Editing Neptunium (section)

====In aqueous solution====
Most neptunium [[coordination complex]]es known in solution involve the element in the +4, +5, and +6 oxidation states: only a few studies have been done on neptunium(III) and (VII) coordination complexes.<ref name="Yoshida77182">Yoshida et al., pp. 771–82.</ref> For the former, NpX<sup>2+</sup> and {{chem|NpX|2|+}} (X = [[chlorine|Cl]], [[bromine|Br]]) were obtained in 1966 in concentrated [[lithium chloride|LiCl]] and [[lithium bromide|LiBr]] solutions, respectively: for the latter, 1970 experiments discovered that the {{chem|NpO|2|3+}} ion could form [[sulfate]] complexes in acidic solutions, such as {{chem|NpO|2|SO|4|+}} and {{chem|NpO|2|(SO|4|)|2|-}}; these were found to have higher [[equilibrium constant|stability constants]] than the neptunyl ion ({{chem|NpO|2|2+}}).<ref name="Yoshida77182" /> A great many complexes for the other neptunium oxidation states are known: the inorganic ligands involved are the [[halide]]s, [[iodate]], [[azide]], [[nitride]], [[nitrate]], [[thiocyanate]], [[sulfate]], [[carbonate]], [[Chromate ion|chromate]], and [[phosphate]]. Many organic ligands are known to be able to be used in neptunium coordination complexes: they include [[acetate]], [[propionate]], [[glycolate]], [[lactic acid|lactate]], [[oxalate]], [[malonate]], [[phthalate]], [[mellitate]], and [[citrate]].<ref name="Yoshida77182" />

Analogously to its neighbours, uranium and plutonium, the order of the neptunium ions in terms of complex formation ability is Np<sup>4+</sup> > {{chem|NpO|2|2+}} ≥ Np<sup>3+</sup> > {{chem|NpO|2|+}}. (The relative order of the middle two neptunium ions depends on the [[ligand]]s and solvents used.)<ref name="Yoshida77182" /> The stability sequence for Np(IV), Np(V), and Np(VI) complexes with monovalent inorganic ligands is [[fluoride|F<sup>−</sup>]] > [[dihydrogen phosphate|{{chem|H|2|PO|4|-}}]] > [[thiocyanate|SCN<sup>−</sup>]] > [[nitrate|{{chem|NO|3|-}}]] > [[chloride|Cl<sup>−</sup>]] > [[perchlorate|{{chem|ClO|4|-}}]]; the order for divalent inorganic ligands is [[carbonate|{{chem|CO|3|2-}}]] > [[Monohydrogen phosphate|{{chem|H|PO|4|2-}}]] > [[sulfate|{{chem|SO|4|2-}}]]. These follow the strengths of the corresponding [[acid]]s. The divalent ligands are more strongly complexing than the monovalent ones.<ref name="Yoshida77182" /> {{chem|NpO|2|+}} can also form the complex ions [{{chem|NpO|2|+|M|3+}}] (M = [[aluminium|Al]], [[gallium|Ga]], [[scandium|Sc]], [[indium|In]], [[iron|Fe]], [[chromium|Cr]], [[rhodium|Rh]]) in [[perchloric acid]] solution: the strength of interaction between the two cations follows the order Fe > In > Sc > Ga > Al.<ref name="Yoshida77182" /> The neptunyl and uranyl ions can also form a complex together.<ref name="Yoshida77182" />