Valence linkings

By this method the atoms of a substance were combined to form a molecule using only valency links to construct the represen tative of the substance. This method can be used only when the connections between the atoms are established by chemical reactions (or spectroscopic data). 

Following the atomic and molecular theories, the study of the chemical compositions of substances led to the doctrine of valence or saturation capacity. Each atom is considered to be capable of combining with a definite number of atoms of its own kind or a different kind. The arrangement of the elements in the Periodic System of Mendeleeff brings out clearly these valence numbers. The introduction of dots and dashes in chemical formulas to represent valence linkings forms a convenient method of representation. 

Sometimes confusion has been caused by taking valence linkings to represent some form of combining power or stability. This question must be made clear and the meaning of valence defined carefully for a proper understanding of the questions to be taken up. In order to do this, chemical energy will be taken up for a moment and the relation between it and valence described. 

It has been also established that Tet s anion is a bidentate ligand. Each ion of Tet links simultaneously with two neighboring Fe(II) ions and forms a single ion (basic valence) and single donator-acceptor (side valence) link, as a result of which polymer-type bridge structures are formed in the CC. Thanks to these structures in particular, the adsorptive Tet-Fe films are resistant to organic solvents and chemical agents. 

Fig. 4. Different types of valence-linked diagrams. Diagram (a) is irreducible and connected, (b) is reducible, while (c) is irreducible and disconnected.

In precise investigation of atomic oscillations in the molecule, it appears that two different types are again distinguishable valence oscillations and deformations or break oscillations. The former consist of the periodic motion of the atoms in the direction of the main valences linking them the oscillation therefore causes an intermittent variable extension and contraction of any bond distance, a few data for which are shown in Table 2. In the break oscillations, however, the deformations of the atom are executed perpendicular to the bond direction and effect a periodic increase and diminution of the valence angle numerical data are also given for these in Table 4. 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

Any heterocycle containing the OCH=CH moiety can in principle extrude the superfluous fragment and form oxirene, as illustrated for a five-membered ring in Scheme 105. Probably the most propitious AB fragment would be nitrogen, but the required 1,2,3-oxadiazole (123) is unknown (see Chapter 4.21), probably because of ready valence tautomerization to diazoethanal (Scheme 106) (this approach has been spectacularly successful with the sulfur analogue of (2) (8UA486)). The use of (123) as an oxirene precursor is thus closely linked to the important diazo ketone decompositions discussed in Section 5.05.6.3.4(f). 

In addition polymerisation a simple, low molecular weight molecule, referred to in this context as a monomer, which possesses a double bond, is induced to break the double bond and the resulting free valences are able to join up to other similar molecules. For example poly(vinyl chloride) is produced by the double bonds of vinyl chloride molecules opening up and linking together (Figure 2.1). 

Additional axial ligands can be linked to the center of phthalocyanines either by valence bonds as in the case of silicium phthalocyanine151 or by coordinative bonds as in the case of bis(pyridino)iron(II) phthalocyanine [PcFe(py)2].131353... 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

These compounds contain two or more independent portions that are not bonded to each other by any valence forces but nevertheless must remain linked. Catenanes are made up of two or more rings held together as links in a chain, while in rotaxanes a... 

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