X-RAY STUDY OF (NH4)7[H2ASW18O60].16H2O - 1ST EXAMPLE OF A HETEROPOLYANION CONTAINING PROTONS AND ARSENIC(III)

The crystal structure of ammonium 18-tungstoarsenate(III), (NH4H2AsW18O60HeH2O, has been determined from three-dimensional X-ray data measured on a computer-controlled four-circle diffractometer at room temperature. The compound crystallizes in the monoclinic space group C2/c; lattice constants are a = 21.937 (5) A, b = 14.239 (5) Å, c = 22.177 (9) Å, (3 = 96.95 (2)°, and Z = 4. The structure has been refined by a full-matrix least-squares method to a final R factor of 0.066 for 6219 observed reflections. No correction could be applied for absorption. The [H2AsW18O60]7-heteropolyanion is built up with two XW9O33 units sharing six oxygen atoms and joined in a C1- fashion. Inside the first XW9O33 unit is As(III) with its lone pair directed toward the second unit. As revealed by NMR, this polyanion contains two protons inside, which cannot be titrated; thus the two XW9O33 units are different: one of them contains arsenic(III) and the other two protons. © 1979, American Chemical Society. All rights reserved.