Heteronuclear diatomic molecules, molecular orbitals

Heteroboranes, structure prediction for. 802-807 Heterocatenation. 741-742 Heterocyclic inorganic ring systems, 775-780 Heterogeneous catalysts, 705 Heteronuclear diatomic molecules. molecular orbitals in, 167-175... 

FIGURE 3.33 A typical d molecular orbital energy-level diagram for a heteronuclear diatomic molecule AB the relative contributions of the atomic orbitals to the molecular orbitals are represented by the relative sizes of the spheres and the horizontal position of the boxes. In this case, A is the more electronegative of the two elements. 

The molecular orbital energy-level diagrams of heteronuclear diatomic molecules are much harder to predict qualitatitvely and we have to calculate each one explicitly because the atomic orbitals contribute differently to each one. Figure 3.35 shows the calculated scheme typically found for CO and NO. We can use this diagram to state the electron configuration by using the same procedure as for homonuclear diatomic molecules. 

The limitation of the above analysis to the case of homonuclear diatomic molecules was made by imposing the relation Haa = Hbb> as in this case the two nuclei are identical. More generally, Haa and for heteronuclear diatomic molecules Eq. (134) cannot be simplified (see problem 25). However, the polarity of the bond can be estimated in this case. The reader is referred to specialized texts on molecular orbital theory for a development of this application. 

This is a simple example of a heteronuclear diatomic molecule which is found in a stable molecular substance. We must first choose the basis set. The only AOs that need to be seriously considered are the hydrogen Is, fluorine 2s and fluorine 2p, written for brevity as ls(H), 2s(F) and 2p(F). The fluorine Is orbital lies very low in energy (700 eV lower than 2p) and is so compact that its overlap with orbitals on other atoms is quite negligible. The fluorine 2p level lies somewhat lower than ls(H), as indicated by the higher ionisation potential and electronegativity of F. Interaction between 2p(F) and 2s(H) is very small and can be neglected for all practical purposes. One is tempted to discard 2s(F), which lies more than 20 eV below 2p(F) the 2s-2p separation increases... 

Heteronuclear diatomic molecules are naturally somewhat more complicated than the homonuclear comprehensive comparisons with homonuclear molecules were given by Mulliken [15]. The atomic orbital coefficients in the molecular orbitals ofheteronu-clear diatomic molecules are no longer determined by symmetry alone, and the electrons in the molecular orbitals may be shared equally between atoms, or may be almost localised on one atom. The molecular orbitals can still be classified as a or n, but in the absence of a centre-of-symmetry the g/u classification naturally disappears. Some heteronuclear molecules contain atoms which are sufficiently similar that the molecular orbitals resemble those shown in figure 6.7. In many other cases, however, the atoms are very different. This is particularly the case for hydride systems, like the HC1 molecule,... 

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

In many cases the molecular orbitals for a heteronuclear diatomic molecule may be worked out in a straightforward manner as for hydrogen chloride. In others, however, certain difficulties arise and we shall take as an example the case of carbon monoxide, the structure of which has been the subject of much controversy. In carbon monoxide, as in the nitrogen molecule, there are fourteen valency electrons and Mullikan has formulated the structure of both molecules as... 

Apply the Aufbau Principle to find molecular orbital descriptions for heteronuclear diatomic molecules and ions with small (EN) values... 

The following is a molecular orbital energy level diagram for a heteronuclear diatomic molecule, XY, in which both X and Y are from Period 2 and Y is slightly more electronegative. This diagram may be used in answering questions in this section. 

Molecular orbitals for heteronuclear diatomic molecules can be made by combining atomic orbitals of the same symmetry and of similar energy from the two atoms. 

Because the atomic orbitals combined in heteronuclear diatomic molecules are not of the same energy, the electron in the molecular orbital is not equally shared between the two atoms. 

Molecular orbitals in heteronuclear diatomic molecules cannot be labelled with the subscripts g and u because the molecule does not possess a centre of symmetry. 

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