A NEUTRON-DIFFRACTION STUDY ON HYDROGEN-BONDING IN THE MINERAL MELLITE (AL2[C6(COO)6].16H2O) AT 15-K

Lattice constants for tetragonal Al2[C6(COO)6] · 16 H2O at 15 K are a = 1555.3(5) and c = 2311.0(8) pm, Z = 8, space group I41 acd. Anisotropic refinement on single-crystal neutron diffraction data led to Rw = 0.039. The crystal structure is made up by barely distorted Al(H2O)3+6 octahedra linked by strong asymmetric hydrogen bonds to the [C6(COO)6]6- anions yielding a complicated three-dimensional framework. Additional water molecules not bound to Al3+ participate in hydrogen bonding as well. Hydrogen bonds involving water molecules bound to Al3+ as proton donors are markedly shorter (O ··· H2O 257.6-266.4 pm) than those with noncoordinated water molecules (271.0 and 272.4 pm). OH bond lengths corrected for thermal motion range from 99.5 up to 101.3 pm. The hexaanion has 222 symmetry with CO bonds being equal (125.5-125.7 pm) within the limits of experimental error. The OCO angles are widened up considerably (124.96 and 126.30°). The C6 ring of the anion deviates slightly, but significantly from an ideal hexagon. © 1991.