Polyhedra, coordination Connectivity

Carbon tends to adopt the position of lowest coordination number on the polyhedron and to keep as far from other C atoms as possible (i.e. the most stable isomer has the greatest number of B-C connections). 

The coordination polyhedron results when the centers of mutually adjacent coordinated atoms are connected with one another. For every coordination number typical coordination polyhedra exist (Fig. 2.2). In some cases, several coordination polyhedra for a given coordination number differ only slightly, even though this may not be obvious at first glance by minor displacements of atoms one polyhedron may be converted into another. For example, a trigonal bipyramid can be converted into a tetragonal pyramid by displacements of four of the coordinated atoms (Fig. 8.2, p. 71). 

The engineering of zinc-binding sites in a-helical peptides, where metal binding stabilizes protein tertiary structure, has been reported by Handel and DeGrado (1990). In these experiments zinc-binding sites are incorporated into a dimeric helix-loop—helix peptide (H3 2) and a protein composed of four helices connected by three short loop sequences (H3 4). a model of one subunit of the H3 2 dimer is found in Fig. 47. In addition to metal complexation by two histidine residues at positions n and n+4 of one a helix, the metal is coordinated by a third histidine residue of an adjacent a helix. The composition of the zinc coordination polyhedron is like that of carbonic anhydrase (i.e., Hiss), and spectroscopic results suggest that all three histidine residues are involved in zinc complexation. This work sets an important foundation... 

The structure of p-CsBeFs is usually described as consisting of chains of corner-connected Bep4 tetrahedra, with Cs atoms/ions interposed between them so that the Cs atoms are 8-coordinated by fluorine, but in a very irregular way. Figure 5 shows that, in fact, this Cs-centred coordination polyhedron is a rather irregular trigonal prism with two lopsided caps. 

Although the connected coordination polyhedron approach is useful in crystal chemistry, it has certain implicit rigidity which makes it difficult to extend to less regular and more complex structures. Several alternative approaches to the description... 

It may often be convenient to describe the crystal structure in terms of the domains of the atoms [40], The domain is the polyhedron enclosed by planes drawn midway between the atom and each neighbor, these planes being perpendicular to the lines connecting the atoms. The number of faces of the polyhedral domain is the coordination number of the atom and the whole structure is a space-filling arrangement of such polyhedra. 

Hydrates of rare earth chlorides also have two different crystal systems a triclinic system for lanthanum, cerium, and praseodymium, as well as a monoclinic system for neodymium to lutetium and yttrium. CeCl3-7H20, as an example of the former system, is different from the above infinite polymer as two cerium atoms are connected by two [i2-bridges to form a dimer. The formula for this dimer is [(H20)7Ce([i2-Cl)2Ce(H20)7]Cl4 as shown in Figure 1.18. Therefore, the coordination number of cerium is nine and the polyhedron takes on a destroyed mono-capped square antiprism configuration. 

---