This thesis extends earlier work at the University of Edinburgh aimed at opening up new flowsheets for the hydrometallurgical recovery of base metals using tetradentate ligands which are capable of transporting metal salts {both a metal cation and its attendant anion(s)}. Prototypes were based on salicylaldimine derivatives of diamines, “salen” ligands in which coordination of a metal dication releases the two phenolic protons which are captured by pendant secondary amine groups generating a preorganised dicationic binding site for the anion(s). The thesis initially deals with the design, synthesis and evaluation of ligands to improve the strength, selectivity and speed of binding of nickel(II) sulfate by incorporating two additional donors in the salen unit to generate N₂X₂O₂^2- binding sites for the nickel yielding pseudo octahedral complexes. The ligands N,N or O,O or S,S o-aminophenyl-substituted 1,2-diaminoethane, 1,2-dioxaethane, 1,3-dioxapropane or 1,2-diethioethane were also used in a screening study with some other divalent metal salts (calcium, cobalt, copper, magnesium, manganese and zinc) commonly found in the feed solutions in commercial processes. Chapter two deals with the nickel(II) coordination chemistry of a series of sexadentate (N₂X₂O₂^2-) ligands. Nickel-ligand-anion complexes have been synthesised for sulfate, nitrate and chloride salts and neutral nickel-ligand complexes have been made. Crystal structures of complexes all contain the same isomer which has a planar mer arrangement of the salicylaldimato XNO^- units. A “nickel only” complex for an X₂N₂O₂^2- ligand with pendent piperidine groups shows that these could provide a cavity to encapsulate a single sulfate anion. All the ligands were found to be very weak extractants and showed slow complexation kinetics and phase transfer of nickel sulfate. The synthesis and characterisation of a series of tridentate ligands related to the sexadentate ligands, with NXO^- binding sites, are reported in chapter three. In theory these could form complexes with a ligand: nickel ratio of 2:1, with a more nearly “ideal” octahedral donor set. Solid state structures of the ligands show them to be pre-organised with an approximately 90° X˙˙˙N˙˙˙O angle. Nickel complexes have been synthesised for sulfate, chloride and acetate salts. Analysis indicates that complexes with ligand: nickel: dianion ratios of 2:1:1 were formed. The tridentate ligands were found to be very weak extractants for nickel sulfate. Chapter four describes the screening of the potentially sexadentate N₂X₂O₂^2-, tridentate NXO^- and the tetradentate “salen” ligands N₂O₂^2- in which the complexation and phase transfer of calcium(II), cobalt(II), copper(II), magnesium(II), manganese(II), nickel(II) and zinc(II) sulfates and chlorides were studied.