Axial-ligand substitution reactions of a head-to-head pivalamidato-bridged Pt(III) binuclear complex bearing equatorial bromide ligands: A mechanistic study

Abstract Axial ligand-substitution reactions of head-to-head (HH) a binuclear pivalamidato-bridged platinum(III) complex bearing equatorial bromide ligands, [(H 2 O)(NH 3 ) 2 Pt(μ-pivalamidato) 2 Pt(Br) 2 (OH 2 )] 2+ ( 1 ), with chloride and bromide ions were thermodynamically and kinetically investigated in acidic aqueous solutions. Reactions of 1 with p -styrenesulfonate and 4-penten-1-ol were also kinetically investigated in order to clarify the reaction mechanisms involving olefins. In contrast to the reactions of the HH tetraammine pivalamidato-bridged platinum(III) binuclear complex, [(H 2 O)(NH 3 ) 2 Pt(μ-pivalamidato) 2 Pt(NH 3 ) 2 (OH 2 )] 4+ ( 2′ ), all substitution processes on 1 involve single-step reactions. The effect of the equatorial halide ligands on the axial ligand substitution reactions of 1 is discussed in relation to 2′ ; the rate-determining first substitution of H 2 O by X − at the Pt(Br 2 O 2 ) site in 1 is followed by a fast second substitution of H 2 O by X − at Pt(N 4 ). In contrast, axial ligand substitution on 2′ by X − proceeds in two consecutive steps. Reactions of 1 with p -styrenesulfonate or 4-penten-1-ol proceed in one step to form only mono-π complexes that are in rapid equilibrium with σ complexes, whereas reactions of 2′ with these olefins proceed in three consecutive steps via di-π complexes. These mechanistic differences are interpreted in terms of enhanced charge localization in the mono-π complexes of 1 ([Pt II (NH 3 ) 2 (μ-pivalamidato) 2 Pt IV (Br) 2 ( p -styrenesulfonate or 4-penten-1-ol)] 2+ ).