The modes of action of the commercial solvent extractants used in extractive hydrometallurgy are classified according to whether the recovery process involves the transport of metal cations, Mⁿ⁺, metalate anions, MX_x ⁿ⁻, or metal salts, MX_x into a water-immiscible solvent. Well-established principles of coordination chemistry provide an explanation for the remarkable strengths and selectivities shown by most of these extractants. Reagents which achieve high selectivity when transporting metal cations or metal salts into a water-immiscible solvent usually operate in the inner coordination sphere of the metal and provide donor atom types or dispositions which favour the formation of particularly stable neutral complexes that have high solubility in the hydrocarbons commonly used in recovery processes. In the extraction of metalates, the structures of the neutral assemblies formed in the water-immiscible phase are usually not well defined and the cationic reagents can be assumed to operate in the outer coordination spheres. The formation of secondary bonds in the outer sphere using, for example, electrostatic or H-bonding interactions are favoured by the low polarity of the water-immiscible solvents.

Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.

Ditopic ligands have been designed and synthesized for the solvent extraction and membrane transport of transition metal salts for application in metal separation and purification processes. Tetradentate salicylaldimine ligands of the H₂salen type bearing pendant tertiary amine groups undergo a zwitterionic transformation when binding metal salts, forming overall neutral assemblies which can be transferred into non-polar, water-immiscible solvents. A comparison of the solid state structures of copper(II) sulfate and neutral copper-only complexes demonstrates how incorporation of the metal ion into the salen²⁻ unit preorganises the anion binding site to accommodate the sulfate group. The extraction/recovery characteristics of copper and sulfate are reported for a ligand which has been modified to lower the strength of copper-binding and thus facilitate copper-recovery and ligand-recycling. An X-ray structure determination of this 2,2′-biphenylene-bridged ligand indicates a predisposition towards the provision of a non-planar N₂O₂ ²⁻ donor-set. This is exemplified by its cis-octahedral nickel(II) sulfate complex in which the sulfate acts as a bidentate ligand. Competitive metal ion transport experiments show the selectivity of this ligand for copper(II) transport over a range of other metals including nickel(II) and zinc(II). The pH profiles for loading of Cu²⁺ (pH_1/2 2.0, i.e. pH at 50 percent extraction) and of SO₄ ²⁻ (pH_1/2 9.7) into the ligand zwitterion in chloroform have been determined and this has allowed the demonstration of a protocol for loading of CuSO₄, stripping of copper and sulfate separately and recycling of the extractant. Such a protocol provides proof-of-concept for application of these ditopic ligands in metal recovery from sulfide leaching or from acid mine drainage streams.

The pH-dependence of simultaneous metal- and sulfate- loading of simple salen derivatives demonstrates the feasibility of their application as extractants for recovery of base metals from the leaching of sulfidic ores. The efficacy of the ligands depends on the templating of the sulfate binding site by the attendant metal ion.