Polarization and covalency

B. G. Lake, H. Gaudin, R. J. Price, D. G. Walters, Metabolism of [3-14C]Coumarin to Polar and Covalently Bound Products by Hepatic Microsomes from the Rat, Syrian Hamster, Gerbil and Humans , Food Chem. Toxicol. 1992, 30, 105- 115. 

Te2" and highly charged ones such as As3" and P3 are especially prone to polarization and covalent character. 

The Shannon-Prewitt ionic radii (r+ + r ) are based on the most ionic compounds, the fluorides and oxides for the radii of the metal cations, and the alkali hahdes for the radii of the anions of the remaining halides. The shortening of silver halide bond lengths is attributahle to polarization and covalency. 

I have not described the calculation of the eigenvalues, which requires the solution of the equations of motion and therefore a knowledge of the force constants. The shell model for ionic crystals, introduced by Dick and Overhauser (1958), has proved to be extremely useful in the development of empirical crystal potentials for the calculation of phonon dispersion and other physical properties of perfect and imperfect ionic crystals. There is now a considerable literature in this field, and the following references will provide an introduction Catlow etal. (1977), Gale (1997), Grimes etal. (1996), Jackson et al. (1995), Sangster and Attwood (1978). The shell model can also be used for polar and covalent crystals and has been applied to silicon and germanium (Cochran (1965)). 

The ratio t/as/2yag is considered to be a measure for the tendency toward spreading of phase a across the surface of phase s (eq 7). In a microscopic picture, the same ratio must also be a measure of the relative strengths of interactions between the atoms of phase a and those of phase s in the interface and among the atoms of phase a themselves. The interactions between the two phases arc complex in nature and may involve dispersion, polar, and covalent interactions depending on the chemical nature of the two phases. 

Of course the bonding situation for Tb + in BaTbQs is possibly more complicated than for Mn + in MnO because the 4/ electrons are almost certainly more tightly bound than the oxygen 2p electrons and spin polarization and covalency effects involving the outer s, p and d orbitals may also play a role [155). Calculations for Tb + and other rare earth ions are in progress (75(5). 

Semi-conductors generally chemisorb hydrogen in a number of forms among which we may distinguish again molecular, polar and covalent states, e.g. hydrogen on zinc oxideand on chromia. ... 

The halogens, particularly fluorine, have very high electronegativity values (see Table 20.17). They tend to form polar covalent bonds with other nonmetals and ionic bonds with metals in their lower oxidation states. When a metal ion is in a higher oxidation state, such as +3 or +4, the metal-halogen bonds are polar and covalent. For example, TiCl4 and SnCl4 are both covalent compounds that are liquids under normal conditions. 

Because they are non-polar and covalent compounds, the alkanes are soluble in non-polar solvents such as benzene or ethoxyethane (diethyl ether) and insoluble in water. 

The shell model can allow the simulation of a significantly covalent material using an ionic model because of the similarity between polarization and covalency - both are just shifts in the electron density distribution, but with different partitioning. In this case, the dipolar shell model can subsume covalent effects because of the low symmetry at oxygen. 

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