Antiprismatic geometry

Both calcium oxalate monohydrate and dihydrate - whewellite and weddelite - contain eight-coordinated Ca2+ in distorted square-antiprismatic geometry, but there is one coordinated water in the former, two in the latter. Whewellite has a layer structure through the sharing of three edges of each coordination square-antiprism with three adjacent polyhedra weddelite has a chain structure (128). The trihydrate also contains eight-coordinated Ca2+ (129). 

The cluster Sng 4 has a monocapped square antiprismatic geometry. On the other hand, the cluster SM5 ]2, 68b, has a trigonal bipyramidal geometry, as shown by X-ray crystallography107. 

Capped argon stirring (CAS), 23 264 Capped octahedron, geometry for metal coordination numbers, 7 574, 575t Capped square antiprismatic, geometry for metal coordination numbers, 7 574, 575t... 

The polyhedron around La(III) in La(Py0)8(C104)3 has been described as a square antiprism, highly distorted toward that of the cube 149). Distorted square antiprismatic geometry has also been found in Eu(thd)3(DMF)2 283). In this complex, two of the thd moieties form one square face, and one thd and two DMF molecules form the other square face. The DMF molecules occupy cis positions on the square face. As in the case of Eu(thd)3 DMSO 283), Eu(thd)3(DMF)2 also shows two nonequivalent conformations in the same unit cell. 

The square antiprismatic or distorted square antiprismatic geometries are found in various inorganic and chelated complexes of the lanthanides and antinides. It has been suggested that the tetrafluorides of cerium (98) praseodymium (99),... 

Among the chelated species, octacoordination is often encountered. The tetrakis-acetylacetonate complex of U(IV) (a-form) and Ce(IV) are isostructural. A two-dimensional X-ray analysis showed a slightly distorted square antiprismatic geometry [109—111) for Ce(acac)4 belonging to space group P2i/c [Ctn] with an average Ce—O (acac) bond length of 2.40 A and an <0—Ce—0 of 72°. The a-form of Th(acac)4 is found to be isomorphous (770) with Ce(acac)4 (Th—0=2.41 A)... 

In both cases, the deviation from the idealized square antiprismatic geometry is caused due to the presence of chemically different ligands bonded to the same metal ion. 

The tetrakis-dithiophosphinate complex [PPI14][Pr(S2PMe2)4], whose crystal structure has also been determined, has a distorted tetragonal antiprismatic geometry with Pr—S = 2.888-3.0150.400 The complex ions [M S2P(OEt)2 4]", have been the subject of an NMR paramagnetic shift study (see Section 39.2.9.4). 

KsIMofCN H jO is isomorphous with K3[W(CN)8]-H20, which has square antiprismatic geometry, although a full structure was not determined for the molybdenum analogue as it decomposed under X-ray irradiation,312 The ESR spectrum of K3[Mo(CN)8] in glycerine solution indicates that all the cyanides are equivalent, consistent with a square antiprismatic structure, or with stereochemical non-rigidity on the ESR timescale.313... 

Transition Metals complex is transformed into trigonal prismatic or square antiprismatic geometry. Furthermore, the complexes tend to be labile in solution. Table 14.4 presents a summary of these differences based on properties of typical transition metal complexes. 

The octalluoroxenaies are the most stable xenon compounds known they can be heated to 400 °C without decomposition. The anions have square antiprismatic geometry. They, too, present a problem to VSEPR theory analogous to that of XeF6 since they should also have a stereochemically active lone pair of electrons that should lower the symmetry of the anion. If the steric crowding theory is correct, however, the presence of eight ligand atom/, could force the lone pair into a stereochemically inert s Orhital. 

The anion (Fig. 33) comprises two parallel rhombs of metal atoms at an interplanar distance of 2.12 A. The two layers are staggered so as to produce an antiprismatic geometry, which may be derived from the trigonal prismatic starting material by the capping of two tetragonal faces and the breaking of the Co-Co bond common to these two faces (shown as a dotted line in Fig. 33). 

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