Valency structure and

Sadler, P.J. (1976) The biological chemistry of gold a metallo-drug and heavy-atom label with variable valency Structure and Bonding, 29, 171—219. 

Fig. 6. The valence structures and styx numbers of BbH9, B4H10, and B6H...

In Fig. 7 we show all Kekule valence structures of benzo[ghi]perylene and their degrees of freedom. The first eight structures have df = 3, the next five Kekule valence structures have df = 2, and the last Kekule valence structure has df = I. The individual Kekule valence structures have quite different count of conjugated circuits R, which can be as high as five (in the first Kekule valence structure) and as low as one (in the last Kekule valence structures). A close look at these... 

INCREASED-VALENCE STRUCTURES AND MULLIKEN-DONOR-ACCEPTOR COMPLEXES ... 

Example of increased-valence structures, and the Lewis structures and three-electron bond structures from which they may be derived, are displayed in Figure 10 for NO2 and N2O4. These two molecules involve four-electron three-centre and five-electron three-centre bonding units. N2O4 also possesses six-electron four-centre bonding units that are components of a ten-electron six-centre bonding unit. 

In this paper I point out the similarities and differences between the BaPb1 xBix03 and the copper systems. This will be done from a chemistry viewpoint considering such issues as covalency, mixed valency, structure, and metastability. 

We have also developed a rich canon of names to describe our rapidly changing perception of bonding in molecules, among which the terms hyperconjugation , negative (anionic) hyperconjugation , Heitler-London (HL) Increased-Valence-Structures, and r-Donor Bonds are those that concern us. We use them to explain upheld and downheld chemical shifts in the P-NMR spectra of compounds that we could not explain by normal [Pg.10]

Valence shell electron pair repulsion theory, 32-39 effective bond length ratios, 34 repulsion energy coefficient, 33 Valency structure and, 5 Valinomycin... 

In the next sections, we see additional illustrations of partial ordering, including partial ordering of isomers, partial ordering of smaller benzenoid hydrocarbons, partial ordering of Kekule valence structures, and use of partial ordering in the search for the phamacophore, the active part of trioactive molecules. Finally, we also illustrate the use of partial ordering for numerical characterization of proteome maps [20]. 

In Figure 2.12, we have illustrated determinants for an additional half a dozen benzenoid hydrocarbons that have three top and three bottom vertices. They have also been mentioned in Clar s booklet except for the last structure, which has no Kekule valence structure and iUnstrates a hypothetical system, referred to as concealed non-Kekulean benzenoids [32] becanse, in some cases, it is not immediately apparent that they have no F. A. Kekule (1829-1896) structures. For some of the benzenoid hydrocarbons shown in Figures 2.11 and 2.12 later in the chapter, there is an even simpler way of finding K, but the approach of John and Sachs is quite general and is often the simplest way of calculating K. 

Gutman and M. Randic, A correlation between Kekule valence structures and conjugated circuits, Chem. Phys. 41 (1979) 265-270. 

Title A Correlation between Kekule Valence Structures and Conjugated Circuits... 

Figure 2-11 Increased-Valence structures and component Eewis octet structures.

Increased-Valence Structures and Molecular Orbital Theory for N2O4... 

In Fig. 12-1, we show how to use the standard Lewis structures to construct some of the increased-valence structures that we have considered previously in Chapter 11. For each molecule, one or more lone-pair electrons have been delocalized into vacant 2-centre bonding orbitals. This technique for generating increased-valence structures (and thereby stabilizing the Lewis structure) can be used whenever the arrangement of electrons shown in structure (4) occurs in a Lewis valence- bond stmcture. This must surely be the case for thousands of molecular systems ... 

In Section 2-5, we have used the electroneutrality principle to deduce that (I) should be the most important of the four increased-valence structures, and have then deduced from (I) that the N-N and N-0 bond-lengths for N2O should be respectively longer than an N-N triple bond, and similar to an N-0 double bond. The bond-lengths reported in Table 13-1 are in accord with this deduction. 

The symmetrical triatomic species N , NOj and CO2 are also isoelectronic with N2O. Their standard Lewis and increased-valence structures are displayed in Fig. 13-1. For each of these systems, resonance between the four increased-valence structures indicates more clearly than does resonance between the standard Lewis stractures that the N-N, N-0 and C-0 bond-lengths of 1.18 A, 1.15 A and 1.16 A are shorter than those of double-bonds (see Table 13-1). The smaller formal charges for increased-valence stmctures (II) and (III) suggest that these are the most important of the four increased-valence structures, and inspection of them alone makes clear why the bond-lengths are shorter than double-bonds. Similar types of increased-valence stmctures should also be the primary stractures for HNCO. Inspection of them in Fig. 13-1 leads to the conclusion that the C-N and C-O bonds are both shorter than double bonds, and the bond-lengths reported in Table 13-1 support this conclusion. 

In these latter structures, the valencies of N", N, and hT are 2, 3 and 4, respectively. We may note that each of the structures (1), (2) and (3) seems to have one more bond than have the corresponding Lewis structures, and therefore we might also designate structures (1), (2) and (3) as increased-valence structures. Alternatively, we may say that the quinquevalent nitrogen atom has increased its valence relative to the maximum of four which is allowed in the Lewis theory. Sometimes, the valence-bond structures such as (1), (2) and (3) are designated as classical valence structures , and we shall refer to them as such here. 

Using the procedures described in Section 14-3, it may be deduced that the maximum nitrogen valence for structure (16) is 4.25, and that a total of 18 electrons participate in bonding between all pairs of atoms. Also, as indicated in the caption for Fig. 11-7, as well the cis increased-valence structure (16) here, there is a mirror-image cis increased-valence structure and two trans increased-valence structures. Resonance between these four increased-valence structures is equivalent to a restricted form of resonance between the 64 octet Lewis structures. 

We shall now examine in more detail some wave-functions for increased-valence structures, and compare them with wave-functions that may be constructed for standard Lewis and Linnett non-paired spatial orbital structures, as well as with the delocalized molecular orbital wave-functions. 

Interpretation of statistical and other data on molecules. In this article in particular we will see how graph theoretical concepts, such as the conjugated circuits, the innate degree of freedom of Kekule valence structures, and the Clar structures, can all be combined to characterize the local and overall aromaticity of benzenoid hydrocarbons, fully benzenoid hydrocarbons, and fully aromatic compounds. In addition, we will see that these approaches that lead to the quantification of Clar s r-sextets model can be justified using chemical arguments. ... 

Figure 34. One-to-one correspondence between the Kekule valence structures and the paths illustrated on one of the Kekule structures of C48H18. At the right the unique path connecting slanted C=C bonds.

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