Low temperature kinetic study of very fast substitution reactions at platinum(II) trans to olefins

Ultra-fast substitution of chloride for bromide, iodide, azide and thiocyanate trans to ethene in Zeise's anion, [PtCl3(C2H4)](-), 1, has been investigated in methanol solvent by use of cryo temperature diode array stopped-flow spectrophotometry. Reactions follow the usual two-term rate law for square-planar substitutions, k(obs)=k(1)+k(2)[Y] (where k(1)=k(MeOH) [MeOH]), with k(1)=118+/-10 s(-1) and k(2)=(5.1+/-0.2) x 10(2), (3.51+/-0.07) x 10(3), (11.8+/-0.2) x 10(3), and (56+/-4) x 10(3) mol(-1) dm(3) s(-1) for Y=Br-, I-, N-3(-) and SCN-, respectively, at 223 K. Activation parameters for MeOH, Br-, I- and N-3(-) are DeltaH(not equal)=23+/-2, 21+/-2, 17+/-1.0 and 11.9+/-1.5 kJ mol(-1) and DeltaS(not equal)=-124+/-10, -96+/-9, -98+/-4 and -111+/-6 J K-1 mol(-1), respectively. Recalculation of k(1) to second-order units gives the sequence of nucleophilicity MeOHCH(2)CHSiMe(3)similar toC(8)H(14), spanning a factor of between 5 and 30 depending on the nucleophile,