Molecular orbital model bond order

The angular-overlap model (AOM) is one such specific model whose parameters have been chosen so as to refer to the central ion-to-ligand bonding process (9). However, it must be remembered that AOM still is a first-order perturbation model (70) but, one may say, a zeroth-order molecular orbital model (5). 

Thus there are five bonding electrons giving a bond order of 2.5, consistent with the bond length of 115 pm, versus 121 pm for the four-electron bond in O2 and 110 pm for the six-electron bond in N2. For these and other related molecules, the double-quartet model is a convenient and useful alternative to the conventional molecular orbital model. Moreover, it shows that for a singly bonded terminal atom such as F or Cl there is a ring of six nonbonding electrons rather than three separate lone pairs. As we will see in Chapters 7 and 8, this conclusion is confirmed by the analysis of electron density distributions. 

Using the molecular orbital model to describe the bonding in F,+, F, and F,", predict the bond orders and the relative bond lengths for these three species. How many unpaired electrons are present in each species ... 

In the course of investigating multiple bonds in molecules and complexes by the valence bond approach, we have recently found that such multiple bonds are more accurately described as bent bonds rather than as a and tt bonds (7-5). In order to understand the potential implications of these results for multiple metal-metal bonds, it is important to brieffy review the basic assumptions of the valence bond model and compare them to those of the more familiar molecular orbital model of bonding. 

In the molecular orbital model, the number of bonds between two atoms is designated as the bond order and depends not only on the number of bonding electrons, but also on the number of antibonding electrons. In general, the bond order can be determined from the following equation.4... 

In summary, a simple molecular orbital model predicts that formation of a 3c-4e bond between A and BC increases the total bond order from 1 to V2. While this calculation ignores issues of overlap and bond length, it indicates that the A -BC bond in ABC should be roughly 0.4 times as strong as the B-C bond in BC. 

We will now apply the molecular orbital model to the helium molecule (He2). Does this model predict that this molecule will be stable Since the He atom has a configuration, s orbitals are used to construct the molecular orbitals, and the molecule will have four electrons. From the diagram shown in Fig. 9.30 it is apparent that two electrons are raised in energy and two are lowered in energy. Thus the bond order is zero ... 

There are definite correlations between bond order, bond energy, and bond length. As the bond order predicted by the molecular orbital model increases, the bond energy increases and the bond length decreases. This is a clear indication that the bond order predicted by the model accurately reflects bond strength, and it strongly supports the reasonableness of the MO model. 

Use the molecular orbital model to predict the bond order and magnetism of each of the following molecules. 

Using the molecular orbital model, write electron configurations for the following diatomic species and calculate the bond orders. Which ones are paramagnetic ... 

The MO model of C2 predicts a doubly bonded molecule, with all electrons paired, but with both highest occupied molecular orhitals (HOMOs) having tt symmetry. C2 is unusual because it has two tt bonds and no cr bond. Although C2 is a rarely encountered allotrope of carbon (carbon is significantly more stable as diamond, graphite, fullerenes and other polyatomic forms described in Chapter 8), the acetylide ion, C2 , is well known, particularly in compounds with alkali metals, alkaline earths, and lanthanides. According to the molecular orbital model, 2 should have a bond order of 3 (configuration TT TT a-g ). This is supported by the similar C—C distances in acetylene and calcium carbide (acetylide) . ... 

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