Steric effects on complex formation between nickel(II) and (2-imidazoleazo)benzene, 2,2′-biimidazole and 2,2′-bibenzimidazole

Abstract

The kinetics and mechanism of reversible complexation of NiII with (2-imidazoleazo)benzene (IAB), 2,2′-biimidazole (Bum) and 2,2′-bibenzimidazole (Bibzm) have been investigated at 15-35°C, I = 0.30 mol dm-3. The stability constants, KM, of the [NiL]2+ species vary in the sequence: [Ni(IAB)]2+ < [Ni(Bibzm)]2+ < [Ni(Biim)]2+. The values of the spontaneous dissociation rate constant (kr) at 25°C decrease in the sequence: [Ni(IAB)]2+ > [Ni(Biim)]2+ > [Ni(Bibzm)]2+. The aquation of [Ni(IAB)]2- is insensitive to acid catalysis, whilst [Ni(Biim)]2+ is relatively more susceptible towards acid-catalysed aquation than [Ni(Bibzm)]2+. The chelate ring in [NiL]2+ (L = IAB, Bum or Bibzm) is sterically strained. The formation of [Ni(IAB)]2+ and [Ni(Bibzm)]2+ may be chelation controlled while the normal Id mechanism is supported by our data for [Ni(Biim)]2+. © 1996 Chapman & Hall.