Kinetics and mechanism of complex formation between O-bonded(glycinato)pentaamminecobalt(III) and (glycinato)tetraethylenepentaminecobalt(III) ions with nickel(II) in aqueous medium

Abstract

The kinetics of the reversible complexation of Ni(OH₂) <img src='http://www.niscair.res.in/jinfo/equation27.gif' border=0> with oxygen bonded glycinatocobalt(III) substrates, N₅Co(glyH)³⁺ (N₅ = 5 NH₃ or tetraethylenepentarnine; glyH = H₂NCH₂COOH) have been investigated by stopped flow technique at 25°C, <em>1 </em>= 0.3 mol dm^-³. The values of formation and dissociation rate constants <em>(k</em>_f<em>, k</em>_r) for the binuclear species, [N₅CoO₂CCH₂NH₂Ni] are 270 ± 37, 271 ± 41 dm³ mol^-1 s^-¹, and 0.140 ± 0.015, 0.100 ± 0.007 s^-1 at 25°C (<em>I</em>= 0.3 mol dm^-3) for te tetraethylene-pentaminecobalt(III) and pentaamminecobalt(III) substrates respectively. Lack of any dependence of <em>k</em>_f on the nature of the cobalt(III) moiety strongly suggests that the formation of the mono bonded species occurs by the entry of the pendant NH₂ group into the coordination sphere , of Ni(II) via rate limiting Ni - OH₂ bond dissociation mechanism (I_d). The binuclear species exist in dynamic equilibrium between the monodentate and chelated forms with chelate form predominating. The small values of the dissociation rate constants despite the intrinsic electrostatic repulsion between the like charge centres also support the chelate nature of the binuclear species.