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A combination of mass spectrometry, DFT calculations and 31 P{ 1 H} NMR spectroscopy has been used to define the mode of action of the commercial cobalt extractant, bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX®272, L 1 H) in Co recovery. The nature of the Co II complexes formed in the water-immiscible phase is determined largely by the propensity of phosphinates to form strong interligand H-bonds in the outer coordination sphere and also to form stable μ 2 -Co-O-P-O-Co bridges. At low Co loading levels, the predominant species is the 4:1 complex, [Co(L 1 ·L 1 H) 2 ], in which coordinated neutral phosphinic acid ligands form strong H-bonds to adjacent anionic phosphinates. At higher Co loading, oligomers such as [a]

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The mode of action of iron(III) uptake from sulphate solutions by the commercial extractant CYANEX®272, bis(2,4,4-trimethylpentyl)phosphinic acid, has been studied using conventional solvent extraction methods, together with 31 P{ 1 H} NMR spectroscopy, electrospray-ionisation mass spectrometry and DFT calculations that have not previously been used to analyse the content of the ISOPAR M water-immiscible phase. The maximum Fe-loading recorded was 177% of the theoretical maximum, based on the formation of an Fe(III) complex [Fe(L)3] with a 1:3 molar ratio of Fe to phosphinate (L). Inductively-coupled plasma optical emission spectroscopy (ICP-OES) indicates that sulfate or hydrogen sulfate ions are co-extracted in a 1:2 molar ratio with iron across the whole of the Fe-loading range, which helps to account for the loadings being greater than 100% of the theoretical value. 31 P{ 1 H} NMR spectroscopy indicates that the limiting factor in Fe-loading is the availability of uncomplexed CYANEX®272 (sulfate is present in large excess). In contrast to the behaviour of Co(II) and Zn(II) extraction by CYANEX®272, there is no evidence for the formation of polymeric Fe(III) complexes and highly viscous solutions at high metal loadings. The identity of the extracted species is likely to be a polynuclear Fe(III) sulphate complex.