ROLE OF STERIC HINDRANCE OF METHYL-GROUPS ON THE RATES OF REDUCTION OF N-METHYL SUBSTITUTED ETHYLENEDIAMINE COMPLEXES OF PLATINUM(IV)

The reduction of platinum(IV) complexes of the type [PtCl4(N-N)] by FeCp2, I-, [Pt(NH2Et)4]2+ or [Pt(en)(NH2Et)2]2+ in the presence of chloride ions in methanol have been investigated (N-N = en, meen, s-dmen, a-dmen, trimen, tmen). The reactions obey second- or third-order rate laws: rate = k(FeCp2)[PtCl4(N-N)][FeCpj, k(I-)[PtCl4(N-N)][I-] or k(Pt)II[PtCl4(N-N)][PtL4(2+)][CI-]. The rate constants increase by increasing the steric hindrance at the amine ends of the N-N ligand, even if in a measure which is quantitatively dependent on the reducing agent {I- > FeCp2 > [Pt(NH2Et)4]2+ almost-equal-to [Pt(en)(NH2Et)2]2+}. The steric hindrance of the terminal amine groups appears to behave as an additive parameter and linear relationships are found which correlate the reactivities (log k(Reductant) of platinum(IV) complexes to a proposed parameter of relative overall steric hindrance of the N-N ligand. The observed reactivity trend Of [PtCl4(N-N)] complexes is thought to have basically a thermodynamic origin. Driving forces should also be responsible for the higher reactivity (ca 20 times) of [Pt(en)(NH2Et)2]2+ with respect to that of [Pt(NH2Et)4]2+.