Ionic and covalent models

Kubicki J.D. and Lasaga A.G. (1988) Molecular dynamics simulations of Si02 melt and glass ionic and covalent models. Am. Mineral. 73, 941-955. 

In Table 4 we see some of the common ligands and their electron counts on the ionic and covalent models. In the former, we dissect an M-X bond into M+ and X and in the latter... 

Kubicki, J. D., and A. C. Lasaga (1988), Molecular Dynamics Simulations of Si02 Melt and Glass Ionic and Covalent Models, Am. Min. 73, 941-955. 

In the covalent model, the oxidation state of the metal, no, is equal to the charge left on the metal after having carried out a fictitious dissociation of the complex in which all the ligands take the two bonding electrons with them ( 1.1.2.2). For a complex whose general formula is [ML XJ, one therefore obtains no = x + q (see equations (1.2) and (1.3)). In the ionic formulation of this same complex, (see equation (1.6)), the charge on the metal is just equal to x + q, so the ionic and covalent models lead to the same oxidation state no for the metal. It follows that the same electronic configuration d is obtained by the two models, since n is equal to the number of valence electrons on the metal (m), minus its oxidation state no (equation (1.4)). 

In this chapter, we look at the 18-electron rule and at the ionic and covalent models that are commonly used in connection with electron counting. We then examine the ways in which binding to the metal can perturb the chemical character of a ligand, an effect that lies at the heart of organometallic chemistry. 

Hougen et discuss the estimation of thermodynamic properties from molecular structure data. Gambill, in numerous articles to be found in the four volumes listed, deals with methods for the prediction of heat capacities of liquids and gases, enthalpies of vaporization, fusion and sublimation, critical temperature and pressure, and p, K, T data, including liquid densities. Dasent < presents methods for the estimation of the standard Gibbs energy of formation of non-existent compounds and compounds of low stability by procedures based on the use of ionic and covalent models. 

Ionic and covalent models give the same electron count (Eq. 2.2). 

TABLE 23 Electron Counting on Ionic and Covalent Models... 

The concept of a chemical bond as a localized interaction between two neighboring atoms has been a central part of chemistry for the past century and a half, yet our current description of chemical bonds is still empirical it is a collage of ill-defined and largely incompatible models that are based on assumptions that do not always correspond to physical reality. The ionic and covalent models are mutually incompatible, and both the Lewis and orbital models have serious flaws [3, 4]. They do not conform to modem views of atomic stmcture, and consequently their predictions sometimes fail. While the bond valence theory belongs to this tradition of localized bond models, it is derived from a realistic, though simplified picture of the atom, one that is compatible with more sophisticated atomic descriptions. It can be used to derive powerful and quantitative theorems about chemical stracture. The mles of both the traditional ionic and covalent models can be derived as two special cases of this model (Sects. 5 and 7.2). 

Ionic and covalent bonding are two extreme models of the chemical bond. Most actual bonds lie somewhere between purely ionic and purely covalent. When we describe bonds between nonmetals, covalent bonding is a good model. When a metal and nonmetal are present in a simple compound, ionic bonding is a good model. However, the bonds in many compounds seem to have properties between the two extreme models of bonding. Can we describe these bonds more accurately by improving the two basic models ... 

Eaq and Caq are the tendency of acid A and base B to undergo ionic and covalent bonding, respectively. Equation (2) resembles that proposed by Drago et al. (18) to model heats of complex formation of acids and bases in solvents of low dielectric constant. Only Lewis acids of ionic radius greater than 1.0 A obey Eq. (2). For all smaller Lewis acids, a third pair of parameters has to be introduced ... 

Surprisingly, therefore, the same topological equations (3.3) and (3.4), provide a description of both ionic and covalent bonding. It does not therefore matter whether a bond is considered to be ionic or covalent in character since both have the same bond valence description. This leads to the important corollary the bond valence model cannot distinguish between ionic and covalent bonding. Within the model, the terms ionic bond and covalent bond are without any formal significance. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Fig. 15. Tangent-sphere models of CF4, SiF4, hypothetical CPI , and SiFg-, based on conventional ionic and covalent radii (columns 1 and 2) and the electride ion model (column 3)...

In the simplest model, bonding can be considered to result from the special stability associated with a filled outer shell of electrons. The noble gases, such as helium, neon, and argon, which already have a filled outer shell of electrons, have little tendency to form bonds. Atoms of the other elements, however, seek to somehow attain a filled outer shell of electrons. The two ways in which they accomplish this goal result in two types of bonding ionic and covalent. 

The resulting substance is sometimes said to contain an ionic bond. Indeed, the properties of a number of compounds can be adequately explained using the ionic model. But does this mean that there are really two kinds of chemical bonds, ionic and covalent ... 

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