Coordination numbers definition

On the assumption that the pairs of electrons in the valency shell of a bonded atom in a molecule are arranged in a definite way which depends on the number of electron pairs (coordination number), the geometrical arrangement or shape of molecules may be predicted. A multiple bond is regarded as equivalent to a single bond as far as molecular shape is concerned. 

However, most complexes of Nb and Ta are derived from the pentahalides. NbFs and TaFs dissolve in aqueous solutions of HF to give [MOFs] " and, if the concentration of HF is increased, [MFg]. This is normally the highest coordination number attained in solution though some [NbFy] - may form, and [TaFv] " definitely does form, in very high concentrations of HF. However, by suitably regulating the concentration of metal, fluoride ion and HF, octahedral... 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

The definition of solvent exchange rates has sometimes led to misunderstandings in the literature. In this review kjs 1 (or fc2lsolvent]), sometimes also referred to as keJ s 1, is the rate constant for the exchange of a particular coordinated solvent molecule in the first coordination sphere (for example, solvent molecule number 2, if the solvent molecules are numbered from 1 to n, where n is the coordination number for the solvated metal ion, [MS ]m+). Thus, the equation for solvent exchange may be written ... 

By definition, the coordination number includes the adjacent atoms, and lone electron pairs are not counted. On the other hand, now we are considering lone pairs as occupying polyhedron vertices. To take account of this, we regard the coordination sphere as including the lone pairs, but we designate them with a //, for example -octahedral = octahedron with two lone electron pairs and four ligands. 

In the course of the tempestuous development of organophosphorus chemistry, interest has only recently been focused on compounds of formally quinquevalent phosphorus having coordination number 3, such as 1, 2, or 3, although one of the other species of this kind has long been postulated as reactive intermediate of solvolysis of phosphorylation reactions. Definite evidence of even proof of the existence of such coordinatively unsaturated phosphorus compounds, however, has been obtained only recently in mechanistic studies, by trapping reactions with suitable cycloaddends, or actually by direct isolation. 

However, for alternative definitions of coordination and coordination numbers, see the following paragraphs. 

Coordination number and packing geometry. On the basis of the previous definition of the coordination number (as the number of first neighbours), and of the corresponding coordination geometries (as listed in Table 3.4) some additional remarks may be useful for the particular case of packing of hard sphere atoms. 

According to the Frank-Kasper definition, the coordination number is unambiguously 12 in the hexagonal close-packed metals and assumes the value 14 in a body-centred cubic metal. Generally in several complex metallic structures this definition yields reasonable values such as 14, even when the nearest-neighbour definition would give 1 or 2. 

By using this definition, increasing values of the (weighted) CNW coordination number are obtained for the structures diamond (4.54), simple cubic (6), body-centred cubic (10.16), face-centred cubic (12) (in agreement with the increasing packing density). 

Atomic environment definition (and symbols) and corresponding coordination numbers andpolyhedra... 

Definition of coordination number. The coordination number of an ion is the number of ions to which it is linked by electrostatic flux. 

The coordination number of a cation is defined as the number of bonds that it forms. Although it was shown in Section 2.6 that the number of bonds is uniquely defined by the partitioning of the Madelung field, extensive calculation is needed to extract this information. It is, therefore, convenient to use a simpler, if more arbitrary, definition. 

The alkaline earths, particularly the lighter ones, have more definite water coordination, and a probable coordination number of 4. The heats of sublimation (Table I) of the magnesium salts are about 50 kcal/mole or higher, those for the calcium halides are over 60 kcal/mole, and for the heavier alkaline earths they run over 70 kcal/mole. For the beryllium halides other than the fluoride, however, the values are 30 to 33 kcal/mole. In... 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

EXAFS has provided detailed information about the local environment of the active Co and Ni sites and the Mo atoms to which they are attached in terms of the types of atoms within two atomic shells away from the atom being characterized. Cobalt and nickel were shown to be definitely bonded to the surfaces of small MoS2 crystallites. Representative structures for the environments of Mo and Co are illustrated in the following diagram. In such structures, Mo has a coordination number (CN) of 6, with six nearneighbor sulfur atoms, three nearby Mo atoms, and one nearby Co or Ni atom. Co-S configurations were either CN = 5 (square pyramidal) or CN = 6 (octahedral), with either one nearby Mo atom (low HDS activity) or two nearby Mo atoms (high HDS activity) (62). 

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