Equilibria in complexes of N-heterocycles. Part 50.¹ the formation and properties in solution of the 1:1 adducts of bis(2,2′-bipyridine)platinum(II) and related ions with hydroxide

Equilibrium constants are given for addition (mol ratio 1:1) of hydroxide to several palladium(II) and platinum(II) cations. [Pt(bpy)₂]²⁺ (bpy: 2,2′-bipyridine) and hydroxide ion form the well-known 1:1 adduct rapidly in water; the rate is given at 25°C by k = 1.6 × 10² mol^-1 dm³ s^-1. The equilibrium constants for adding hydroxide to [Pt(bpy)₂]²⁺ and to [Pt(3,3′-[²H]₂-bpy)₂]²⁺ are the same, but differ considerably from those for [Pt([²H]₈-bpy)₂]²⁺ and for [Pt(6,6′-[²H]₂-bpy)₂]²⁺ with hydroxide. In this reaction, the infrared spectrum (1700-900 cm^-1) of the cation initially [Pt(bpy)₂]²⁺ in the aqueous solution shows distinct changes between pH = 7.0 and pH = 11.0, notably in the aromatic region, where the intensity of the band due to C=N at ca. 1600 cm^-1 is much reduced. At pH = 7.0, the methyl groups in [Pt(5,5′-dmbpy)₂]²⁺ (dmbpy: dimethyl-2,2′-bipyridine) or the 4,4′-isomer are equivalent in ¹H magnetic resonance but become inequivalent in the 1:1 adducts with hydroxide. In the presence of (-)-methylbenzylamine, both [Pt(terpy)Cl]Cl (terpy: 2,2′,2″-terpyridine) and [Pt(bpy)₂]²⁺ manifest a strong Pfeiffer effect, but not the 'exciton' coupling characteristic of cis-octahedral ions. These observations are interpreted in terms of addition of hydroxide to the bipyridine ligands.