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)(6)(2+) (M = Ni-II and Co-II) and oxygen-bonded (tetren)Co(pycH)(3+) (tetren = tetraethylenepentamine, pycH = N-protonated pyridine-2-carboxylate) have been investigated by stopped-flow spectrophotometry at 25 degrees C, I = 0.3 mol dm(-3). Both the protonated (pyridine-N) and deprotonated forms of the Co-III 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 for the formation of the binuclear complex are consistent with an I-d mechanism. Interestingly, the dissociation of the binuclear Co-II complex (tetren)Co(pyc)Co4+ was acid-catalysed while that with Ni-II was acid-independent. A suitable explanation for this anomalous behaviour has been discussed.