Screening and Bond Order

The relationship between nuclear charge and atomic wave functions is of the form Z oc //2/3. Core-charge imbalance could therefore be compensated for with an effective charge of Ze = re2/3 on one of the atoms. For p-block atomic pairs, this screening factor does indeed lead to the correct solution. For hydrides of p-block elements in periodic row n, compensating charge is defined as Ze = (knx/n)2/3, with screening constants / g = 0.84 and kz = 0.70. 

The observation that point-charge simulation reproduces the exact same result as a Heitler-London calculation, but only for first-order bonds, confirms the previous conclusion that covalent interactions are mediated by the sharing of a maximum two electrons per bond, allowed by the exclusion principle. In view of this stipulation the conventional assumption, that several pairs of electrons contribute to the formation of high-order bonds, should therefore 

In C2H6 the number of valence electrons per C atom matches the number of neighbouring ligands in C2H4 there is an excess of one and in C2H2 an excess of two electrons per ligand. 

In point-charge simulation this electronic rearrangement is of no immediate consequence except for the assumption of a reduced interatomic distance, which is the parameter needed to calculate increased dissociation energies. However, in Heitler-London calculation it is necessary to compensate for the modified valence density, as was done for heteronuclear interactions. The closer approach between the nuclei, and the consequent increase in calculated dissociation energy, is assumed to result from screening of the nuclear repulsion by the excess valence density. Computationally this assumption is convenient and effective. 

To obtain the bonding curve for high order bonds it is only necessary to replace the factor l/R in eq. (5.30) by k /R, where k is the screening constant for bonds of order v. These screening constants are, not surprisingly, related to those for the j -block hydrides, e.g. fc2 = (0.84)2. Screening constants for all bond orders are summarized by the linear relationship  

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The relationship between screening and bond order is obvious. Bond... 

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