Distorted square prism

The second class of hexanuclear clusters also contains an octahedron of metal atoms, but they are coordinated by twelve halide ligands along the edges (Fig. 16.64b). Niobium and tantalum form clusters of this type. Here the bonding situation is somewhat more complicated The metal atoms are surrounded by a very distorted square prism of (bur metal and four halogen atoms. Furthermore, these compounds are electron deficient in the same sense as the boranes—there are fewer pairs of electrons than orbitals to receive them and so fractional bond orders of are obtained. 

Ideally such a perovskite structure contains 3x3 = 9 oxygen atom sites. It was found, however, that the 123 unit cell contains only 7 oxygen atoms. The vacant oxygen sites are in the plane of Y (oxygen coordination. 8, slightly distorted square prism) and in the basal copper plane (Ba oxygen coordination 10). In this way two different Cu sites result, forming the chains and planes of the 123 structure ... 

Angular coordinates (6 [Pg.209]

PI Z = 2 Dx = 1.910 R = 0.039 for 2,724 intensities. The Ca2+ is eight-coordinated by a distorted, square anti-prism of oxygen atoms from two inositol and four water molecules. The myoinositol mole-... 

Another unique structure is found in the rhombohedral room-temperature modifications of NiS (millerite) and NiSe, shown in Fig. 50. The anions form infinite trigonal prisms. The nickel atoms are located slighty above the square faces of these prisms. The coordination polyhdra of the nickel atoms are distorted square pyramids of anions. However, in millerite the nickel atoms of the other two faces of the sulfur prisms are as close as 2.53 A which roughly corresponds to half bonds. There is, therefore, little doubt that NiS is metallic which would be in agreement with the observed Pauli paramagnetism (55). 

The malonate complex of Eu(III), Eu2(malonate)3 8H2O is interesting since there are two different coordination geometries, and only five of the water molecules are coordinated [167], One of the Eu ions is eight-coordinate in the form of a distorted square antiprism and the other Eu ion is nine-coordinate, distorted tricapped trigonal prism. In the analogous neodymium complex, six water molecules are coordinated because of the difference in ionic radius. The neodymium complex is nine-coordinate with monocapped antiprismatic geometry [168]. 

P2, Z =2 Dx = 1.771, 1.773 R = 0.038 for 1,321 intensities, 0.036 for 2,283 intensities. The proton attached to 0-3 is that removed on salt formation. The ring is planar within 0.05 A (5 pm). The shapes of the side chains of the two ions are different, with 0-4-C-4-C-5-0-5 torsion angles of 56 and 170°, respectively. The other conformational angle of the side chain, 0-5-C-5-C-6-0-6, is 47° in both molecules. The calcium ion is eight-coordinated, in a distorted, square anti-prism, by six oxygen atoms from three L-ascorbate ions and by two water molecules. 

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