Neptunyl(V) oxalate, or neptunium(V) oxalate, is a compound consisting of neptunyl and oxalate ions with formula (NpO2)2C2O4. It is known to form several hydrates with compositions (NpO2)2C2O4·xH2O, where x is between 4 and 6. It also forms several coordination complexes, such as Na2NpO2(C2O4)OH·H2O or Na4(NpO2)2(C2O4)3·6H2O, which are sometimes referred to as neptunium(V) oxalates as well. Neptunyl(V) oxalate and its complexes have been investigated in the study of cation-cation interactions, a type of bonding that frequently occurs between neptunyl cations.
Synthesis A simple method for the preparation of neptunyl(V) oxalate is the reaction of neptunyl(V) nitrate (NpO2NO3) with either sodium oxalate (Na2C2O4), potassium oxalate (K2C2O4), or ammonium oxalate ((NH4)2C2O4). When they react in stoichiometric quantities, neptunyl(V) oxalate hydrates are precipitated:
6 H2O + 2 NpO2NO3 + M2C2O4 → (NpO2)2C2O4·6H2O + 2 MNO3 (M=Na, K, NH4) It is also formed when neptunyl(V) ions (NpO+2) interact with oxalopolyperoxouranylate nanoclusters at high neptunyl concentrations, and when neptunium(IV) oxalate decomposes in air.
Properties Neptunyl(V) oxalate forms several hydrates with composition (NpO2)2C2O4·xH2O, where x=4, 5, or 6. The compounds (NpO2)2C2O4·4H2O and (NpO2)2C2O4·5H2O have been studied for their magnetic properties. (NpO2)2C2O4·5H2O was determined to have a magnetic moment of 2.560 μB per neptunium atom as part of a broader study of neptunyl-containing compounds. (NpO2)2C2O4·4H2O has been found to undergo a metamagnetic transition at 11.6 K.
Structure The structure of the hexahydrate, (NpO2)2C2O4·6H2O, has been characterized as part of the study of cation-cation interactions. In it, neptunyl ions are linked through these bonds to form zigzag chains, and are further linked into layers by bridging oxalate groups. Each neptunium atom in the neptunyl ions has a coordination geometry of pentagonal bipyramidal, and is bonded to two water molecules, two oxygen atoms from an oxalate group, and one oxygen atom from a neighboring neptunyl group. The oxalate groups are bidentate (bonding in two spots) to each bonding neptunyl atom, and bond with it side-on. Four water molecules lie between these layers per formula unit, and link these layers through hydrogen bonding. This chain-like structure leads to (NpO2)2C2O4·6H2O's low solubility of 0.62 g/L.
Complexes Neptunyl(V) oxalate is known to form a complex with imidazole, (NpO2)2C2O4(Im)6·5Im·H2O (Im=imidazole), which was one of the first known examples of a compound where imidazole directly binds with an actinide atom. The (NpO2)2C2O4(Im)6 complex is made up of two neptunyl groups bridged by an oxalate group which bonds with them side-on, as well as six imidazole groups, three bonding to each neptunium atom.
Many monooxalate ([NpO2]+:[C2O4]2− ratio = 1:1) complexes are known, with general formula MNpO2C2O4·xH2O (M=Na, K, Cs, NH4, x=1–3). The compounds CsNpO2C2O4·xH2O and NH4NpO2C2O4·xH2O adopt similar structures, both being made out of cyclic neptunyl trimers. In these trimers, neptunyl ions are linked through cation-cation interactions, with each donating one cation-cation interaction and accepting another. These trimers are linked to each other via oxalate groups, each neptunium atom bonding side-on with two oxalate groups, and each oxalate group bonding side-on with two neptunyl groups, creating a neptunyl oxalate framework. The sodium member of this series, NaNpO2C2O4·3H2O, adopts a different structure. Neptunium atoms have a coordination geometry of pentagonal bipyramidal, being coordinated to one water molecule and two bidentate (bonding in two spots) oxalate groups. Oxalate groups connect neptunyl groups together to form one-dimensional infinite chains. These monooxalate compounds often decompose upon prolonged storage due to conversion to (NpO2)2C2O4·6H2O.
Neptunyl(V) oxalate compounds with [NpO2]+:[C2O4]2− ratios greater than 1:1 are also known, such as Na4(NpO2)2(C2O4)3·xH2O (x=2, 6) and Co(NH3)6NpO2(C2O4)2·xH2O (x=1.5, 3, 4, 5). The compounds Na4(NpO2)2(C2O4)3·2H2O and Na4(NpO2)2(C2O4)3·6H2O have a [NpO2]+:[C2O4]2− of 2:3, and can be called sesquioxalates. Na4(NpO2)2(C2O4)3·2H2O has a chain structure, made up of dimers (groups of two) of neptunyl ions which are joined together through bridging oxalate ions. Neptunium has a coordination geometry of pentagonal bipyramidal; the equator is made up of five oxygen atoms from three oxalate groups. The neptunyl groups in dimers are connected by oxygen atoms coming from oxalate. On the contrary, Na4(NpO2)2(C2O4)3·6H2O adopts a sheet structure. Like Na4(NpO2)2(C2O4)3·2H2O, neptunyl groups are pentagonal bipyramidal with the equator made up of five oxygen atoms from three oxalate groups. Each oxalate group bridges the neptunyl group to a different neptunyl group, connecting the neptunyl ions together; two oxalate groups are bidentate while the third one is monodentate (bonding in one spot).
The Co(NH3)6NpO2(C2O4)2·xH2O compounds can be called dioxalate complexes. In aqueous solution, dioxalate complexes can exist as NpO2(C2O4)2(H2O)3−. The oxalate groups in this ion can undergo rapid ligand exchange with other oxalate groups:
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