Covalent component

With increasing B content, the covalent component of the bonding in boride lattices increases owing to the appearance of direct B—B bonds and a decrease in the metallic bond character, e.g., in the structural series of the CUAI2 family ... 

It can be seen from Table 1 that the compounds LiF and NaF have almost pure ionic character. Magnesium oxide MgO, however, has a more complicated bonding character with considerable contribution of the covalent component. 

Thus it is evident from all these studies that the nature of the C—Li bond varies from compound to compound hence any generalization of the nature of bonding is to be taken cautiously. As Schleyer and Streitwieser have discussed in the past, the C—Li bond is essentially ionic however, the covalent components cannot be neglected . The unnsnal behavior of the C—Li bond has been a subject of discussion from the initial years of applying theoretical methods, and the debate continues in an interesting manner due to the developments of new theoretical methodologies. In fact, we support the implications of Bickelhaupt that there is a covalent contribution to the C—Li bonding, however small this turns out to be in specific examples . 

The truth lies somewhere between these two extremes, and the assessment of covalent and ionic contributions to bonding has attracted considerable attention. Pauling devised the electroneutrality principle and suggested that the relative importance of ionic and covalent components was such that the overall charge on any one atom did not exceed 1. In the context of the [Cr(H20)6]3+ ion, this leads to a description in which the metal centre is neutral and each oxygen bears a charge of + 2. The Cr-0 bonds may be described as 50 % covalent. 

Ion-dipole interactions also include coordinative bonds, which are mostly electrostatic in nature in the case of the interactions of nonpolarisable metal cations and hard bases. Coordinate (dative) bonds with a significant covalent component, as in [Ru(bpy)3]2+, are also often used in supramolecular assembly and, as we will see in Chapters 10 and 11, the distinction between supramolecular and molecular species can become rather blurred. 

The contraction of borazole leads to an enhancement of the polarity of all bonds (endocyclic B-N and exocyclic B-H and N-H) (Table 13), in contrast to the benzene molecule, whose contraction is accompanied by an increase in the covalent component of the C-H bonds. The charge transfer between the C84 cage and the B3N3H6 and C6H6 guests is small. 

For convenience and to avoid confusion, we will symbolize a purely covalent bond between A and B centers as A — B, while the notation A—B will be employed for a composite bond wave function like the one displayed in Equation 3.4. In other words, A—B refers to the real bond while A — B designates its covalent component. 

Thus, the ionic component has decreased while the covalent component has increased in Pmo-ci, relative to the Hartree—Fock configuration o-o-. ... 

In the case where the fragments A and B are polyatomic (e.g., A = B = H3C, in H3C-CH3) and only the A-B bond is being described in a VB manner, the orbitals that are used to construct the determinants may be FOs that have tails on the adjacent atoms (e.g., the H atoms of a given CH3 group), but remain localized in the sense that an orbital of fragment A has no coefficient on B, and vice versa. In such a case, the description of the A-B bond will still require explicit consideration of ionic and covalent components, and the so-defined FOs will be considered throughout the text as a particular case of localized orbitals. 

Non dynamical electron correlation is the part of the total correlation that is taken into account in a CASSCF calculation that correlates the valence electrons in valence orbitals. Physically, the non dynamical electron correlation is a Coulomb correlation that permits the electrons to avoid one another and reduce their mutual repulsion as much as possible with respect to a given zero order electronic structure defined by the Hartree-Fock wave function. In VB terms, the non dynamical correlation ensures a correct balance between the ionic and covalent components of the wave function for a given electronic system. The dynamical correlation is just what is still missing to get the exact nonrelativistic wave function. 

One of the most valuable features of theoretical methods based on classical VB structures is their ability to calculate the energy of a diabatic state. For practical uses, some diabatic bond energy curves of chemical interest can be, for example, the separate dissociation energy curves of the ionic and covalent components of a bond, or the energy curves of the effective VB structures of a... 

Although the cohesive forces in such an idealized metal as described would be nondirectional (as in ionic solids), the orientation effects of d orbitals contribute a directional-covalent component to the bonding in transition metals that requires a more sophisticated definition for metallic bonding. The intemuclear distances in the close packed, or nearly close packed, stmcmres of most metalhc elements ate small enough that the valence orbitals on the metal atoms can overlap (in the valence-bond model) or combine to form COs (in the MO or Bloch model). 

The above division should not be taken too literally. In fact, most of the metal hydrides have a mixture of different types of bonding. Eor example, in LiH the bonds are mainly ionic but still have a significant covalent component [35]. 

Note that this bond can be viewed as having an ionic, as well as a covalent, component. The electrostatic attraction between the partially charged H and X atoms will lead to a greater bond strength. Thus (H—X)act will be... 

The three possible types of bonds (a) a covalent bond formed between identical atoms (b) a polar covalent bond, with both ionic and covalent components and (c) an ionic bond with no electron sharing. 

Quantitatively the MC simulations predict a 2.1-kcal/mol barrier between the contact and solvent-separated ion pairs, with the latter about 4 kcal/mol lower in free energy. These results should be considered preliminary on several counts. In particular, greater study of the dependence of the results on the cutoff procedure for the potential functions is desirable. Also, the constancy of the potential function parameters for between 2.5 and 7 A is a questionable approximation. However, some compensation in changes between the ion-ion and ion-water interactions is expected, that is, if the ion-ion interaction has a covalent component, the increased attraction would be somewhat offset by... 

---