Other Reactions of Organic Ligands

A large number of M and M + cations catalyse the loss of carbon dioxide from oxaloacetic acid to give pyruvic acid  

The catalysts include Be +, Co +, Ni +, Cu +, Zn +, Al +, Y +, La +, Gd +, and Lu +. The reaction mechanism proposed is the fast formation of a 1 1 complex followed by rate-determining loss of carbon dioxide from this complex. The cations thus affect the rate of loss of carbon dioxide in two ways, through the value of the pre-equilibrium complex-formation constant and through the consequences of their electron-withdrawing effect on the actual rate of carbon dioxide loss from the complex. The activation enthalpy is actually higher for the cation-catalysed reaction than for that of free oxaloacetic acid - the observed greater rates in the presence of the cations arise from a large, favourable TA5 difference. There is a LH vs. AS correlation for all these cation-catalysed reactions.  

Pyrazole (7) undergoes rather more rapid iodination when it is complexed with nickel(n). The pH-dependence of iodination rates also differs for free pyrazole and its nickel complex. This difference arises from the additional pH-dependent equilibrium between aquo- and hydroxo-ligands on the nickel in the complex. The substituent effect of the —CH2—Co(dmgH)2Py moiety in 

In basic solutions, the following intramolecular ligand condensation between ethylenediamine and co-ordinated nitrile occurs (X = ClorBr)  

An A -ray structural determination of the product suggests that condensation has taken place at the nitrogen trans to the chloride in the starting compound. The first step in the mechanism is deprotonation, shown to be fast in aneillary H n.m.r. experiments. Then follows nucleophilic attack at the nitrile carbon atom by the co-ordinated amine. The rate law is 

---