Mechanistic studies on monodentate–ligand substitution of five-coordinate trigonal-bipyramidal platinum (II) complexes with tris[2-(diphenylphosphino)ethyl]phosphine

Reaction of the five-coordinate trigonal-bipyramidal platinum(II) complex, [Pt(pt)(pp3)](BF4) (pt = 1-propanethiolate, pp3 = tris[2-(diphenylphosphino)ethyl]phosphine), with I− in chloroform gave the five-coordinate square-pyramidal complex with a dissociated terminal phosphino group and an apically coordinated iodide ion in equilibrium. The thermodynamic parameters for the equilibrium between the trigonal-bipyramidal and square-pyramidal geometries, [Pt(pt)(pp3)]+ + I− ⇄ [PtI(pt) (pp3)], and the kinetic parameters for the chemical exchange were obtained as follows: View the MathML sourceKex298=7.5×10-1mol-1kg, ΔH 0 = − 10 ± 2.4 kJ mol−1, ΔS 0 = − 36 ± 10 J K−1 mol−1, View the MathML sourcekex298=1.3×104s-1, ΔH‡ = 34 ± 4.7 kJ mol−1, ΔS‡ = − 50 ± 21 J K−1 mol−1. The square-planar trinuclear platinum(II) complex was formed by bridging reaction of one of the terminal phosphino groups of trigonal-bipyramidal [PtCl(pp3)]Cl with trans-[PtCl2(NCC6H5)2] in chloroform. From these facts, ligand substitution reactions of [PtX(pp3)]+ (X = monodentate anion) are expected to proceed via an intermediate with a dissociated phosphino group. The rate constants for the chloro-ligand substitution reactions of [PtCl(pp3)]+ with Br− and I− in chloroform approached the respective limiting values as concentrations of the entering halide ions are increased. These kinetic results confirmed the preassociation mechanism in which the square pyramidal intermediate with a dissociated phosphino group and an apically coordinated halide ion is present in the rapid pre-equilibrium.