Kinetics of displacement of dimethyl sulphoxide (dmso) from [Pt(N–N)(dmso)X]+ by X–[N–N = 1,10-phenanthroline, 2,2′-bipyridyl, or 2,2-bis(2′-pyridyl)-1,3-dioxolane; X = Cl or Br]

Abstract

On reaction with Cl^– or Br^– in methanol, dimethyl sulphoxide (dmso) is displaced from [Pt(N–N)(dmso)X]⁺. When N–N is 1,10-phenanthroline or 2,2′-bipyridyl the reactions are relatively rapid and the reaction with Cl^– obeys the rate law –d[complex]/dt=(k₁+k₂[Cl^–]+k₃[Cl^–]²)[complex]. The reaction with bromide can only be studied under conditions where the second-order dependence on [Br^–] is not expected. The reactions with N–N = 2,2-bis(2′-pyridyl)-1,3-dioxolane (bipyoxo) are much slower and, even at the highest chloride concentrations used, the dependence on [Cl^–] is strictly first order. The kinetics are explained in terms of addition of a chloride to form an intermediate that can undergo an associative displacement of dmso by a second chloride in addition to the normal mode of reaction. It is suggested that this intermediate is truly five-co-ordinate. The absence of the second-order dependence in the reactions of the bipyoxo complex is ascribed to a blocking of an axial position on the platinum by the oxygen. An alternative mechanism in which the intermediate remains four-co-ordinate and has the di-imine acting as monodentate is considered and rejected. Comparisons are made with the behaviour of [Pt(N–N)(C₂H₄)Cl]⁺.