s"1 and the pK of cis-[Pt(NH3)4(S03)(S03H)]+ is approximately 2 (both figures extrapolated from the 45-55 °C experimental range to 25 °C). The mechanism appears to involve simultaneous detachment of an S03 molecule and its trans-NH3 ligand (accompanied by rapid hydration of S03 to sulfate) to generate the observed planar Pt(NH3)3SQ3 product.

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