Reversible formation of binuclear complexes between pyridine-2-carboxylato(tetraethylenepentamine)cobalt(III) and nickel(II)/cobalt(II) in aqueous solution. Anomalous behaviour in the dissociation of binuclear complexes

The kinetics of the reversible formation of binuclear species between M(OH2)62+ (M = NiII and CoII) and oxygen-bonded (tetren)Co(pycH)3+ (tetren = tetraethylenepentamine, pycH = N-protonated pyridine-2-carboxylate) have been investigated by stopped-flow spectrophotometry at 25°C, I = 0.3 mol dm-3. Both the protonated (pyridine-N) and deprotonated forms of the CoIII complex were involved in the formation of the binuclear complex (tetren)Co(pyc)Co4+, whereas only the deprotonated form of the complex was involved in the formation of (tetren)Co(pyc)Ni4+. The rate date forthe formation of the binuclear complex are consistent with an Id mechanism. Interestingly, the dissociation of the binuclear CoII complex (tetren)Co(pyc)Co4+ was acid-catalysed while that with NiII was acid-independent. A suitable explanation for this anomalous behaviour has been discussed.