The Secondary Valences

Experience shows, however, that even after saturation of the primary valences inside the resulting molecules, forces still remain operative, which are generally known as residual or secondary valences. The first quantitative statements about these intcrmolecular forces were made by van der Waals in his theory of real gases when he introduced the internal pressure, a/72 so that we frequently speak of van der Waals forces. 

The present state of our knowledge allows us to assume that these van der Waals forces between saturated molecules owe their existence to three different effects  

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Plasticization is the process in which the plasticizer molecules neutralize the secondary valence bonds, known as van der Waal s force between the polymer molecules. It increases the mobility of the polymer chains and reduces the crystallinity. These phenomena become evident in reduced modulus or stiffness, increased elongation and flexibility, and lowering of the brittle or softening temperature of the plasticized product. The effect of plasticizers on polymers is the subject of the first chapter by E. H. Immergut and H. F. Mark. 

On the basis of X-ray measurements it is assumed that a micelle composed of 100-170 simple cellulose chains has a length of at least 600A and a width of 50-60A. An outline of the micellar structure of cellulose, according to Meyer and Mark [31], is sketched in Fig. 77. The secondary valencies (a) unite individual... 

Gloor and Spurlin [33] are of the opinion that compounds are formed between the nitrocellulose and the powdered metal by means of the secondary valencies. They have demonstrated that the higher the molecular weight of the nitrocotton, the smaller the quantity of metal powder necessary to cause the gel to coagulate. 

The fact that the co-ordination number for so many elements is six, and is generally independent of the nature of the co-ordinated groups, has made A. Werner suggest that the number is decided by available space rather than affinity, and that six is usually the maximum number which can be fitted about the central atom to form a stable system. Consequently, the co-ordination number represents a property of the atom which enables the constitution of molecular compounds to be referred back to actual linkings between definite atoms. A molecular compound is primarily formed through the agency of secondary valencies and, just as primary valencies determine the number of univalent atoms or their equivalent which can be linked to a central atom, so secondary valencies determine the number of mols. which can be attached to the central atom. The secondary valency is often active only towards definite mol. complexes, and hence the formation of additive compounds with other mol. complexes does not occur. Accordingly, the number of secondary valencies which are active towards different molecules is not always the same. 

The formation of sulphuric and chlorosulphuric acids by the union of sulphur trioxide with water and hydrogen chloride respectively is brought about by the secondary valencies as indicated in the schemes 03S+0H2=03S. . . OH2 and 03S+C1H=03S. . . C1H. When one of the reacting molecules contains douhle-linked atoms, the auxiliary valencies may not he sufficiently strong to preserve the integrity of the new mol., and, the atoms of the addition product may be rearranged. For example, this is the case with sulphur trioxide. Thus ... 

These observations, together with the results of solution conductivity measurements, can be explained if six groups, either chloride ions or ammonia molecules, remain bonded to the cobalt ion during the reaction and the compounds are formulated as shown in Table 1.4, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions. Werner proposed the term secondary valence for the number of groups bound directly to the metal ion in these examples the secondary valences are six in each case. 

In 1893 Alfred Werner 50) conceived his revolutionary hypothesis. He perceived the metal atom as having two types of valencies Haupt-valenz or primary or ionizable and Nebenvalenz or secondary or non-ionizable. Each metal has a set number of secondary valencies or a set coordination number. He claimed that the primary valence must be satisfied only by negative ions, whereas the secondary valencies may be satisfied by negative ions or neutral groups. These secondary valencies... 

On the basis of this evidence alone it is logically possible that one isomer could be tetrahedral. Early coordination chemists, however, assumed that the directions of the "secondary valencies were fixed, which would preclude this possibility. X-ray structural analysis shows that, in the case of Pt complexes, they were correct. 

Werner recognized that the secondary valence bonds in his theory were directed in space, with the result that geometrical and optical isomersion is possible. For example, there are nine isomers with the empirical formula Co(NH3)3(N02)3, corresponding to different spatial arrangements of the MH3 and NO2 group. In 1916... 

The first step in crystallite formation is the creation of a stable nucleus brought about by the ordering of chains in a parallel array, stimulated by intramolecular forces, followed by the stabilization of long-range ordo- by the secondary valence forces, which aid the packing of molecules into a three-dimensional ordered structure. 

Plasticizers reduce the hardness and brittleness of polymers. They increase the distance between the molecular chains, thus reducing the secondary valence forces and shifting the transition interval to lower temperamres, see Sect. 1.1. There are two ways to reach these goals, internal and external plasticization. 

When the macromolecules show a preferred orientation direction, more primary valency bonds take effect in this direction, replacing part of the secondary valency bonds, which are lower by about one power of ten. This results in differing physical properties in both the direction of orientation and at right angles to it. 

In 1893 the Swiss chemist Alfred Werner (1866—1919) proposed a theory that successfully explained the observations in Table 23.3. In a theory that became the basis for understanding coordination chemistry, Werner proposed that any metal ion exhibits both a primary valence and a secondary valence. The pritnaij valence is the oxidation state of the metal, which is +3 for the complexes in Table 23.3. — (Section 4.4) The secondary valence is the number of atoms bonded to the metal ion, which is also called the coordination number. For these cobalt complexes, Werner deduced a coordination number of 6 with the ligands in an octahedral arrangement around the Co ion. 

The secondary valence is directed toward fixed positions in space. [Note that this is the basis for the stereochemistry of metal complexes (Chapter 3).]... 

Returning now to the theory, one finds that Werner represented C0CI3 5NH3 as structure VI. He did this in accord with postulate (2), which states that both the primary and secondary valence tend to be satisfied. In C0CI3 5NH3, there are only five ammonia molecules to satisfy the secondary valence. Therefore, one chloride ion must serve the dual function of satisfying both a primary and a secondary valence. Werner represented the bond between such a ligand and the central metal by a combined dashed-solid line -------. Such a chloride is not readily precipitated from solution by Ag. The... 

Werner s coordination theory, with its concept of secondary valence, provides an adequate explanation for the existence of such complexes as [Co(NH3)6]Cl3-Some properties and the stereochemistry of these complexes are also explained by the theory, which remains the real foundation of coordination chemistry. Since Werner s work predated by about twenty years our present electronic concept of the atom, his theory does not describe in modem terms the nature of the secondary valence or, as it is now called, the coordinate bond. Three theories currently used to describe the nature of bonding in metal complexes are (1) valence bond theory (VBT), (2) crystal field theory (CFT), and (3) molecular orbital theory (MOT). We shall first describe the contributions of G. N. Lewis and N. V. Sidgwick to the theory of chemical bonding. 

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