Coordination Trends in Alkali Metal Crown Ether Uranyl Halide Complexes:  The Series [A(Crown)]2[UO2X4] Where A = Li, Na, K and X = Cl, Br

UO2(C2H3O2)2·2H2O reacts with AX or A(C2H3O2 or ClO4) (where A = Li, Na, K; X = Cl, Br) and crown ethers in HCl or HBr aqueous solutions to give the sandwich-type compounds [K(18-crown-6)]2[UO2Cl4] (1), [K(18-crown-6)]2[UO2Br4] (2), [Na(15-crown-5)]2[UO2Cl4] (3), [Na(15-crown-5)]2[UO2Br4] (4), [Li(12-crown-4)]2[UO2Cl4] (5), and [Li(12-crown-4)]2[UO2Br4] (6). The compounds have been characterized by single-crystal X-ray diffraction, powder diffraction, elemental analysis, IR, and Raman spectroscopy. The [UO2X4]2- ions coordinate to two [A(crown)]+ cations through the four halides only (2), through two halides only (3), through the two uranyl oxygens and two halides (3, 4), or through the two uranyl oxygen atoms only (5, 6). Raman spectra reveal νU - O values that correlate with expected trends. The structural trends are discussed within the context of classical principles of hard−soft acid−base theory.