Trigonal antiprisms

The lone pair of the tertiary amine nitrogen atom (N7) capping the trigonal antiprism is, directed ut the center of a trigonal antipnsmatic face and thus interacts directly with the [Pg.265]

There are extensive possibilities for the formation of geometric and optical isomers in eight-coordinate complexes. Thus far. apparently only one pair has been completely characterized The diglyme [= di(2-methoxyethyl)etherl adduct of samarium iodide. Sml rO(CH2CH2OCH3)2]2. has been isolated in both cis and irons forms. The trans complex (Fig. 12.39a) has a center of symmetry Thus, the I—Sm—1 angle is exactly 180. and the molecule is a bicapped trigonal antiprism. The cis isomer (Fig. (2.39b) has ihe lower symmetry of a distorted dodecahedron with I—Sm—I angles of 92 > ... 

Six bonds may be arranged symmetrically in space in three ways to the corners of a regular octahedron, to those of a trigonal prism, or to those of a trigonal antiprism (an octahedron stretched or compressed along one of the tri-... 

There are two allotropic forms of polonium, both primitive, with one atom per unit cell. a-Po is simple cubic (3PO) and each Po atom has six nearest neighbors (3.366 A) at the vertices of a regular octahedron. The simple cubic structure is the same as that of NaCl (Figure 3.6) with all atoms identical. The structure of (3-Po is a primitive rhombohedral lattice. Each Po atom has six nearest neighbors forming a slightly flattened trigonal antiprism (or a flattened octahedron). 

The N atoms in O layers are at the centers of flattened octahedra (trigonal antiprisms) formed by six Be atoms. The notation is 4 2PBeIPBe NPBeON. The layers are filled. The double layer has filled A and B positions for the small Be and N atoms. Alternatively, to avoid the overfilling the double layer, all layers can be considered as one-third filled with two double layers, giving 1/3P1/3O1/3. 

Further twisting of the trigonal antiprism along the ternary axes generates a trigonal prism. Such peculiar packing has been found in the dianions Pt3(CO)6]]n2 (n = 2, 3, 4, 5) (29, 95), which contain a repeated trigonal... 

Fig. 18 Formation of the trigonal antiprism 26 upon coordination of six pyrazolone ligands 25 and six gallium(III) ions [67]...

Complex [Fe6(L8)6] (23) can be described as having idealized D3-molecular symmetry. The iron centers define the apices of a distorted trigonal antiprism in which six tripodal, tris(bidentate) ligands (L8)3 make up the equatorial faces, leaving the top and bottom triangles unoccupied. All six iron(III) ions are... 

The tetragonal distortion most commonly involves an elongation of one C4 axis and, in the limit, two trans ligands are lost completely, leaving a square, 4-coordinate complex. The trigonal distortion transforms the octahedron into a trigonal antiprism. 

Three other forms of octacoordination, which occur less often and are essentially restricted to actinide and lanthanide compounds, are the hexagonal bipyramid (D6h) (1-VI), the bicapped trigonal prism (D3 ) (1-VII) and the bicapped trigonal antiprism (Dm) (1-VIII). The hexagonal bipyramid is restricted almost entirely to the oxo ions, where an OMO group defines the axis of the bipyramid, though it is occasionally found elsewhere. 

The binary phase (Mo6S8) contains quite distorted Mo6 octahedra, elongated on a three-fold axis to become trigonal antiprisms. This can be attributed to the fact that there are only 20 cluster electrons. This number is insufficient to form a full set of Mo—Mo bonds, that is, to populate all the 12 bonding orbitals with the result that there is a distorted octahedron with somewhat longer Mo—Mo bonds compared to the 24-electron clusters. In the Chevrel phases the additional metal atoms add electrons to the Mo6 core and it becomes more regular in structure with shorter Mo—Mo bonds. 

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