Square antiprism

Chlorates and bromates feature the expected pyramidal ions X03 with angles close to the tetrahedral (106-107°). With iodates the interatomic angles at iodine are rather less (97-105°) and there are three short I-O distances (177-190 pm) and three somewhat longer distances (251-300 pm) leading to distorted perovskite structures (p. 963) with pseudo-sixfold coordination of iodine and piezoelectric properties (p. 58). In Sr(I03)2.H20 the coordination number of iodine rises to 7 and this increases still further to 8 (square antiprism) in Ce(I03)4 and Zr(I03)4. 

The most symmetrical structure possible is the cube Oh but, except in extended ionic lattices such as those of CsCl and CaF2, it appears that inter-ligand repulsions are nearly always (but see p. 1275) reduced by distorting the cube, the two most important resultant structures being the square antiprism D4h and the dodecahedron Did (Fig. 19.10). 

Figure 19.10 (a) Conversion of cube to square antiprism by rotation of one face through 45° (b) Conversion of cube into dodecahedron. 

Several carboxylates, both simple salts and complex anions, have been prepared often as a means of precipitating the An ion from solution or, as in the case of simple oxalates, in order to prepare the dioxides by thermal decomposition. In K4[Th(C204)4].4Fl20 the anion is known to have a 10-coordinate, bicapped square antipris-matic structure (Fig. 31.8b). -diketonates are precipitated from aqueous solutions of An and the ligand by addition of alkali, and nearly all are sublimable under vacuum. [An(acac)4], (An = Th, U, Np, Pu) are apparently dimorphic but both structures are based on an 8-coordinate, distorted square antiprism. 

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... 

The functions I accordingly correspond to an oblate antiprism and II to a prolate antiprism. There is a simple explanation for the difference in orientation of the principal axes. The theorem that the sum of the squares of the values of the functions for a complete set (a subshell) is constant requires that the shape parameters vary in a satisfactory way with change in orientation of the principal axes. For the prolate set (II) the maximum value in the plane orthogonal to the principal axis of the function lies in the basal plane of rhe antiprism, and thus serves to increase the electron... 

Possible stereochemistries for eight-coordinate [Ln(H20)8]3+ are the dodecahedron, the square antiprism, and the cube, although the last seems less likely (295). The proposed nine-coordinate [Ln(H20)9]3+ tri-capped trigonal prism transition state is similar to the [Ln(H20)9]3+ structures observed in the solid state. Thus, the possible water exchange... 

When three different kinds of spherical ions are present, their relative sizes are also an important factor that controls the stability of a structure. The PbFCl type is an example having anions packed with different densities according to their sizes. As shown in Fig. 7.5, the Cl- ions form a layer with a square pattern. On top of that there is a layer of F ions, also with a square pattern, but rotated through 45°. The F ions are situated above the edges of the squares of the Cl- layer (dotted line in Fig. 7.5). With this arrangement the F -F distances are smaller by a factor of 0.707 (= /2) than the CP-CP distances this matches the ionic radius ratio of rF-/rcl- = 0.73. An F layer contains twice as many ions as a CP layer. Every Pb2+ ion is located in an antiprism having as vertices four F and four... 

Cl- ions that form two square faces of different sizes. Pb2+ ions are located under one half of the squares of the F- ions an equal number of Pb2+ ions are situated above the other half of the squares, which in turn form the base faces of further antiprisms that are completed by another layer of Cl- ions. In this way, the total number of Pb2+ ions is the same as the number of F- ions the number of Cl- ions also is the same because there are two Cl layers for every F- layer. Together, these layers form a slab that is limited by Cl ions on either side. In the crystal these slabs are stacked with staggered adjacent Cl-layers. As a consequence, the coordination sphere of each Pb2+ ion is completed by a fifth Cl- ion (dashed in Fig. 7.5). 

This bismuth-III structure is also observed for antimony from 10 to 28 GPa and for bismuth from 2.8 to 8 GPa. At even higher pressures antimony and bismuth adopt the body-centered cubic packing of spheres which is typical for metals. Bi-III has a peculiar incommensurate composite crystal structure. It can be described by two intergrown partial structures that are not compatible metrically with one another (Fig. 11.11). The partial structure 1 consists of square antiprisms which share faces along c and which are connected by tetrahedral building blocks. The partial structure 2 forms linear chains of atoms that run along c in the midst of the square antiprisms. In addition, to compensate for the... 

What is the composition of a column of square antiprisms joined by common square faces ... 

Pr )2(PPh3)7](PFg)2-3249 and [Au, i(PPlioIV1 e)i(l]3, 3250 The structure of the mixed-ligands clusters corresponds to a C3v metal skeletal framework and the homoleptic to a centered bicapped square antiprism (Figure 32). Other stoichiometries are known as, for example, [AunCl2(PPh3)8]Cl.3251... 

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