Editing Neptunium (section)

===Coordination complexes===
There is much interest in the [[coordination chemistry]] of neptunium, because its five oxidation states all exhibit their own distinctive chemical behavior, and the coordination chemistry of the actinides is heavily influenced by the [[actinide contraction]] (the greater-than-expected decrease in [[ionic radii]] across the actinide series, analogous to the [[lanthanide contraction]]).<ref name="Yoshida745">Yoshida et al., pp. 745–750.</ref>

====Solid state====
Few neptunium(III) coordination compounds are known, because Np(III) is readily oxidized by atmospheric oxygen while in aqueous solution. However, [[sodium formaldehyde sulfoxylate]] can reduce Np(IV) to Np(III), stabilizing the lower oxidation state and forming various sparingly soluble Np(III) coordination complexes, such as {{chem|Np|2|(C|2|O|4|)|3}}·11H<sub>2</sub>O, {{chem|Np|2|(C|6|H|5|AsO|3|)|3}}·H<sub>2</sub>O, and {{chem|Np|2|[C|6|H|4|(OH)COO]|3}}.<ref name="Yoshida745" />

Many neptunium(IV) coordination compounds have been reported, the first one being {{chem|(Et|4|N)Np(NCS)|8}}, which is isostructural with the analogous uranium(IV) coordination compound.<ref name="Yoshida745" /> Other Np(IV) coordination compounds are known, some involving other metals such as [[cobalt]] ({{chem|CoNp|2|F|10}}·8H<sub>2</sub>O, formed at 400&nbsp;K) and [[copper]] ({{chem|CuNp|2|F|10}}·6H<sub>2</sub>O, formed at 600&nbsp;K).<ref name="Yoshida745" /> Complex nitrate compounds are also known: the experimenters who produced them in 1986 and 1987 obtained single crystals by slow evaporation of the Np(IV) solution at ambient temperature in concentrated [[nitric acid]] and excess 2,2′-[[pyrimidine]].<ref name="Yoshida745" />

The coordination chemistry of neptunium(V) has been extensively researched due to the presence of [[cation–cation interaction]]s in the solid state, which had been already known for [[actinyl]] ions.<ref name="Yoshida745" /> Some known such compounds include the neptunyl [[dimer (chemistry)|dimer]] {{chem|Na|4|(NpO|4|)|2|C|12|O|12}}·8H<sub>2</sub>O and neptunium [[glycolate]], both of which form green crystals.<ref name="Yoshida745" />

Neptunium(VI) compounds range from the simple oxalate {{chem|NpO|2|C|2|O|4}} (which is unstable, usually becoming Np(IV)) to such complicated compounds as the green {{chem|(NH|4|)|4|NpO|2|(CO|3|)|3}}.<ref name="Yoshida745" /> Extensive study has been performed on compounds of the form {{chem|M|4|AnO|2|(CO|3|)|3}}, where M represents a monovalent cation and An is either uranium, neptunium, or plutonium.<ref name="Yoshida745" />

Since 1967, when neptunium(VII) was discovered, some coordination compounds with neptunium in the +7 oxidation state have been prepared and studied. The first reported such compound was initially characterized as {{chem|Co(|NH|3|)|6|NpO|5}}·''n''H<sub>2</sub>O in 1968, but was suggested in 1973 to actually have the formula {{chem|[Co(|NH|3|)|6|][NpO|4|(OH)|2|]}}·2H<sub>2</sub>O based on the fact that Np(VII) occurs as {{chem|[NpO|4|(OH)|2|]|3-}} in aqueous solution.<ref name="Yoshida745" /> This compound forms dark green prismatic crystals with maximum edge length 0.15–0.4&nbsp;[[millimeter|mm]].<ref name="Yoshida745" />

====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" />