Dicaesium hexachloroplutonate

Dicaesium hexachloroplutonate (or DCHP)^[1] is a compound of caesium, plutonium, and chlorine with formula Cs₂PuCl₆. It has been used in the synthesis of other plutonium compounds,^{[3]: 1184, 1190–1191} and in the purification of plutonium metal.^[1]

Dicaesium hexachloroplutonate

Names
Other names Dicesium hexachloroplutonate DCHP[1] Dicaesium plutonium hexachloride Dicesium plutonium hexachloride[2]
Identifiers
CAS Number	24352-13-4
3D model (JSmol)	Interactive image
InChI InChI=1S/6ClH.2Cs.Pu/h6*1H;;;/q;;;;;;2*+1;+4/p-6 Key: HSTAMIWUDFNTGM-UHFFFAOYSA-H
SMILES [Cs+].[Cs+].Cl[Pu-2](Cl)(Cl)(Cl)(Cl)Cl
Properties
Chemical formula	Cs2PuCl6
Molar mass	723 g·mol−1
Density	4.10 g/cm3
Solubility	Soluble in dilute acids[2]
Related compounds
Other anions	Caesium hexachloroplatinate
Other cations	Tetramethylammonium hexachloroplutonate(IV)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Synthesis

DCHP is prepared by first suspending a plutonium sample in water. Then, concentrated hydrochloric acid is added to the mixture. To ensure that all the plutonium is in the +4 oxidation state, an aqueous solution containing sodium chlorite is mixed in with the plutonium solution. Immediately upon adding caesium chloride solution, Cs₂PuCl₆ is precipitated, and is left to sit for 12 hours. The Cs₂PuCl₆ powder is then isolated by vacuum filtration, washed with concentrated HCl, and then left to dry for 8 hours at 120 °C in an oven.^[4]

Properties

Dicaesium hexachloroplutonate forms pale yellow crystals^[3]: 1109 which adopt the K₂GeF₆-type structure, forming trigonal crystals. Each plutonium atom is bonded to six chlorine atoms as discrete hexachloroplutonate anions (PuCl2−6), and each caesium atom is bonded to twelve chlorine atoms. The hexachloroplutonate ion has octahedral molecular geometry, and in DCHP, the Pu-Cl bond length is 2.62 Å. The Cs-Cl bonds have a length of 3.71 Å.^{[3]: 1190 [5]}

DCHP is a stoichiometric compound, and unlike other plutonium compounds, such as plutonium(IV) nitrate, DCHP is non-hygroscopic.^[2]

Reactions

Dicaesium hexachloroplutonate can undergo several reactions, and as such, it is used to prepare other plutonium compounds. For example, upon reaction with cyclopentadienylthallium in acetonitrile solution, it forms the coordination complex (η⁵-C₅H₅)₃PuCl:

Cs₂PuCl₆ + 3 Tl(C₅H₅) → (η⁵-C₅H₅)₃PuCl + 2 CsCl + 3 TlCl

It also reacts with magnesium cyclopentadienide in tetrahydrofuran to give an emerald green product, most likely (η⁵-C₅H₅)₃Pu(THF) (THF=tetrahydrofuran), which loses its tetrahydrofuran ligand upon sublimation to form (η⁵-C₅H₅)₃Pu. Plutonium(III) chloride, PuCl₃, also reacts with magnesium cyclopentadienide, though the reaction occurs much more slowly, likely because PuCl₃ has a polymeric structure while Cs₂PuCl₆ contains discrete PuCl2−6 groups.^{[3]: 1190–1191}

Dicaesium hexachloroplutonate can also be used to synthesize plutonium tetrachloride adducts. Upon reaction with amides or phosphine oxides, it forms the adducts PuCl₄L₂ or PuCl₄L₃ (L=amide or phosphine oxide).^[3]: 1184

Dicaesium hexachloroplutonate reacts with ammonia to form the complex PuCl₄·~7–8NH₃. It is an ammine complex, meaning the ammonia groups are bonded to the plutonium atom. The complex decomposes over time to give the solid PuCl₄·~5NH₃, which is stable at room temperature.^[6]

Uses

As a standard

Because of several of its properties, such as its non-hygroscopicity, high equivalent weight, and an absence of non-stoichiometry, it has been proposed for use as a plutonium primary standard. It has been used as a reference for plutonium in its +4 oxidation state.^[2][4]

In plutonium separation

Before it shut down, DCHP was used at Rocky Flats Plant in the separation of americium from plutonium metal. It was used as an oxidant to oxidize plutonium metal to plutonium(III) chloride in a molten calcium chloride matrix. Any americium would then react with plutonium(III) chloride and be extracted into the calcium chloride matrix.^[1]