Molecules heteronudear

FIGURE 3.9 Molecular orbital diagrams for some heteronudear molecules and ions of second-row elements. 

It is interesting to note that both CO and CN are isoelectronic with the N2 molecule. That is, they have the same number and arrangement of electrons as the N2 molecule. However, as we will see later, these species are quite different from N2 in their chemical behavior. The properties of many homonudear and heteronudear molecules and ions are presented in Table 3.1. 

F g. 5 3 Heteronudear molecule. A B, with large electronegativity difference. Covalency is insignificant (he bond is essentially ionic. 

Vij stoichiometric number for the jth species in the ith reaction cr symmetry number, 1 for a heteronudear molecule, 2 for homonu-... 

The conclusions reached here apply equally well to general multi-electron homonuclear molecules. In this case the value of in the dissociation limit becomes equal to the sum of the contributions of the atomic fragments. The A-integrated exchange-correlation hole for a reference electron on one of the atomic fragments will then be equal to the A-integrated hole of the atom itself. The properties of the exchange-correlation in heteronudear molecules are discussed in the next section. 

The heteronudear molecule. 4B is thereforeless stable than the homonu-dear molecules and Bg. At decreasing temperature the equilibrium composition is more and more diifted in favour of A and Ba. We may say that isotopic substitution lowers the stability of the system. 

In addition to the homonudear molecules, the elements of the second period form numerous important and interesting heteronudear species, both neutral molecules and diatomic ions. The molecular orbital diagrams for several of these species are shown in Figure 3.9. Keep in mind that the energies of the molecular orbitals having the same designations are not equal for these species. The diagrams are only qualitatively correct. 

For a homonuclear diatomic molecule, the bond is purely covalent (a, = 0, ac = 1) whereas fora heteronudear diatomic molecule the bond shows mixed covalent-ionic character (a 0, ac 0). In the limit as the separation between the atomic energy levels on the two atoms becomes very large the bond becomes purely ionic (at = 1, ac = 0). 

Molecular orbitals in heteronudear diatomic molecules. 167-175 in homonudear diatomic molecules, 160-166 of metallocenes. 670-673 in octahedral complexes, 414-418... 

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