Molecular orbital heteronuclear diatomic case

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 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. 

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,... 

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... 

When the atoms in a heteronuclear diatomic molecule are close to one anoth in a row of the periodic table, the molecular orbitals have the same relative order of energies as those for homonuclear diatomic molecules. In this case you can obtain the electron configurations in the same way, as the next example illustrates. 

NIR studies have been applied also to heteronuclear diatomics like CO and NO [95-97]. On alkali and alkaline earth cations the results are essentially similar to those obtained with homonuclear probes. In the case of CO and NO strengthening of the internal bonding is observed, caused by the withdrawal of electron density from slightly antibonding molecular orbitals in correspondence with quantum chemical calculations [70,72]. In spite of the uncertain Do determination on cobalt and copper ion-exchanged zeolites A the CO dissociation energy turns out to be decreased, which may be understood by backdonation of electronic charge into the carbonyl tt orbital [97,98]. 

In contrast to homonuclear diatomics, the molecular orbitals of heteronuclear diatomics cannot be constructed from a linear combination of two identical atomic orbitals. The atomic orbitals may differ in energy and type and hence do not contribute equally to the molecular orbitals. In the general case,... 

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