Co-ordinating polyhedra

Chalcogenides. The crystal structures of titanium sulphides have been reviewed.41 Ti3S8 has several structural features in common with TiS2 including trigonal-prismatic co-ordination polyhedra about S atoms and cubic co-ordination polyhedra about Ti atoms. Whereas six cube faces are shared with trigonal prisms in TiS2 only,... 

We can finally conclude that the number of chemical systems which appear to reject point-defect populations as a mode of accommodating their non-stoicheiometric behaviour is large and varied and here we have touched upon only a few which make use of planar faults or parallel lamellar or foliar intergrowth structures. The results presented show that physical terms, such as elastic strain, are of importance in controlling the microstructures of such phases, but whether they form or whether they coexist with some form of point-defect clusters may well depend in a sensitive way to the anion-cation bonding within the individual co-ordination polyhedra which made up the structure. The continuing research in this area is certain to produce new and unexpected results before complete answers to the problems posed here are found. 

Figure 12 The co-ordination polyhedra of the K+ ions in potassium d-gluconate monohydrate, (a) Form A. (b) Form B.

Structures for two bismuthides, Ba2Bi777a and Ca5Bi3,777b show the presence of bismuth in nine-fold co-ordination in the former, while two different co-ordination polyhedra, showing eight- and nine-fold co-ordination, occur in the latter. 

Structural Studies.—The structures of some compounds of scandium and yttrium, in addition to those of some of the lanthanides and actinides, have been reviewed. The crystal structure of scandium selenate pentahydrate, Sc(Se04)3,5H20, has been determined by single-crystal JT-ray diffraction. The scandium atoms were shown to be octahedrally co-ordinated by the oxygen atoms of the selenate groups and water molecules. The mean Sc—O distances in the scandium co-ordination polyhedra vary from 2.08 to 2.10 A and the mean Sc—O bond lengths in the selenate groups vary from 1.61 to 1.64 A. Two formula units were found in each cell. 

In essentially ionic crystals it is normally the co-ordinating polyhedra of anions about cations with which we are concerned since the anions are almost always the largest ions present and it is the co-ordination of them about the cations, rather than that of the cations about the anions, which is determined by the radius ratio the co-ordination about the anions is governed by the number of cations available. In the structure of sodium chloride, for example, both Na+ and Cl- ions are admittedly octahedrally co-ordinated by ions of the other kind but it is... 

II. 538 A, p =95.15°, and Z = 8. The structure contains two crystallo-graphically independent Mg atoms each co-ordinated to six water molecules the seventh water molecule is linked by hydrogen bonds only. No oxygen atoms are shared by co-ordination polyhedra and the crystal structure can be described in terms of layers parallel to (010) or to (001). ... 

The optimum hybridization schemes for coordination compounds are summarized in Table 3. In Fig. 1 the hybridization characteristics of spherical co-ordination polyhedra are summarized together with those for polyhedra with... 

Co-ordination polyhedra can be described as nido, arachno and hypho if they can be represented as fragments of spherical polyhedra with one, two and three vertices missing respectively. The grossly non-spherical nature of these polyhedra means that their hybrid orbitals are no longer adequately described in terms of the methodology developed above. It might appear that it is possible to describe the... 

Fig. 6. The positions of the nodal lines in hybrids for spherical co-ordination polyhedra...

Perhaps the most striking feature of the complexes described in this section is the great variety of the structural types exhibited. Co-ordination numbers of the metal atoms vary from three to eight, and a wide range of different co-ordination polyhedra is thereby displayed. With one exception, the inorganic complexes of chromium have the metal atom in the tripositive state, but for molybdenum and tungsten the higher oxidation states, from three- to six-positive, are all well represented. Comparative studies of related molecules are few. 

Complexes of Copper(n).—In terms of the number of papers devoted to its stereochemistry, copper(ii) is the most intensively studied transition-metal ion. Square, distorted tetrahedral, trigonal-bipyraraidal, square-pyramidal, and square-bipyramidal co-ordination polyhedra are all well represented in the structures described below. In a number of cases the co-ordination is too irregular to permit classification. The absence of regular tetrahedral and octahedral stereochemistries is striking. 

The occurrence of these non-bonding orbitals can be associated with either spherical or non-spherical co-ordination polyhedra. The choice of co-ordination polyhedra depends on several factors and notably ... 

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