ROLE OF THE LIGANDS IN AFFECTING THE STOICHIOMETRY AND THE RATE OF REACTIONS BETWEEN PLATINUM(IV) AND PLATINUM(II) COMPLEXES

The stoichiometries and the rates of the reactions between trans-[Pt(en)(N-N)Cl2]2+ and [Pt(en)(N'-N')]2+ in the presence of bromide ions have been studied in aqueous solutions (N-N and N'-N' = en, 1,3-pn, 1,2-pn, meen, dmen, tmen). The platinum(IV) reaction product is either trans-[Pt(en)(N-N)Br2]2+ or trans-[Pt(en)(N'-N')Br2]2+, depending on the bulkiness of N-N and N'-N'. The reaction rates obey a pseudo-first-order rate law with k(obs) almost-equal-to {k(c)[Br-]+ k(d)[Br-]2}[Pt(en)(N'-N')2+]. The ligands significantly affect the rates, mainly via their steric hindrance. An increase of the steric hindrance near the platinum(IV) centre only moderately enhances its reactivity towards reduction, whereas a strong lowering of the reactivity of platinum(II) towards oxidation is caused by an increase of the steric hindrance of the ligands coordinated to this atom centre. The quantitative kinetic effect displayed by changes of such ligands either on platinum(IV) or on platinum(II) complexes is also dependent on the particular platinum(II) or platinum(IV) complex used as the standard reducing or oxidizing reagent. An explanation of the observed behaviour is suggested in terms of an atom-transfer redox mechanism.