Ideal covalent bonding

All the inconsistencies can be traced back to the often incompatible requirements of electron-pair covalency as the manifestation of the ideal covalent bonding state on the one hand, and covalency as the opposite to ionicity on the other. On the basis of the ideas advocated in Section III. 1, it is comparatively easy to choose between the two alternatives and give the definition ... 

Thus, (4.9a) gives the general condition for an idealized covalently bonded closed-shell Lewis-like duodectet structure (4.9b) with no formal charge on the central metal atom. (The more general conditions for coordinative [dative] ligands and other departures from the idealized Lewis-like formula will be discussed below.)... 

This is only rigorous for an ideal covalent bond. For heteronuclear interactions, one intuitively recognises that the more electronegative element will attract a greater share of the overlap density. The Mulliken procedure ignores... 

Organic molecules are generally composed of covalent bonded atoms with several well-defined hybridization states tending to have well-understood preferred geometries. This makes them an ideal case for molecular mechanics parameterization. Likewise, organic molecules are the ideal case for semiempirical parameterization. 

The shortest cation-anion distance in an ionic compound corresponds to the sum of the ionic radii. This distance can be determined experimentally. However, there is no straightforward way to obtain values for the radii themselves. Data taken from carefully performed X-ray diffraction experiments allow the calculation of the electron density in the crystal the point having the minimum electron density along the connection line between a cation and an adjacent anion can be taken as the contact point of the ions. As shown in the example of sodium fluoride in Fig. 6.1, the ions in the crystal show certain deviations from spherical shape, i.e. the electron shell is polarized. This indicates the presence of some degree of covalent bonding, which can be interpreted as a partial backflow of electron density from the anion to the cation. The electron density minimum therefore does not necessarily represent the ideal place for the limit between cation and anion. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Table 4.6. Geometries and NBO descriptors 0/MH2 and MH3 metal hydrides of the third transition series of various spin multiplicities (IS + I), illustrating the correlations of metal charge (Qu) with metal hybrid d character (%d, taken as the average of a and 3 hybrids for open-shell species), bond length (Ruw) and angle (9hmh)> and average absolute deviation (Dev. = average %mh — 90" ), from idealized covalent geometry...

Hydrogen-bonding has a huge influence on the physical properties of molecules. Boiling is the conversion of a liquid (where the molecules are free to move, but linked by intermolecular bonds) to a gas, where (in an ideal gas) the molecules are so distant from each other that they do not interact. Boiling, therefore, does not break the strong covalent bonds within molecules, but rather the weaker intermolecular bonds between them. 

A comparison of the covalent connectivity associated with each of these architecture classes (Figure 1.7) reveals that the number of covalent bonds formed per step for linear and branched topology is a multiple (n = degree of polymerization) related to the monomer/initiator ratios. In contrast, ideal dendritic (Class IV) propagation involves the formation of an exponential number of covalent bonds per reaction step (also termed G = generation), as well as amplification of both mass (i.e. number of branch cells/G) and terminal groups, (Z) per generation (G). 

Mathematically, the number of covalent bonds formed per generation (reaction step) in an ideal dendron or dendrimer synthesis varies according to a power... 

Owing to its single composition and pure covalent bonding, diamond is a standard solid material, and takes the role of the most appropriate sample in explaining the effects upon structure-sensitive properties when the structure of solid material deviates from the ideal state. 

Of course, when multiple pairs of electrons participate in double or triple covalent bonds, those electrons stay within the same bonding axis. Lone pairs repel other lone pairs more strongly than they repel bonding pairs, and the weakest repulsion is between two pairs of bonding electrons. Two lone pairs separate themselves as fcir apart as they can go, on exact opposite sides of an atom if possible. Electrons involved in bonds also separate themselves as far apart as they can go but with less force than two lone pairs. In general, all electron pairs try to maintain the maximum mutual separation. But when an atom is bonded to many other atoms, the ideal of maximum separation isn t always possible because bulky groups... 

The bond energy for the complex M—A is related to the energies of the ideal ionic and covalent bonds. Hi and He, respectively, and the fraction Cf of the ionic bond in the adsorption bond, by the expression 128)... 

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