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 Ni^II with (2-imidazoleazo)benzene (IAB), 2,2'-biimidazole (Biim) and 2,2'-bibenzimidazole (Bibzm) have been investigated at 15-35 °C, I = 0.30 mol dm⁻³. The stability constants, K_M, 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 (k_r) 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, Biim or Bibzm) is sterically strained. The formation of [Ni(IAB)]²⁺ and [Ni(Bibzm)]²⁺ may be chelation controlled while the normal I_d mechanism is supported by our data for [Ni(Biim)]²⁺.