Covalent bonding description

Let us consider, for example, the relation between the geometry and the covalent bond description for a molecule of hydrogen cyanide, HCN (a deadly poisonous gas, by the way). The dot formula for this molecule... 

Pearson and Edwards noted that some substrates in Sn2 reactions seem more susceptible to the Bronsted basicity of a nucleophile, while other substrates seem more susceptible to its polarizability. Pearson proposed that bases be divided into two categories soft (polarizable) and hard (nonpolarizable). On the basis of equilibrium data, Pearson concluded that H, Li, Na, Mg, and Ca are hard acids, while Cu, Ag, Hg, I, Br, I2, and Br2 are soft. Pearson noted that hard acids show greater association with hard bases, while soft acids prefer soft bases. One explanation for this behavior is the ionic/covalent bond description, in which it is assumed that hard acids and hard bases are attracted primarily through ionic interactions, while soft acids and soft bases associate through covalent bonds. In addition, the two... 

Assuming that the 2c-2e covalent-bond description is sufihcient for the interpretation of chemical bonding within the framework of anionic clathrates, the... 

Interatomic potentials began with empirical formulations (empirical in the sense that analytical calculations based on them... no computers were being used yet... gave reasonable agreement with experiments). The most famous of these was the Lennard-Jones (1924) potential for noble gas atoms these were essentially van der Waals interactions. Another is the Weber potential for covalent interactions between silicon atoms (Stillinger and Weber 1985) to take into account the directed covalent bonds, interactions between three atoms have to be considered. This potential is well-tested and provides a good description of both the crystalline and... 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

Molecular orbital (MO) theory (Section 1.11) A description of covalent bond formation as resulting from a mathematical combination of atomic orbitals (wave functions) to form molecular orbitals. 

In order to attempt a more quantitative description one may start from the early theoretical considerations of Boudart9 who was first to tackle the problem of predicting the change in heats of adsorption with changing work function O. According to his early semiempirical electrostatic model when the work function of a surface changes by AO then the heat of adsorption, -AHad, of covalently bonded adsorbed species should change by ... 

Forty six years ago, on the basis mainly of empirical arguments, I formulated a description of the interatomic forces in metals (2) that had some novel features. I pointed out that according to this view the metallic bond is very closely related to the ordinary covalent (shared-electron-pair) bond some of the electrons in each atom in a metal are involved with those of neighboring atoms in an interaction described as covalent-bond... 

The Zintl-Klemm concept evolved from the seminal ideas of E. ZintI that explained the structural behavior of main-group (s-p) binary intermetaUics in terms of the presence of both ionic and covalent parts in their bonding description [31, 37]. Instead of using Hume-Rother/s idea of a valence electron concentration, ZintI proposed an electron transfer from the electropositive to the electronegative partner (ionic part) and related the anionic substructure to known isoelectronic elemental structures (covalent part), e.g., TK in NaTl is isoelectro-nic with C, Si and Ge, and consequenUy a diamond substructure is formed. ZintI hypothesized that the structures of this class of intermetallics would be salt-like [16b, 31 f, 37e]. 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

There are also molecules that are exceptions to the octet rule because one of the atoms has fewer, rather than more than, eight electrons in its valence shell in the Lewis structure (Figure 1.19). These molecules are formed by the elements on the left-hand side of the periodic table that have only one, two, or three electrons in their valence shells and cannot therefore attain an octet by using each of their electrons to form a covalent bond. The molecules LiF, BeCl2, BF3, and AIC13 would be examples. However, as we have seen and as we will discuss in detail in Chapters 8 and 9, these molecules are predominately ionic. In terms of a fully ionic model, each atom has a completed shell, and the anions obey the octet rule. Only if they are regarded as covalent can they be considered to be exceptions to the octet rule. Covalent descriptions of the bonding in BF3 and related molecules have therefore... 

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allowed us to explain isotopes (Chap. 3), the octet rule for main group elements (Chap. 5), ionic and covalent bonding (Chap. 5), and much more. However, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, why many of the transition metals have ions of different charges, how the shapes of molecules are determined, and much more. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

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