Synthesis and structure of ditetrabutylammonium tetrachlorodioxoneptunium(VI)

The reaction of a stock solution of NpO₂² + in 2 M HCl with two equivalents of tetrabutylammonium chloride in 6 M HCl results in the preparation of the title compound. This compound, [NBu₄]₂[NpO₂Cl₄], is isostructural with the uranium analogue, [NBu₄]₂[UO₂Cl₄]. The anion of the complex, [NpO₂Cl₄]²–, adopts a pseudooctahedral geometry common to six-coordinate actinyl complexes in which the neptunium metal is coordinated by two trans axial oxo groups with NpO(oxo) bond lengths of 1.733(5)Å, and four equatorial chloride ligands with Np–Cl bond lengths in the range 2.637(2)–2.676(2)Å. The neptunyl ONpO bond angle is 178.6(3)°. Charge balance of the neptunyl tetrachloride anion is maintained by two tetrabutylammonium cations. This complex crystallizes in the monoclinic space group P2₁/n (a = 15.416(4), b = 15.362(4), c = 18.479(5)Å, = 108.147(6)°, V = 4158.4(19)ų, Z = 4).