Covalent Models

Consider the closely related ion [FeCHiO/e] ". The only difference is in the formal oxidation state of the metal ion. If an ionic model is assumed (9.6), the charge on the metal center is +2. A purely covalent model results in the placing of a formal quadruple negative charge upon the iron center (9.7). To satisfy the electroneutrality principle, and establish a near-zero charge on the metal, each oxygen atom is... 

On the basis of the shell model, two apparently different models of the chemical bond were proposed, the ionic model and the covalent model. 

Formal charge and oxidation number are two ways of defining atomic charge that are based on the two limiting models of the chemical bond, the covalent model and the ionic model, respectively. We expect the true charges on atoms forming polar bonds to be between these two extremes. 

Sharma, R.R., Das, T.P., and Orbach, R. 1967. Zero-field splitting of S-state ions. II. Overlap and covalency model. Physical Review 155 338-352. 

R. S. Drago. Applications of Electrostatic-Covalent Models in Chemistry. Surfside Gainesville, 1994. 

Vanadium(n) Complexes.—Dehydration of VSO. THjO has been shown to proceed via the formation of VS04,mH20 (where n = 6, 4, or 1) and V(OH)-(SO4), which were characterized by X-ray studies. The polarographic behaviour and the oxidation potential of the V -l,2-cyclohexanediamine-tetra-acetic acid complex, at pH 6—12, have been determined.Formation constants and electronic spectra have been reported for the [Vlphen),] " and [V20(phen)] complexes. The absorption spectrum of V ions doped in cadmium telluride has been presented and interpreted on a crystal-field model. The unpaired spin density in fluorine 2pit-orbitals of [VF ] , arising from covalent transfer and overlap with vanadium orbitals, has been determined by ENDOR spectroscopy and interpreted using a covalent model. " ... 

As mentioned above, the initial model (39) was chosen so as to break the enthalpy of adduct formation into electrostatic and covalent contributions, i.e., — AH =EaEb- -CaCb. The fact that more than one solution can exist enables us to attempt to break-up the enthalpy of adduct formation into a physically meaningful model other than the electrostatic and covalent model initially employed. One apparently obvious break-up of the enthalpy of adduct formation which is of chemical interest would be that of sigma, tr, and pi, n, contributions ... 

In our original work, we used an ionic-covalent model to interpret the E and C parameters. It has been suggested that our E and C parameters are a quantitative manifestation of the hard-soft model. "Softness (or hardness") can be considered (67) as a measure of the ratio of the tendency of a spedes to undergo covalent interaction to the tendency of the species to undergo electrostatic interaction. The relative "softness or hardness is depicted in the C/E ratio. The ratios for the acids and bases can be calculated from the data in Tables 3 and 4. If the ratio C/E is comparatively large, the add or base would be classified as type B or soft. Inasmuch as the relative ratios of C/E tells the relative importance of the two effects for various donors and acceptors, we agree that the hardness or softness discussed in the HSAB model is given by this ratio. 

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. 

The usually accepted approach is to adopt an ionic model for the superoxide ion on the surface. In this model, an electron is transferred from the surface to the oxygen to form 02, and there is an electrostatic interaction between the cation at the adsorption site and the superoxide ion. A calculation of the g tensor based on this model (Section 111,A,1) accounts for nearly all the data from adsorbed 02 and is consistent with the evidence that the spin density on both oxygen nuclei is the same (Section III,A,2). However, there are examples of oxygen adsorbed on the surface where the g values do not fit the predictions of the ionic model (Section IV,E) and also a few cases where the spin density on the two oxygen nuclei is found to be different. In these situations it seems likely that a covalent model in which a a bond is formed between the cation and the adsorbed oxygen, is more relevant. 

There are halides for which it is impossible to construct a strictly covalent model with all atoms in octet configuration, because they contain too many halogen atoms, e.g. PF5, SF6, IF7, PC15, SC14, SeCl4 and IC13. If, in PF5, five P F bonds are accepted, ten electrons would belong to each P atom the S atom in SF6 would have 12 electrons, and the I atom in IF7 14 electrons. 

CFCM central-field covalency model L total orbital angular momentum... 

SRCM symmetry-restricted covalency model r linewidth... 

The reduction of the free-ion parameters has been ascribed to different mechanisms, where in general two types of models can be distinguished. On the one hand, one has the most often used wavefunction renormalisation or covalency models, which consider an expansion of the open-shell orbitals in the crystal (Jprgcnscn and Reisfeld, 1977). This expansion follows either from a covalent admixture with ligand orbitals (symmetry-restricted covalency mechanism) or from a modification of the effective nuclear charge Z, due to the penetration of the ligand electron clouds into the metal ion (central-field covalency mechanism). 

The symmetry-restricted covalency model (SRCM) leads to AFk (x N4 and Af a IV2,... 

In most high-pressure experiments, only the covalency models have been taken into account. The likely reason for this is the arbitrary nature of the sphere radius Rs used in the electrostatic model. 

Nearly identical results were obtained also for Y202S Eu3+ (Liu et al., 1998a) and Gd2C>2S Eu3+ (Chi et al., 1998a, 1998b). In both cases the reduction of the free-ion parameters was about -0.40% for F2 and -0.20% for f up to pressures of 13 GPa and 14.8 GPa, respectively. Considering the two different covalency models, the authors concluded that the SRC model describes the nephelauxetic effect more reasonably than the CFC model. 

Similar to the rare-earth trichlorides, also different ternary MYX compounds have been studied thoroughly under high pressure. The results for the pressure-induced changes of the Slater parameter F2 and the spin-orbit coupling parameter of these and other compounds are presented in table 5. Due to the difficulties with the DS model, the evaluation of the parameter shifts has been performed only in terms of the two covalency models. Assuming small changes for the free-ion parameters, the relative changes were approximated by ... 

Clearly the ionic model (first column) prohibits too much it prohibits, Sidgwick notes, "even so stable a molecule as carbon tetrafluoride. On the other hand, the covalent model (second column) allows too much it allows, Sidgwick observes, "ample room, not only for the number of attached atoms in actual stable molecules, but for far more than are ever found even in the imaginary Cl6 there would be space left between the iodine atoms. ... 

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