Hexagonal bipyramid structure

The above shows the tendency for uranyl-containing esters to exist as hexagonal bipyramidal structures containing two inner core water moleciiles per uranium atom. 

For example, both Hiickel aromatics B and C conform to the Wade-Mingos electron counting rules [2] (see Chapter 1.1.2) and to the structural systematics developed for boranes and heteroboranes [1] the hexagonal bipyramid with the apices removed is in agreement with an arachno electron count of 18 SE for (CH)g [2],... 

Among the known structures, the hexagonal bipyramidal (Has) geometry seems to be less common than others and it has only been definitely established for a few uranyl complexes (94—97). Probably an axial colHnear grouping, as in 0—U—0, is necessary for obtaining a geometry for the complexes. None of the octa-coordinated lanthanide complexes has a Hea geometry. 

The structure of the 2,6-dihydroxybenzoate, [U02 (HO)2C6H3C02 2(H2O)2]-8H2O consists of an irregular hexagonal bipyramidal arrangement with two bidentate carboxylate groups and two H20 molecules in trans positions in the equatorial plane.2780... 

The unique properties of the stable d5 Mn2+ ion is reflected by the fact that MnS and MnSe crystallize in three modifications, in the rocksalt, the cubic sphalerite and the hexagonal wurtzite structure. While in the NiAs structure of MnTe the cations occupy the octahedral holes of a hexagonal close-packing of anions they occupy half of the tetrahedral holes of this packing in the ZnO type modification of MnS and MnSe. The non-metallic character is evident already from the fact that the structure is undistorted (c/a = 1.61 for MnS and 1.63 for MnSe) and that the cations really are at the centres of one set of tetrahedral holes and not at the centre of the bipyramidal holes composed of two tetrahedra of the two different sets. 

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