Square antiprism coordination

In a cation with square-antiprism coordination, four d orbitals may be used for bond formation so that we expect this coordination to occur with d°, d1 and d2. For a d2 configuration, however, the trigonal-prismatic coordination of the cation seems to be more stable so that only d° and d1 representatives are found with a square-antiprismatic neighbourhood of anions. 

Because the cuboctahedral clusters are the most widespread and the largest scale ones, we will therefore consider them in detail. All eight normal anion positions (Fp) in the fluorite unit cell containing such a cluster are vacant. Twelve atoms F form the cubocta-hedron (8 12 0). The position in the centre of the unit cell could be occupied by one additional fluorine atom F" (8 12 1). The anion cluster is surrounded by an octahedron of R " " cations located in face centres (Figure 14.12). The coordination polyhedron of R " " cations is a square antiprism (coordination number equal to 8). 

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. 

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... 

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... 

Figure 2.9 Schematic structure of two dif- (d) cube. The dpp parameter defines the aver-ferent polyoxometalate complexes with inter- age distance between the two oxygen-based esting coordination symmetry (a) LnW10 and square planes. The djn is the average 0-0 (b) [LnPd"12(AsvPh)8032]5-, and their coor- distance within the oxygen-based square dination polyhedra (c) square antiprism and planes.

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). 

Bi-0 2.54(l)-2.68(2) A], In contrast, the nine-coordinate capped square antiprism geometry for bismuth in [Bi(N03)3(H20)3] (18-crown-6) does not involve the expected multidentate ether coordination to bismuth, but rather a hydrogen-bonded interaction of the crown ether with the hydrated bismuth center chelated by bidentate nitrate groups [Bi-0 2.38(2)—2.56(2) A] 32, implying that the hexado-... 

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