Coordination pentagonal pyramidal

Using a different metal salt, Cd(NOj)2, instead of CdClj oxidation reaction results in a slightly higher yield of the cadmium texaphyrin complex [60, 65]. However, upon purification, the product is obtained as a mixture of crystalline and non-crystalline solids. A single crystal X-ray diffraction study of the crystalline portion of the sample gave an unexpected result. The structure obtained (Fig. 20) revealed a six-coordinate pentagonal pyramidal cadmium (II) complex 156 (c.f. Scheme 20) where one of the two possible axial ligation sites is occupied by a benzimidazole [65]. The five donor atoms of the pentadentate texaphyrin macrocycle complete the coordination sphere about the cadmium with the cadmium... 

The a-rhombohedral form of boron has the simplest crystal stmcture with slightly deformed cubic close packing. At 1200°C a-rhombohedral boron degrades, and at 1500°C converts to P-rhombohedral boron, which is the most thermodynamically stable form. The unit cell has 104 boron atoms, a central B 2 icosahedron, and 12 pentagonal pyramids of boron atom directed outward. Twenty additional boron atoms complete a complex coordination (2). 

Table 4.3 indicates that octahedral coordination is a common mode for Li. Less usual is planar 6-fold coordination (Fig. 4.8a), pentagonal pyramidal coordination (Fig. 4.8i) or irregular 6-fold coordination (Fig. 4.9a). Examples of 7-fold coordination are in Fig. 4.9b and c. Lithium has cubic 8-fold coordination in the metallic form and in several of its alloys with metals of large radius. It is also 8-coordinate in the dilithionaphthalene complex shown in Fig. 4.9d here the aromatic... 

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

Carbon is known with all coordination numbers from 0 to 8 though compounds in which it is 3- or 4-coordinate are the most numerous. Some typical examples are summarized in the Panel (p. 291). Particular mention should also be made of hypercoordinate non-classical carbo-nium ions such as 5-coordinate CHj", square pyramidal CsHs (cf. the isoelectronic cluster B3H9, p. 154), pentagonal pyramidal C6Me6 " (cf. iso-electronic Bf,Hio, p. 154) and the bicyclic cation 2-norbomyl, C7H] 1... 

Complexes with mixed 0-and A -donor ligands such as edta and Schiff bases are well known and [Fe(edta)(H20)] and [Fe(salen)Cl] are examples of 7-coordinate (pentagonal bipyramidal) and 5-coordinate (square-pyramidal) stereochemistries respectively. 

Fig. 16. The sodium, water, benzo-15-crown-5 (XIV) complex showing the sodium ion in a pentagonal pyramidal coordination...

The distorted pentagonal-pyramidal coordination has been identified also in Mo and W 0X0 diperoxo siloxane complexes (14-17). Crystal strnctnres for species 14, 15 and 16 have been solved, while for the tungsten dinnclear componnd, strnctnral identification was suggested on the basis of similar IR and Raman spectra obtained for complexes 16 and 17 . ... 

Sn2(02CCF3)40]2-C6H6,6S7 are quite unusual, and exhibit pentagonal pyramidal and square-based pyramidal coordination polyhedra, respectively, for bivalent tin. The former represents... 

In the case of three bidentate sulfur ligands, e.g. [Bi(pedt)3]2-EtOH (pedt = 1-pyrrolidinecarbondithioate), more complex structures are formed. The dimer contains two differently coordinated bismuth atoms one is trigonal prismatic with an extra position on a rectangular face and the other bismuth atom is pentagonal pyramidal. The overall molecular structure is (80).270... 

The action of H202 on acidic dichromate solutions gives an unstable blue species which can be extracted into ether (equation 128).2,518,1428 On the addition of pyridine the blue compound Crvl0(02)2py crystallizes. The molecules are distorted pentagonal pyramids with sideways-bonded peroxo groups (298) 1429 some workers with less refined data have reported rather different bond distances.1430 In water the blue species is considered to be CrO(62)2(OH2), in ether it is Cr0(02)20Et2, and amines such as aniline, bipy and phen can replace the pyridine. With bidentate bipy the coordination sphere becomes essentially a pentagonal bipyramid (299),... 

Change in LFSE" upon changing o six-coordmate complex to a five-coordinate (square pyramidal) or a seven-coordinate (pentagonal bipyramidal) species ... 

This is a convenient, facile, and high-yield preparative route for quantitative preparation of the complexes above no special equipment is required. UI3(THF)4 crystallizes in a P2x/c space group. It is mononuclear with pentagonal-pyramidal coordination geometry around the central uranium atom. This compound is stable until 75°C, then THF molecules are removed by steps, forming UI3 at 162°C. Lewis base adducts of uranium tri-iodide, such as UI3(THF)4 above, are synthetically useful precursors for trivalent uranium chemistry (see Sec. 5.3.2) [360]. 

---