Cubes eight-coordinate compounds

Spot tests, 1, 552 Square antiprisms dodecahedra, cubes and, 1, 84 eight-coordinate compounds, 1,83 repulsion energy coefficients, 1, 33, 34 Square planar complexes, 1,191, 204 structure, 1, 37 Square pyramids five-coordinate compounds, 1,39 repulsion energy coefficients. 1,34 Squares... 

In the case of eight-coordination, the cube is fairly rare the compounds Na3[MFg] (M = Pa, U, Np) constitute the best-established examples. As for the trigonal prism versus the octahedron, interligand repulsion can be reduced by distortion of the cube to yield the square antiprism or dodecahedron (this is best demonstrated by playing with models). These two are about equally common. For example, in the case of Mo(CN)g-both square antiprismatic and dodecahedral coordination are found in the solid state in solution, the ion is stereochemically nonrigid (or fluxional) and appears to convert rapidly from one geometry to the other. 

The eight-fold coordination type with the ligands situated at the corners of a cube maximizes non-bonded repulsions (20, 58, 59,64), and it, therefore, is not an appealing coordination t 7pe. The cube is, indeed, unknown as a structure type among the molecular coordination compounds of the d-transition elements although it is prevalent among inonic compounds with the CaF2 and CsCl lattice types. 

The majority of elemental substances, and a large number of compounds, have metallic properties (see Section 3.3). Metallic elemental substances are characterised by three-dimensional structures with high coordination numbers. For example, Na(s) has a body-centred cubic (bcc) structure in which each atom is surrounded by eight others at the corners of a cube, each at a distance of 371.6pm from the atom at the centre. The Na atom also has six next-nearest neighbours in the form of an octahedron, with Na-Na distances of 429.1pm. A fragment of this lattice is shown in Fig. 7.14. These distances may be compared with the Na-Na bond length of 307.6 pm in the Na2 molecule, which can be studied in the gas phase by vaporisation of sodium metal. 

Sphalerite (or zincblende ) has a lattice with zinc atoms at the corners and face centers of a unit cube and sulfur atoms at the centers of four out of the eight smaller cubes into which the large cube can be divided. Both zinc and sulfur are in regular tetrahedral coordination. The cleavage surface is the (110) surface, and this nonpolar face of ZnS (and other zinc-blende-structure binary compounds) is by far the best understood of all semiconductor surfaces (Kahn, 1983). 

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