Formation of a synergist adduct:

Based on the given discussion, a few generalization about the formation of a synergist adduct can be drawn.

i) One of the extractants, a ligand or an acid (HA), neutralizes the charge on the metal ion preferably by forming a chelate. The list of ligands is very large. The acids can be organophosphorus or carboxylic acids.

ii) The solvating reagent (S) should displace residual coordinated water from the neutral metal complex rendering it less hydrophilic. The solvent should not be itself hydrophilic and coordinated less strongly than HX. The commonly used solvating molecules are organophosphorus compounds and nitrogen containing bases.

iii) A maximum coordination number of the metal and the geometry of the ligand should be favourable.

iv) A little more complexity in the formation of a synergistic adduct is being presented by taking the extraction equilibria of Zn (II) and Cd (II) in the chloroform solution of propionic, butyric and valeric acid within the presence of amines. The extraction of both the metal ions independently in carboxylic acid and amine is very low (~ 10 - 20%). It is observed that the addition of amine to the solution of the acid enhances the extraction of metal ion to an unpredictable extent. This may probably be ascribed to the formation of less hydrophilic species due to the removal of the water molecules from the coordination sphere of the metal ion. That has been individually established that amines form an integral part of the extractable species of the metal ion. Because of low dielectric constant of chloroform it is expected that the bond between the proton and the acid anion is much stronger in chloroform than in water; consequently, neither could one rule out the hypothesis of the absence of any reaction between the acid proton and the amine. Through nmr spectroscopy, it has been confirmed that there is a creation of a salt of the acid with amine. It has also been experimentally confirmed and known that the acid in the organic phase mostly exists as a dimer.