Potentially bidentate 4-diazopyrazol-5-one ligands carrying a range of substituents can readily be prepared by coupling arene diazonium salts on to the appropriate pyrazolones. Hydrocarbon-soluble versions are shown to be suitable reagents for the recovery of copper by solvent extraction from ammoniacal leach solutions, and potentially have greater resistance to chemical degradation than the β-diketones which have been used. Cobalt(III), nickel(II), copper(II) and zinc(II) complexes with 4-(4-tert-butylphenylazo)-3-methyl-1-phenyl-5-pyrazolone (HL) have been characterised by X-ray crystallography. The significant deviations from planar coordination geometry which are observed in [CuL₂] and [CuL′₂], where HL′ is 3-methyl-1-phenyl-4-phenylazo-5-pyrazolone, arise from interligand repulsion, and account for the relatively weak complexation and ease of stripping of copper from this class of extractant. The cobalt(III) complex [CoL₃]·3MeOH and the high spin nickel(II) complex [NiL₂(MeOH)₂]·2MeOH both have approximately octahedral geometries, but show significantly different bite distances in the chelate rings. The zinc complex [ZnL₂] has a pseudo-tetrahedral structure.