Identification of the Limiting Species in the Plutonium(IV) Carbonate System. Solid State and Solution Molecular Structure of the [Pu(CO₃)₅]^6- Ion

The identity of the limiting Pu(IV) species under high carbonate concentrations has been determined to be Pu(CO3)56-. Single crystals of [Na6Pu(CO3)5]2·Na2CO3·33H2O were obtained from a 0.15 M solution of Pu(IV) in 2.6M Na2CO3. The asymmetric unit contains a complex network consisting of [Pu(CO3)5]6- anions and Na+ cations which are linked through interactions with CO32- and H2O ligands. Each Na+ ion is coordinated to the O atoms of H2O or CO32- ligands such that the Na+ ions achieve either octahedral or square-based pyramidal coordination. The [Pu(CO3)5]6- anion can be viewed as a pseudohexagonal bipyramid with three CO32- ligands in a equatorial plane and two in axial positions. EXAFS data for Pu(IV) in 2.5M Na2CO3 solution were collected at the Pu LIII edge. Curve fitting reveals three shells in the FT spectrum. The first shell contains ten O atoms with Pu−O = 2.42(2) Å; the second shell was fit with five C atoms with Pu- - -C = 2.89(2) Å; and the third shell was fit with five O atoms with Pu- - -O = 4.17(2) Å. These distances and coordination numbers correspond well with the average Pu−O, nonbonding Pu- - -C, and distal Pu- - -O distances of 2.415(7), 2.874(9), and 4.117(10) Å, respectively, found in the [Pu(CO3)5]6- ion in the solid-state structure of [Na6Pu(CO3)5]2·Na2CO3·33H2O. The UV−vis diffuse reflectance spectrum of a single crystal of [Na6Pu(CO3)5]2·Na2CO3·33H2O ground into a powder is nearly identical to the solution absorption spectrum of the limiting Pu(IV) carbonato complex in 2.5 M Na2CO3 solution. The peak by peak correspondence by position and relative absorption strengths, along with solution EXAFS data, suggest that the Pu(IV) chromophore is the same in both cases, and confirms that the [Pu(CO3)5]6- ion is the limiting species in high carbonate solutions. Crystal data for [Na6Pu(CO3)5]2·Na2CO3·33H2O: monoclinic space group P21/c (No. 14), a = 9.860(2) Å, b = 17.427(2) Å, c = 18.210(1) Å, β = 101.42(1)°, Z = 2, R1 = 0.0337, wR2 = 0.0775.