Coordination number separator

Since A Aw is the change in interaction energy per 1,2 pair, it can be expressed as some multiple of kT per pair or of RT per mole of pairs. It is also conventional to consolidate the lattice coordination number and x into a single parameter x, since z and x are not measured separately. With this change of notation Az Aw is replaced by its equivalent xRT, and Eq. (8.41) becomes... 

In principle this type of isomerism (also known as polytopal isomerism) is possible with any coordination number for which there is more than one known stereochemistry. However, to actually occur the isomers need to be of comparable stability, and to be separable there... 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

Two other, closely related, consequences flow from our central proposition. If the d orbitals are little mixed into the bonding orbitals, then, by the same token, the bond orbitals are little mixed into the d. The d electrons are to be seen as being housed in an essentially discrete - we say uncoupled - subset of d orbitals. We shall see in Chapter 4 how this correlates directly with the weakness of the spectral d-d bands. It also follows that, regardless of coordination number or geometry, the separation of the d electrons implies that the configuration is a significant property of Werner-type complexes. Contrast this emphasis on the d" configuration in transition-metal chemistry to the usual position adopted in, say, carbon chemistry where sp, sp and sp hybrids form more useful bases. Put another way, while the 2s... 

Although the lanthanide cations most commonly give complexes with coordination numbers of 8 or 9, the [ML3(LH)3] species predominate on extraction with phosphorus(V) acids and it has been suggested that the bulk of the L- LH chelate ring favors a lower coordination number 4 The phosphorus(V) acids are very effective reagents for the extraction and separation of lanthanides.124 (See chapter 3.2). 

Fourier transformation of Cu EXAFS data gathered on the Cu(MPG) complex reveals two separate peaks representing shells at distances of 1.9 and 2.3 A. When tested for Ns (coordination number), metal-ligand distance (R as), and Debye-Waller parameter difference (Aa2as) followed by comparison to known model compounds, results show that the presence of both a Cu-(N, O) and Cu-S shell is necessary to obtain an adequate fit to the EXAFS data. Therefore it was concluded that a Cu-S bond is present in the compound. 

In mercury rich alloys such as M Hg12 (M = K, Rb), a particular situation occurs due to high coordination number (CN) polyhedra (up to CN 22) in their structures. In these compounds the electropositive atoms are located in the centers of these polyhedra and are thus spatially separated from each other. The covalent Hg-Hg interaction, as discussed above, is of minor importance in these amalgams. 

A Inclusion of structural information. The names described so far detail ligands and central atoms, but give no information on stereochemistry. The coordination number and shape of the coordination polyhedron may be denoted, if desired, by a polyhedral symbol. These are listed in Table 4.4. Such a symbol is used as an affix in parentheses, and immediately precedes the name, separated from it by a hyphen. This device is not often used. 

This is illustrated in Figure 4.2. The peak capacities of the two dimensions are shown as the number of adjacent Gaussian profiles that can be packed into the space along the respective separation coordinates. The separation plane is divided into rectangular boxes that represent the resolution units in the 2D plane. The total peak capacity 2d is therefore approximately equal to the number of such boxes. 

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