Covalence degree

The high covalent degree of the carbon-zinc bond and the small polarity of this bond leads to a moderate reactivity of these organometallics toward many electrophiles. Only powerful electrophiles react in the absence of a catalyst. Thus, bromolysis or iodolysis... 

The high covalent degree of the carbon-zinc bond and the small polarity of this bond lead to a moderate... 

It has been observed that Newman s method for determining intrinsic parameters A can be used for classifying the compounds according to their covalency degree. 

Although analytic expressions for the potential constants exist, they are rarely calculated directly. The covalency degree, uncertainties of effective ligand charges and other conceptual drawbacks make such an approach problematic. The potential constants are more often taken as free parameters of the theory which enter the final formulae of electron spectroscopy, electron spin resonance and magnetochemistry. The potential constants in different representations of the crystal field potential obey simple proportionality relationships which can be found in special monographs [10-13]. For example, the potential expressed through the Racah operators... 

The particles interaction in ionic lattices is manifested by increasing the bond covalence degree. This deformation can be intuitively represented by the modification of the s mimetry of the electronic cloud of a particle imder the action of another particle, i.e., by polarization notion. 

From the presented examples the existence of some different situations of polarization is indicated, as based on the type of combination studied. Thus, in the alkaline earth oxides the polarization is symmetrical around each particle, and therefore, the effect is the decrease of coordination number simultaneously with the increase of covalent degree of bond. 

Typical results for a semiconducting liquid are illustrated in figure Al.3.29 where the experunental pair correlation and structure factors for silicon are presented. The radial distribution function shows a sharp first peak followed by oscillations. The structure in the radial distribution fiinction reflects some local ordering. The nature and degree of this order depends on the chemical nature of the liquid state. For example, semiconductor liquids are especially interesting in this sense as they are believed to retain covalent bonding characteristics even in the melt. 

For other compounds, the agreement is not always so good. The assumption that the lattice is always wholly ionic is not always true there may be some degree of covalent bonding or (where the ions are very large and easily distorted) some appreciable van der Waals forces between the ions (p.47). 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Since the peptide units are effectively rigid groups that are linked into a chain by covalent bonds at the Ca atoms, the only degrees of freedom they have are rotations around these bonds. Each unit can rotate around two such bonds the Ca-C and the N-Ca bonds (Figure 1.6). By convention the angle of rotation around the N-Ca bond is called phi (<[)) and the angle around the Ca-C bond from the same C atom is called psi (y). 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

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