Bond valence concept

Steiner, Th. (1995a). Lengthening of the N-H bond in N-H... N hydrogen bonds. Preliminary structural data and implications of the bond valence concept. Chem. Comm. 1331-2. 

When only little information is available on both the surface structure and the type of complexes formed it is reasonable to investigate the adsorption with the simple one-pKn model of a pseudo homogeneous surface and to describe the complexation with Eqs. (55) to (59). Review [29] and Refs. [52-54] can be consulted for details of this type of description. In review [29] also the bond valence concept is used to describe complexation. No attempts have been made to predict complexation constants so that these have to be treated as adjustable parameters. 

The numerical precision of the bond-valence concept can be further improved if the universality of the scaling length (B = 0.37 A) is abandoned, thus leaving one more parameter for fitting the experimental data. Research by Adams [45] indeed shows that B depends somewhat on the chemical nature of the atoms involved (e.g., ionization energy, electron affinity, polarizability, etc.), and a corresponding set of improved bond-valence parameters has been proposed, taking into account the bond softness. The latter idea is taken from the theory of (absolute) chemical hardness by Pearson, later covered in Section 2.14. 

Applying the time-dependent bond-valence concept for AIN yields excellent numerical results. Under standard conditions, AIN crystallizes with the wurtzite structure in the hexagonal system and, just as for cubic zinc-blende AIN, there is tetrahedral coordination for both aluminum and nitrogen. For a correct structural simulation, a bond-valence parameter of To = 1.786 A for the Al-N combination (close to the tabulated one from Table 1.4) ensures that the experimental Al-N distances are exactly reproduced. After having set up a supercell with 576 Al and N atoms, all atoms are randomly shifted by ca. 0.5 A to partially destroy the structure. When they are given kinetic energies corresponding to room temperature, however, some thousand time steps of... 

Brown ( ) extended the bond valence concept to individual ions and ligands and defined the residual bond valence as a measure of the acid or base strength depending on whether the... 

The calculation consists in calculating the electrostatic term and in describing explicitly the parameters involved in the constants K p. The use of the bond valence concept simplifies the calculation by providing a way of removing the influence of other ligands in the coordination sphere. 

The use of the bond valence concept to model the stmctures of perovskites is already well established and has been put to a number of practical uses over the last decade. The crystal structures obtained from modeling using bond valence concepts can be used for a variety of purposes including, but not limited to (1) starting points for Rietveld refinements, (2) an initial structure model for density functional theory (DFT) calculations, (3) estimate the structure stability of target compositions for the synthesis of new materials, and (4) investigating of the nature of the chemical bonding present in extended solids. The extensirai of this approach to pyrochlores. 

The discussions above aim to show that the success of the bond valence concept has a physical basis, as bond valences can be understood as a mass (or principal quantum number n, or atomic number,. ..) dependent functional of electron density. The particular close correlation with the electron density at the bond critical point in predominantly ionic compounds (or more generally in compounds for which the Laplacian V bq, remains >0) also provides a better understanding why the bond valence concept though in principle applicable to a wide range of bond types and originally building on Pauling s concepts for covalent compounds or metals, is more appropriate for at least partially ionic compounds. 

Our principal goal has been to translate the deepest truths of the Schrodinger equation into a visualizable, intuitive form that makes sense even for beginning students, and can help chemistry teachers to present bonding and valency concepts in a manner more consistent with modern chemical research. Chemistry teachers will find here a rather wide selection of elementary topics discussed from a high-level viewpoint. The book includes a considerable amount of previously unpublished material that we believe to be of broad pedagogical interest, such as the novel Lewis-like picture of transition-metal bonding presented in Chapter 4. 

These results can be interpreted successfully in terms of Pauling s valence bond order concept. In the framework of this model, a chemical bond between X and H in diatomic molecule XH or between H and B in a HB molecule can be characterized by empirical valence bond orders Pxh and Phb decreasing exponentially with bond distance ... 

The loss of outer electrons from the atoms to a conduction band, in such a simple picture, bears a similarity to the redox process which is supposed to occur in molecular bonds, and which is at the basis of the oxidation state or formal valence concept. If this is the case, the metallic valence should coincide with one of the valences that one encounters in compounds, possibly with the most stable one. 

This book is divided into four parts. Part I provides a theoretical derivation of the bond valence model. The concept of a localized ionic bond appears naturally in this development which can be used to derive many of its properties. The remaining properties, those dependent on quantum mechanics, are, as in the traditional ionic model, fitted empirically. Part II describes how the model provides a natural approach to understanding inorganic chemistry while Part 111 shows how the limitations of three-dimensional space lead to new and unexpected properties appearing in the inorganic chemistry of solids. Finally, Part IV explores applications of the model in disciplines as different as condensed matter physics and biology. The final chapter examines the relationship between the bond valence model and other models of chemical bonding. 

The concept of bond valence, which, as will be shown below, is the same as the bond flux derived in Chapter 2, grew out of attempts to refine Pauling s principles determining the structures of complex ionic crystals (Section 1.7). In this empirical evolution of Pauling s model, both the electrostatic and short-range components were developed simultaneously. Only later did it become apparent that it was also possible to derive the properties of the electrostatic component independently using the ionic theory. 

An approach to chemical bonding that is currently attracting attention is that based on an analysis of electron densities calculated from quantum mechanics or measured using X-ray diffraction. Since the electron density shows how the electrons are distributed, it gives a better physical picture of the nature of chemical bonding than other models. It has been admirably described by Bader (1990) and, for inorganic solids, by Pendas et al. (1997, 1998) and Luana et al. (1997), but it is only necessary here to give a brief account of the approach to show why it is difficult to relate its concepts to those of the bond valence model. 

Jansen, L. and Block, R. (1991). On the concept of bond valence sums and its applicability in the analysis ofhigh-T superconductors. Physica C181,149-59. 

Tho author has repeatedly [6j pointed out that the valency conception of non-localized bonds (resonance among several valence-bond structures, many centre molecular orbitals) lias no reality and only arises from a neglect of electronic repulsions. It is noteworthy that on this basis the possibility of a contribution of structures IIIc and d (IVc %od d) to Ufa and b (IVa and b) and vice versa would not enter and, thus, this objection against the discussed interpretation of the hydrogen bond would be removed ... 

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