I. David Brown

Bond Valence Theory

This chapter shows that a unified concept of a chemical bond can be derived from a theoretical picture of the atom in which the Coulomb forces are described using the electric field rather than the electric potential. The localized chemical bond and its valence arises naturally from this picture, allowing the theorems of electrostatics to be used to describe the formation and properties of any chemical structure composed of localized bonds. There is no distinction made between ionic and covalent bonds. An empirical correlation links the theoretical bond valence to the experimental bond length. The resulting picture of chemical structure predicts where bonds will form, how long they will be, and in what direction they will point. It indicates the conditions for chemical stability, suggesting which reactions a compound might undergo either in solution or at a surface. Electronic anisotropies are handled in an ad hoc manner, in which the VSEPR theory of lone pairs is extended to cases where the lone pairs are inactive or only partially stereoactive. Steric constraints leading to compressed or stretched bonds are quantified by observing the difference between the real and theoretical structures. The potential of the bond valence theory is only beginning to be exploited.

Influence of pressure on the lengths of chemical bonds

An expression to describe the force that a chemical bond exerts on its terminal atoms is proposed, and is used to derive expressions for the bond force constant and bond compressibility. The unknown parameter in this model, the effective charge on the atoms that form the bond, is determined by comparing the derived force constants with those obtained spectroscopically. The resultant bond compressibilities are shown to generally agree well with those determined from high-pressure structure determinations and from the bulk moduli of high-symmetry structures. Bond valences can be corrected for pressure by recognizing that the bond-valence parameter, R(0), changes with pressure according to the equation dR(0)/dP = 10(-4) R(04)/(1/B-2/R(0) AA GPa (-1).

Topology and Chemistry

The determinants of chemical bonding are the chemical properties of the atoms and the constraints of three-dimensional (3-D) space into which the atoms must fit, but topology provides a convenient way of describing the resultant structure. This paper explores the topologies of various scalar fields associated with atoms in molecules and crystals and what they can tell us about chemical bonding. The scalar fields examined are the electron density, the electrostatic potential, and two simplified electrostatic potentials in which the contributions of the electron cores have been removed, namely the Madelung and the covalent field. Not all of the information contained in these fields is present in the topology but, since the topology is insensitive to the details of the field, it can often be determined using simplified calculations. Although the same topological model is used to explore all four fields, each field has its own distinctive topology and each provides different information about the nature of chemical bonding and structure. The analysis of these topologies, when combined with simple electrostatic theory and a few empirical observations, leads to a quantitative model of localized chemical bonding. In the process, the analysis provides insights into the nature of chemical bonding.

On measuring the size of distortions in coordination polyhedra.

There is no unique index that measures the size of the distortion found in a coordination polyhedron because different indices can result in a different ordering depending on the mode of the distortion (i.e. the third and higher moments of the bond-length distribution). This paper proposes the increase in the average bond length as a suitable index as this is directly related to the increase in volume of the coordination polyhedron and hence of the unit cell. Some examples are discussed.

Preparation, structure, and properties of mercury layer and chain compounds

In this paper is reviewed the preparation, structure, and electrical properties of salts of the polyatomic mercury cations Hg32+ and Hg42+, the infinite chain compounds Hg3−δMF6 (M=As, Sb, Nb, and Ta), and the layer compounds Hg3NbF6 and Hg3TaF6.

X-ray study of structural modulations in Cs2HgCl4

High resolution x-ray diffraction at temperatures from 7 to 300 K shows that at room temperature caesium tetrachloromercurate Cs2HgCl4 has the β-K2SO4 structure but on cooling exhibits a sequence of incommensurate and commensurate modulations along a* and c* axes. Two modulation phases, one of which is incommensurate, are found between 184 and 221 K with wavevectors along a* and, below 184 K, six further modulation phases, one of which is incommensurate, are found with wavevectors along c*.

Divalent metal halide double salts in equilibrium with their aqueous solutions: I. Factors determining their composition

Abstract Using a few simple assumptions based on the hard and soft acid and base model and the bond-valence model, we explain why the five most common double salt compositions xMeX 2 · yMe ′ X 2 · z H 2 O ( Me , Me ′ = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd; X = Cl, Br) found in equilibrium with the corresponding saturated solutions at room temperature will have x : y : z equal to 2:1:12, 1:1:8, 1:1:6, 1:2:6, and 1:4:10 and predict that these will belong to nine structural types. About 90% of the double salts observed under these conditions have one of these compositions and the remainder have closely related structures. We also make predictions about the chemical species expected in saturated solutions.

Divalent metal halide double salts in equilibrium with their aqueous solutions: II. Factors determining their crystal structures

Abstract The double salts of many divalent metal halides that exist in equilibrium with their saturated aqueous solutions are composed of discrete or polymerized complex ions in simple ratios (e.g., 1:1 or 1:2). By treating the complex ions as spheres, ellipsoids, or cylinders whose dimensions can be easily estimated, one can predict possible crystal structures for these compounds. The observed cell dimensions generally lie within 0.8 A of those predicted by the model. We show that the existence of some compositions can only be understood when crystal packing is considered. In particular, we are able to predict why interstitial water appears in some compounds.

What is the best way to determine bond-valence parameters?

Two recent systematic determinations of bond-valence parameters addressed the problem of the correlation between R 0 and b in different ways raising the question of which is to be preferred.

Are covalent bonds really directed

Abstract The flux theory of the chemical bond, which provides a physical description of chemical structure based on classical electrostatic theory, correctly predicts the angles between bonds, to the extent that they depend on the intrinsic properties of the bonded atoms. It is based on the justifiable assumption that the charge density around the nucleus of an atom retains most of its spherical symmetry even when bonded. A knowledge of these intrinsic bond angles permits the measurement and analysis of the steric angular strains that result from the mapping of the bond network into three-dimensional space. The work ends by pointing out that there are better ways of characterizing bonds than describing them as covalent or ionic.