Covalent contribution

The first two terms on the right-hand side have both eleetrons on the same eentre, they describe ionic contributions to the wave function, H+H . The later two terms describe covalent contributions to the wave function, H H. The HF wave function thus contains equal amounts of ionic and covalent contributions. The full Cl wave function may be written in terms of AOs as... 

The HF wave funetion eontains equal amounts of ionie and eovalent eontributions (Section 4.3), For covalently bonded systems, like H2O, the HF wave funetion is too ionie, and the effect of electron correlation is to increase the covalent contribution. Since the ionic dissociation limit is higher in energy than the covalent, the effect is that the equiUbrium bond length increases when correlation methods are used. For dative bonds, such as metal-ligand compounds, the situation is reversed. In this case the HF wave function dissociates correctly, and bond lengths are normally too long. Inclusion of... 

Thus, increasing the temperature exerts an influence on the fluoride complexes that is similar to the influence of outer-sphere cations and is expressed by an increase of the covalent contribution to the bonds between fluoride complexes and outer-sphere cations. 

Increasing the temperature generally promotes an increase in the covalent contribution to the bond between complex anions and outer-sphere cations ... 

Clearly not all these atomic and bond properties are independent of each other and it can be difficult to disentangle one from another. Nevertheless we will find these properties useful for discussing the properties of molecules, as we do for some typical molecules of the period 2 elements in this chapter. In particular, the amount of accumulated or shared density, which we assume is approximately measured by the bond critical point density, represents what is commonly called the covalent contribution to the bonding. The atomic charges represent what is commonly called the ionic contribution. 

Finally, it is of interest to compare the estimates of covalency contributions for Ir(IV) hexahalides deduced by Allen et al. (11) from spectroscopic data, with those obtained by Owen and Thornley (85, 86) from ESR results. These latter authors attributed the reduction of below the free-ion value, entirely to symmetry restricted covalency, deriving the expression 0bsd = N (Cd +s , >), where the normalising constant, N , is equal to (1 —4a S + and [Pg.153]

Note that the bond order index defined by Mayer accounts for the covalent contribution to the bond (this is why of late it is often mentioned as shared electron pair density index, SEDI). As such, the index cannot be expected to produce the integer values corresponding to the Lewis picture if a bond has a significant ionic contribution. The bond order index defined in this way measures the degree of correlation of the fluctuation of electron densities on the two atoms in question [7]. 

For soft cations, such as Ag+ and Pb2+, covalent contributions are much more important, and consequently the observed order of complex stabilities is quite different from that for alkali cations NH > O > S for Pb2+ and NH, S > O for Ag+. Dissection of the overall effect into enthalpy and entropy contributions (Table 15) reveals the complicated nature of the heteroatom effect. For K+ and Ba2+, the more favourable entropy contribution for N and S ligands is more than offset by the unfavourable change in enthalpy of binding. 

The electrostatic contribution is highly influenced by the dielectric constant of the medium since no relationship is found between the dielectric constant and the half-wave potential (not even for ions with noble gas structure, such as K ), it may again be concluded that covalent contributions should not be neglected in any solvation phenomena. 

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 . 

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