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

Abstract Reaction of the five-coordinate trigonal-bipyramidal platinum(II) complex, [Pt(pt)(pp 3 )](BF 4 ) (pt = 1-propanethiolate, pp 3  = 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)(pp 3 )] +  + I −  ⇄ [PtI(pt) (pp 3 )], and the kinetic parameters for the chemical exchange were obtained as follows: K ex 298 = 7.5 × 10 - 1 mol - 1 kg , Δ H 0  = − 10 ± 2.4 kJ mol −1 , Δ S 0  = − 36 ± 10 J K −1  mol −1 , k ex 298 = 1.3 × 10 4 s - 1 , Δ H ‡  = 34 ± 4.7 kJ mol −1 , Δ S ‡  = − 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(pp 3 )]Cl with trans -[PtCl 2 (NCC 6 H 5 ) 2 ] in chloroform. From these facts, ligand substitution reactions of [PtX(pp 3 )] + (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(pp 3 )] + 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.