Secondary bonding theory

The structures of the element trihalides EX3 are covered in a number of textbooks on structural inorganic chemistry (4, 5), and these will not be discussed in great detail here. It is, however, worth mentioning some of the salient structural features. In most cases, a molecular trigonal pyramidal EX3 unit consistent with VSEPR theory predictions is readily apparent in the solid-state structure, although there are usually a number of fairly short intermolecular contacts or secondary bonds present. A general description of the structures as molecularly covalent but as having a tendency toward macromolecular or polymeric networks is therefore reasonable. Only in the case of the fluorides is an ionic model appropriate. 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

With the discovery of x-ray diffraction and the opportunity this gave to determine exactly where the atoms are in a crystal, there arose an unexpectedly direct way to ascertain the measure of reality behind the Werner theory and its implied equivalence of some primary and secondary bonds. 

For a chemical interpretation of this picture, note that the r-electron density is concentrated between carbon atoms 1 and 2, also between carbon atoms 3 and 4. Thus, the predominant structure of butadiene has double bonds between these two pairs of atoms. Each double bond consists of a r-bond, in addition to the underlying a-bond. However, this is not the complete story, because we must also take into account the residual r-electron density between carbons 2 and 3 and beyond the terminal carbons. In the terminology of valence-bond theory, butadiene would be described as a resonance hybrid with the predominant structure CH2—CH-CH=CH2, but with a secondary contribution from CH2—CH= CH— CH2 . The reality of the latter structure is suggested by the ability of butadiene to undergo 1,4-addition reactions. 

In the polymerization of acrylates, interaction of lithium with carboxyl groups of incoming monomer and the penultimate unit of the polymer chain is the dominant feature of theories of stereoregulation [168,191]. The stable form of the chain end is proposed to involve secondary bonding of the lithium to the penultimate carboxyl group thus holding it in a fixed configuration, viz. 

The structure-determining role of the secondary bonds even in simple inorganic molecules and ions cannot be underestimated. Their formation results in increased coordination numbers and distortions of the coordination geometries. This matter was discussed in terms ofvalence bond and VSEPR theories, and a remarkable analysis of the coordination geometries o/Sb(III) halides influenced by secondary bonds could serve as a model for similar discussion of other main group elements. 

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. 

Adhesion is a subject with many important practical applications. The practical properties of an adhesive bond are a consequence of the bonding between the atoms and molecules involved (Adhesion fundamental and practical). Much of the theory of adhesion that can give an insight of practical importance makes use of concepts used in chemistry when discussing bonding between atoms and molecules. The distinction is widely made between strong primary bonds between atoms and weak secondary bonds between molecules. This article gives a brief explanation of terms commonly used for both. 

Adsorption theory of adhesion K W ALLEN Adsorption via primary or secondary bonds... 

Their disagreement was slight when Meyer reiterated their views in a second paper. Shortly afterward, in October, 1928, Staudinger formally criticized their views and dubbed their model "the new Micelle Theory" (15). The most important issues to Staudinger were the importance of secondary bonding to physical properties and priority. 

The entropic barrier theory was developed by Sadler and Gilmer (95,96). This latter theory is based upon the interpretation of kinetic Monte Carlo simulations and concomitant rate-theory calculations. The phrase Monte Carlo suggests chance events, or in this case, random motion. While individual motions of the molecules are governed by chance, they move according to rules laid out on the computer such as excluded volume considerations and secondary bonding energies and/or repulsive forces. 

Adsorption theory. The surface-active parts of the adhesive interact with the substrate, forming close secondary bonds and thus creating the adhesion. 

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