Constant valency

The effect of a particular element on the odour of its compound seems also to lend support to the residual affinity theory, for it is only the elements which possess residual affinity in certain of their compounds, which function as osmophores. Oxygen, nitrogen, sulphur, phosphorous, halogens, arsenic, antimony, bismuth, etc., whose valencies vary under certain conditions are powerfully osmophoric whereas carbon, hydrogen, and many others which have a constant valency are practically non-osmophoric, and it is very instructive to note that the element is osmophoric when it is not employing its full number of valencies and therefore has free affinity. 

The approximate correctness of the relative values of fav given in the table is supported by the observed values of the metallic radii. Values of R for ligancy 12 are shown in Figure 1, with a straight line with slope that represents the change in radius with atomic number for constant valence and type of orbital, as given in Figure 3 of an earlier paper.17 The points lie above the line by amounts that indicate a monotonic decrease in valence to both sides of molybdenum. 

At about the time that Claus proposed his ammonia theory, the concept of valence was being formulated and developed by a number of chemists — in particular, Kekule, Frankland, Williamson, Odling, Kolbe and Couper. During the late 19th and early 20th centuries the principal difficulty in the field of valence was its application to all types of chemical compound, and one of the main controversies involved whether or not a given element could possess more than one valence. Since coordination compounds pose a number of basic constitutional problems, it is not surprising that they became involved in the question of variable vs. constant valence. 

Most of the pioneers in the theory of valence readily admitted the possibility of variable valence. On the other hand, August Kekule (1829—1896)45 adopted and rigidly adhered to the principle of constant valence. In spite of the mass of data that soon accumulated to contradict this, he insisted that atomicity (his term for valence) was a fundamental property of the atom which is just as constant and unchangeable as the atomic weight itself . The simplicity of this principle, however, was more than outweighed by the complicated and unrealistic formulas required to maintain it, and eventually Kekule stood virtually alone in its defense. 

With the discoveries that led to the founding of coordination chemistry by Alfred Werner and others at the start of the 20th century, and the concomitant demise of constant valency theories, the utility of additive nomenclature once again became evident and was the nomenclature pattern used by Werner and his contemporaries in their publications. 

Kekule tried to buttress his theory of constant valence by dividing compounds into atomic compounds and molecular compounds Compounds in which all the elements are held together by the affinities of the atoms that mutually saturate one another could be called atomic compounds. They are the only ones that can exist in the vapor state. We must distinguish a second category of compounds that I shall designate molecular compounds. ... 

Formula I, according to which ammonium salts were regarded as molecular compounds, was proposed by Kekul6 as an attempt to preserve his dogma of constant valence. Formula II, proposed by Frankland and the advocates of variable valence, involved the formation of five bonds by nitrogen, a situation recognized as impossible by modern orbital theory. 

The Kekul constant valence Formula VIII, which viewed oxonium compounds as molecular compounds formed by adding acid to an organic... 

The bending frequency is estimated as 270 80 cm" from the following information. A value of 272 cm" derived from the three-constant valence-force field (17) using Vg and (k6/t )/kj = 0.040 transferred from isoelectronic CaFg (18). The... 

For the case of constant valence, the counterion of higher atomic number (lower hydrated radius or smaller solvated volume) at low concentration and ordinary temperature. Thus,... 

For the case of constant valence, the counterion with greater polarizability at ordinary temperature and low concentration. A measure of polarizability is the Debye-Htickel parameter d, which gives the distance of closest approach between ions of opposite sign. Thus, highly polarized counterions, such as Ag+ and T1+, have abnormally low d values and are preferred to alkali ions by the cation exchange resins with sulfonic acid groups. 

At low aqueous concentrations, ambient temperatures and constant valence, the extent of exchange increases with increasing atomic number (decreasing hydrated radius) of... 

Table 3.8 Semiempirical Expressions for Effective Spin-Spin Constants Valence States of Homonuclear Molecules... 

I would suggest that the formation of metal chelate complexes, with a four or six-coordinate metal partly bound to an optically active protein and partly bound to a substrate molecule can explain this stereospecificity. The optically active coordination compounds of metals, such as cobalt, have extraordinarily high molecular rotation, and so the difference in chelation powers of the d and I forms of a substrate may be very great. As Dr. Chaberek has pointed out (Lecture 33), this chelation may involve both metals of constant valency, e.g.. Mg, Zn, and those of variable valency. Metallic ions of both types are proven essential trace metals in biological systems. 

Numerical ab initio calculations for selected examples with polarized basis sets and Cl of reasonably size confirmed that the size of the matrix elements within the active space matrix is negligible. In contrast, the elements of that involve both the active and inner shells are large, since is primeuily due to the shielding of nuclei by inner-shell electrons [11]. It is therefore common practice in many semiquantitative applications, to account for the effect of the fixed-core electrons by replacing the factor gPgZ r in by the empirical value of the atomic spin-orbit coupling constant valence p orbitals on... 

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