Valency theory

VSEPR theory See valency, theory of. vulcanite See ebonite. 

The theory of molecular symmetry provides a satisfying and unifying thread which extends throughout spectroscopy and valence theory. Although it is possible to understand atoms and diatomic molecules without this theory, when it comes to understanding, say, spectroscopic selection rules in polyatomic molecules, molecular symmetry presents a small barrier which must be surmounted. However, for those not needing to progress so far this chapter may be bypassed without too much hindrance. 

The simplest, and perhaps the most important, information derived from photoelectron spectra is the ionization energies for valence and core electrons. Before the development of photoelectron spectroscopy very few of these were known, especially for polyatomic molecules. For core electrons ionization energies were previously unobtainable and illustrate the extent to which core orbitals differ from the pure atomic orbitals pictured in simple valence theory. 

We have seen above how X-ray photons may eject an electron from the core orbitals of an atom, whether it is free or part of a molecule. So far, in all aspects of valence theory of molecules that we have considered, the core electrons have been assumed to be in orbitals which are unchanged from the AOs of the corresponding atoms. XPS demonstrates that this is almost, but not quite, true. 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

Kotani, M., Proc. Shelter Island Conference on Quantum Mechanical Methods in Valence Theory, p. 139. Best orbital for the hydrogen molecule."... 

The outermost electrons are used in the formation of chemical bonds (Chapter 2), and the theory of bond formation is called valence theory hence the name of these electrons. 

The valence theory of metals and intermetallic compounds is still in a rather unsatisfactory state. It is not yet possible to make predictions about the composition and properties of intermetallic compounds with even a small fraction of the assurance with which they can be made about organic compounds and 7— 553041. Lee brtx Nobel en /pjy. 

Now that we have considered some of the ways in which the idea of resonance has brought clarity and unity into modem structural chemistry, has led to the solution of many problems of valence theory, and has assisted in the correlation of the chemical properties of substances with the information obtained about the structure of their molecules by physical methods, we may well inquire again into the nature of the phenomenon of resonance.1... 

A fuller and interesting account of molecular ( and of equivalent ) orbitals will be found in Murrell, Kettle, Tedder, Valency Theory, 2nd ed., Wiley, New York, 1969, p 190. 

One-electron pictures of molecular electronic structure continue to inform interpretations of structure and spectra. These models are the successors of qualitative valence theories that attempt to impose patterns on chemical data and to stimulate experimental tests of predictions. Therefore, in formulating a one-electron theory of chemical bonding, it is desirable to retain the following conceptual advantages. 

August Kekule ignored Frankland s work and claimed the valence theory for himself. It was 20 years before scientists recognized that Frankland had founded the theory of valency, while Kekule had contributed to it at a later date. 

In contrast to the lack of recognition for his valency theory, Frankland s work in organometallic compounds attracted considerable attention. When the city of Manchester opened England s first provincial university, Frankland was appointed its chemistry professor. Frankland was a self-made man, and Manchester was a city of self-made men made rich by Britain s textile industry. Its university was a new kind of institution for Britain. It was wholly secular, and its professors were chosen by merit, rather than by the established Church of England. Furthermore, the students—all male, of course—were admitted without regard to religion, rank, or social status. 

Most ESR studies of organic radicals were carried out in the 1950s and 1960s. They provided important tests of early developments in valence theory. The results of these early studies are nicely summarized in a review by Bowers.11 Applications of hyperfine splittings to structure determination are discussed in many of the texts and monographs referenced in Chapter 1. 

However, we should emphasize that the NBO/NRT concepts of hybridization, Lewis structure, and resonance differ in important respects from previous empirical usage of these terms. In earlier phases of valence theory it was seldom possible to determine, e.g., the atomic hybridization by independent theoretical or experimental procedures, and instead this term became a loosely coded synonym for the molecular topology. For example, a trigonally coordinated atom might be categorized as sp2-hybridized or an octahedrally coordinated atom as d2sp3-hybridized, with no supporting evidence for the accuracy of these labels as descriptors of actual... 

At the most elementary level of valence theory, chemical bonds (and the associated NBOs) are expected to retain approximately fixed forms during internal rotations. At this level one can simply visualize torsional interactions in terms of frozen NBOs moving on a frozen rigid-rotor geometrical framework, with NBO populations... 

The nature of the noncovalent bonds that link molecules into supramolecular clusters has inspired discussion and speculation throughout the history of valence theory. Werner s transition-metal studies originally led to the concept of near-valence ... 

The use of hybrid atomic orbitals in qualitative valence theory has, in the past, rested on two points (i) an empirical justification of their use involving the concept of the valence state of an atom and (ii) a simple linear transformation technique for the construction of the explicit forms of the orbitals. In this section we show that both of these points can be replaced. The justification can be replaced by a derivation and this derivation can be used to suggest variational forms which render the linear transformation technique redundant. 

To return to our main line of thought, the development of variational techniques for valence theory, there is an obvious parallel between the use of the usual (complex) AOs for the calculation of atomic electronic structures and the hybrid AOs for the molecular case ... 

We have not mentioned open shells of electrons in our general considerations but then we have not specifically mentioned closed shells either. Certainly our examples are all closed shell but this choice simply reflects our main area of interest valence theory. The derivations and considerations of constraints in the opening sections are independent of the numbers of electrons involved in the system and, in particular, are independent of the magnetic properties of the molecules concerned simply because the spin variable does not occur in our approximate Hamiltonian. Nevertheless, it is traditional to treat open and closed shells of electrons by separate techniques and it is of some interest to investigate the consequences of this dichotomy. The independent-electron model (UHF - no symmetry constraints) is the simplest one to investigate we give below an abbreviated discussion. 

The upshot of these conclusions is that the use of a minimal basis of GHOs provides the formal, conceptual and numerical foundation for a variety of the aspects of valence theory. In particular, the GHOs act as a theoretical foundation for a number of facets of the theory of valence, which are not normally considered to be intimately connected ... 

Thus in 1899, Johannes Thiele extended his valence theory of double bonds to include colloids. Thiele suggested that in such materials as polystyrene the molecules of styrene were bound together merely by association of the double bonds. He referred to this association as "partial valence" (21). In 1901, Rohm concluded that the transformation of acrylic esters into polymers was from an "allotropic alteration" and not a chemical reaction (22). Schroeter, working with salicylides just as Kraut, Schiff, and Klepl before him, concluded that the tetrameric salicylide was formed by "external forces about the monomeric molecules", and that the chemical structures of the monomers were unaltered (23). Thus the association theory rapidly grew in popularity. 

In addition to atomism, the principal chemical theories of the nineteenth century included electrochemical dualism, the radical theory, the type theory, and the structure theory, the latter strongly identified with what chemists called the "law of linking" of carbon atoms. The valence theory evolved as a way of tying together the notions of chemical equivalence and chemical structure, and it carried along the old problem that some chemical elements (e.g., nitrogen) exhibit different combining values with another element in different circumstances. 

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