Covalent bond directional character

The common feature of both layer types is the character of the boundary sheets. The boundary sheets of a sandwich compound as well as both sheets of a two-dimensional polymer consist of ions that are able to form covalent bonds directed... 

Similarly, in studies of lamellar interfaces the calculations using the central-force potentials predict correctly the order of energies for different interfaces but their ratios cannot be determined since the energy of the ordered twin is unphysically low, similarly as that of the SISF. Notwithstcinding, the situation is more complex in the case of interfaces. It has been demonstrated that the atomic structure of an ordered twin with APB type displacement is not predicted correctly in the framework of central-forces and that it is the formation of strong Ti-Ti covalent bonds across the interface which dominates the structure. This character of bonding in TiAl is likely to be even more important in more complex interfaces and it cannot be excluded that it affects directly dislocation cores. 

This type of argument leads us to picture a metal as an array of positive ions located at the crystal lattice sites, immersed in a sea of mobile electrons. The idea of a more or less uniform electron sea emphasizes an important difference between metallic bonding and ordinary covalent bonding. In molecular covalent bonds the electrons are localized in a way that fixes the positions of the atoms quite rigidly. We say that the bonds have directional character— the electrons tend to remain concentrated in certain regions of space. In contrast, the valence electrons in a metal are spread almost uniformly throughout the crystal, so the metallic bond does not exert the directional influence of the ordinary covalent bond. 

Ions stack together in the regular crystalline structure corresponding to lowest energy. The structure adopted depends on the radius ratio of cation and anion. Covalent character in an ionic bond itnposes a directional character on the bonding. 

To sum up all the experimental evidence whereas AU11L7X3 and smaller molecular cluster compounds are covalently bonded and show no tendency toward metallic binding, the bonding in Au55(PPh3),2Cl6 is delocalized, non-directional, and substantially metallic in character. 

The concepts which we need for understanding the structural trends within covalently bonded solids are most easily introduced by first considering the much simpler system of diatomic molecules. They are well described within the molecular orbital (MO) framework that is based on the overlapping of atomic wave functions. This picture, therefore, makes direct contact with the properties of the individual free atoms which we discussed in the previous chapter, in particular the atomic energy levels and angular character of the valence orbitals. We will see that ubiquitous quantum mechanical concepts such as the covalent bond, overlap repulsion, hybrid orbitals, and the relative degree of covalency versus ionicity all arise naturally from solutions of the one-electron Schrodinger equation for diatomic molecules such as H2, N2, and LiH. 

An obvious use of an electronegativity scale is to predict the direction of electrical polarity of a covalent bond with ionic character. Table 2.2 tells us that the C—H bond in alkanes (CnH2n+2) is polar in the same sense as the 0—H bonds in water, although to a much lesser degree ... 

The properties of a substance depend in part upon the type of bonds, between the atoms of the substance and in part upon the atomic arrangement and the distribution of the bonds. The atomic arrangement is itself determined to a great extent by the nature of the bonds the directed character of covalent bonds (as in the tetrahedral carbon atom) plays an especially important part in determining the configura-... 

All ionic bonds have some covalent character. To see how covalent character can arise, consider a monatomic anion (such as Cl ) next to a cation (such as Na ). As the cation s positive charge pulls on the anion s electrons, the spherical electron cloud of the anion becomes distorted in the direction of the cation. We can think of this distortion as the tendency of an electron pair to move into the region between the two nuclei and to form a covalent bond (Fig. 2.11). Ionic bonds acquire more covalent character as the distortion of the electron cloud on the anion increases. 

Following Pauling, we have admitted extra orbitals on monovalent atoms involved in molecular systems with some metallic character or very delocalized bonds. In conjunction with a spin-free valence bond formalism, these extra orbitals have allowed us to devise new kinds of VB structures, the Pauling s structures, as we call them. These structures permit the monovalent atoms to form two covalent bonds simultaneously, as a consequence of electron transfer from neighbors and, thus, give information about delocalization of charge in the system, that is not directly inferred from the usual Kekule or ionic structures. Therefore, the Pauling s structures complement the VB description of molecular systems. 

Covalent Bonding Covalent bonds have directional character. Such bonds are generally formed by non-metallic elements. In such compounds the structures can be rationalised by coordination number and bonding geometry of atoms present, e.g.,... 

What has been said about the directional character of bonds in discrete ions or molecules applies, to a large extent, to covalent bonds in crystals for, as we shall see (Chap. 12), many crystals are conveniently regarded as being made up of continuous networks of complexes. 

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