Neptunyl(V) oxalate, or neptunium(V) oxalate, is a compound consisting of neptunyl and oxalate ions with formula . It is known to form several hydrates with compositions , where x is between 4 and 6. It also forms several coordination complexes, such as or , 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.
A simple method for the preparation of neptunyl(V) oxalate is the reaction of neptunyl(V) nitrate () with either sodium oxalate (), potassium oxalate (), or ammonium oxalate (). When they react in stoichiometric quantities, neptunyl(V) oxalate hydrates are precipitated:
It is also formed when neptunyl(V) ions () interact with oxalopolyÃÂperoxoÃÂuranylate nanoclusters at high neptunyl concentrations, and is probably formed when neptunium(IV) oxalate decomposes in air.
Neptunyl(V) oxalate forms several hydrates with composition , where x=4, 5, or 6. The compounds and have been studied for their magnetic properties. was determined to have a magnetic moment of 2.560 per neptunium atom as part of a broader study of neptunyl-containing compounds. has been found to undergo a metamagnetic transition at 11.6 K.
The structure of the hexahydrate, , 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 's low solubility of 0.62 g/L.
At 180 ðC, the hexahydrate, , loses a water molecule:
Decomposition of begins at 260 ðC. When formed as an intermediate in the decomposition of neptunium(IV) oxalate, it eventually decomposes to neptunium(V) oxide, which decomposes further into neptunium(IV) oxide between 693âÂÂ970 K.
Neptunyl(V) oxalate is known to form a complex with imidazole, (Im=imidazole), which was one of the first known examples of a compound where imidazole directly binds with an actinide atom. The 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 (: ratio = 1:1) complexes are known, with general formula (M=Na, K, Cs, , x=1âÂÂ3). The compounds and 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, , 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 .
Neptunyl(V) oxalate compounds with : ratios greater than 1:1 are also known, such as (x=2, 6) and (x=1.5, 3, 4, 5). The compounds and have a : of 2:3, and can be called sesquioxalates. 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, adopts a sheet structure. Like , 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 compounds can be called dioxalate complexes. In aqueous solution, dioxalate complexes can exist as . The oxalate groups in this ion can undergo rapid ligand exchange with other oxalate groups:
Where superscripts denote individual oxalate ions. This ion can be isolated from solution in the form of these compounds. The structures of both and contain dimers of composition , hexaamminecobalt(III) () ions, and waters of crystallization. Again, neptunyl ions are pentagonal bipyramidal, with five oxygen atoms coming from three oxalates, and the neptunyl ions forming dimers are linked by oxalate oxygen atoms. Each of the four oxalate groups bonds with one neptunyl group to form a 5-membered ring, but two of them additionally form a bond to the other neptunyl group. While they both are made out of the same components, they have a different packing. On the contrary, adopts a chained structure instead. While it again has neptunyl in a pentagonal bipyramidal fashion, with five oxygen atoms from three oxalate groups, two of these oxalate groups bridge to neighboring neptunyl groups, one oxalate being bidentate and one monodentate, and the third oxalate is bidentate, attaching to the side of the chain. The structure of the pentahydrate, , is unknown, though it is possible that some water molecules bond directly to neptunium atoms. It is believed that neptunium atoms have a coordination geometry of hexagonal bipyramidal. Isostructural compounds containing plutonium instead of neptunium (plutonyl(V) oxalates) are also known.
A neptunium(V) oxalate compound containing hydroxide ions, , is also known. It has been formed by the hydrothermal reaction between a neptunium(V) stock solution, oxalic acid, calcium chloride, sodium hydroxide, and hydrogen chloride. Its structure consists of chains of composition separated by sodium ions and water molecules. The chains contain neptunyl dimers which are held together by bridging hydroxide groups. These dimers are bonded with neighboring dimers via oxalate groups, which bond monodentate to one neptunyl group and bidentate to another.