The acid-catalysed hydrolysis of nitro- and nitrito-pentammine complexes of cobalt(III), rhodium(III),and iridium(III)

Abstract

The kinetics of the replacement of nitro- and nitrito-ligands from the complexes M(NH₃)₅X²⁺(where X = NO₂ or ONO and M = Co, Rh, or Ir) at various temperatures have been studied. The nitro-complexes are hydrolysed in concentrated acid and the logarithm of the corresponding first-order rate constant is proportional to Hammett's acidity function. The nitrito-complexes are hydrolysed in relatively dilute solutions of strong acids. The logarithm of first-order rate constant is proportional to log [H⁺]² or 2H₀. The Arrhenius parameters and kinetics are interpreted as follows: (a) The nitro-complexes are hydrolysed by a loss of NO⁺ ion from the transition state; water is not involved in the transition state, and (b) the nitrito-complexes react via a mechanism involving mono- and diprotonated forms. The equilibria are very sensitive to ionic strength. It is difficult to ascertain from the kinetics whether or not water is involved in the rate-determining step. The similarity of Arrhenius parameters, irrespective of central metal ion, suggests that oxygen–nitrogen bond breaking occurs rather than metal–oxygen.