Bonding theories electrostatic

The model that largely replaced valence bond theory for interpreting the chemistry of coordination compounds was Ihe crystal field theory, first proposed in 1929 by Hans Bethe.11 As originally conceived, it was a model based on a purely electrostatic... 

Note the differences between crystal field theory and valence bond theory. In crystal field theory, there are no covalent bonds, no shared electrons, and no hybrid orbitals—just electrostatic interactions within an array of ions. In complexes that contain neutral dipolar ligands, such as H20 or NH3, the electrostatic interactions are of the ion-dipole type (Section 10.2). For example, in [Ti(H20)g]3+, the Ti3+ ion attracts the negative end of the water dipoles. 

To justify such a description within quantum mechanics we are led into a consideration of ionic and covEilent contributions to an approximate wave function. If covalent contributions are minor, the bond is said to be ionic in character, and the electrostatic model is considered to be applicable. Unfortunately, the terms ionic character and covalent character are used with various meanings. This is so, in part, because the rapid development of chemical bond theory has caused a drift of the meanings of these terms over the past two decades. Pauling s definitions, as presented in his book (1585, p. 48), no doubt represent the intent of most workers as of 1940. He concluded that there is a covalent bond between two atoms X and Y if the dissociation energy of X—is the mean of the dissociation energies of X— X and Y— Y. If the dissociation energy of X— Y exceeds this mean, the excess is attributed to additional ionic character of the bond. This criterion furnishes the basis for his scale of electronegativity, and ionic character is inter-... 

N. D. Coggeshall. J. Chem. Phys. 18, 978-83 (1950). Theory electrostatic calculation of intensity change, energy of H bond. 

M. Magat. J. chim. phys. 52, 272-8 (1955). IR, Raman review, and H bond length, electrostatic theory. 

One phenomenon not well accounted for by other approaches is seen in Table 6-8. It shows a series of four acids and five bases in which both E and C increase. In most descriptions of bonding, as electrostatic (ionic) bonding increases, covalent bonding decreases, but these data show both increasing at the same time. Drago argued that this means that the E and C approach explains acid-base adduct formation better than the HSAB theory described earlier. 

The participation of fluorine is less common, but several fluoride-containing trihalide ions have been isolated as crystalline salts (Table 17,3). The triiodide ion presents exactly the same problem to classical bonding theory as does xenon difluoride, and although the triiodide ion was discovered in 1819, only eight years after the discovery of iodine itself, chemists managed to live with this problem for almost a century and a half without coming to grips with it. The explanation offered most often was that the interaction was electrostatic—an ion-induced dipole interaction. The existence of symmetrical triiodide ions as well as unsymmetrical triiodide ions makes this interpretation suspect, and the existence of ions such as BrF and IF makes it untenable. 

Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

The interatomic forces between atoms result in an atomic aggregate with sufficient stability to form chemical bonds within a molecule. According to the valence bond theory, a chemical bond is formed when an electron in one atomic orbital pairs its spin with that of an electron supplied by another atomic orbital, these electrons are then shared between two or more atoms so that the discrete nature of the atom is lost. Three main types of chemical bond are considered covalent, electrostatic (ionic) and metallic bonds. 

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