Reaction of coordinated ligands: Complex formation between nickel (II) and pyridine- 2-carboxylato-/pyridine- 3-carboxylato-pentaamminecobalt (III) ions and aquation of the cobalt (III) substrates catalysed by nickel (II) and copper (II) - A kinetic and mechanistic study

Abstract

The carboxylatopentaarnminecobalt (III) ions derived from pyridine-2-, and pyridine-3-carboxylates bind, Ni(II) and Cu(II) to form binuclear species. The kinetics of reversible formation of the binuclear species (NH₃)₅CoO₂C - (2/)pyNi⁴⁺ (Py = C₅H₄N) was investigated by stopped-flow technique at 10-35°C, [Ni²⁺]_T: 11 = 0.005-0.05; and <em>1= </em>0.3 mol dm³. The <em>k</em>_obsvalues fitted 10 the relationship <em>k</em>_o_bs= <em>k</em>_f<em>K</em>_eq[Ni^F+]/(1 + <em>K</em>_eq [Ni²⁺] consistent with the reaction Scheme N₅CoO₂CC₅H₄N²⁺ + Ni(OH₂) <img src='http://www.niscair.res.in/jinfo/equation27.gif' border=0> <img src='http://www.niscair.res.in/jinfo/equation110.gif' border=0> N₅CoO₂CC₅H₄N-Ni(OH₂)<img src='http://www.niscair.res.in/jinfo/equation112.gif' border=0> <img src='http://www.niscair.res.in/jinfo/equation113.gif' border=0> N₅CoO₂CC₅H₄NNi(OH₂)<img src='http://www.niscair.res.in/jinfo/equation111.gif' border=0>. This is in contrast to the reversible complexation of Ni(II) with the pyridine-3-analogue under comparable condition for which chelation step is not involved. The resulting binuclear complexes of pyridine-2-carboxylato complex promoted aquation at the cobalt (III) centre (NH₃)₅CoO₂C-(2-)pyM⁽ⁿ⁺²⁾⁺<img src='http://www.niscair.res.in/jinfo/equation114.gif' border=0>(NH₃)₅CoOH<img src='http://www.niscair.res.in/jinfo/equation28.gif' border=0> + py(- 2 - )CO₂Mⁿ⁺ the effect of the metal ions being in the order Cu²⁺ > Ni²⁺. The rate data for dissociation of binuclear species as well as catalytic activities of the metal ions in promoting water for carboxylate substitution at the cobalt (III) centre are consistent with the fact that the metal ions are chelated by pyridine-nitrogen and cobalt (III) bound carboxylate group of pyridine-2-carboxylate.