Molecules with Multiple Bonds

According to the description in Chapter 20, the double bond in C2 consists of two pi bonds. We now construct approximate wave functions for larger molecules containing double bonds. 

Describe the bonding in ethene (ethylene), whieh has the following structural formula 

Since the nuclei must be placed on the symmetry axes of the hybrid orbitals to have good overlap, the C-C-H bond angles equal 120°, in agreement with the VSEPR prediction. The double bond consists of a sigma bond and a pi bond. 

Describe the bonding in diatomic carbon using hybrid orbitals like those used in ethene. Do you think this description would be superior to that in Section 20.3  

We now model a carbon-carbon triple bond as a sigma bond and two pi bonds. 

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EXAMPLE 3.6 Accounting for the structure of a molecule with multiple bonds... 

Molecules with multiple bonds between atoms (e.g., unsaturated hydrocarbons) ... 

On the other hand, in molecules with multiple bonds, each "component bond" of double and triple bonds is numbered independently, and keeping in mind the geometrical distribution of points we have just referred to. For example. 

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes. The square prismatic [Re2Cl8]2 ion, shown in Figure 3-29, played an important role in the history of the discovery of metal-metal... 

It seems to us reasonable to believe that cautious empirical projection and common sense still have a dominant role to play in forecasting the behavior of molecules with multiple bonds between metal atoms and that when these approaches come into conflict with the results of ab initio calculations, the latter should be viewed with reservations. 

Molecules of heavier main-group elements show significant differences to compounds of elements from the first-octal row of the periodic system. The coordination number of the lighter elements is rarely larger than 4 while many stable molecules of the heavier elements with coordination number 5 and 6 are known. Another difference concerns the stability of molecules with multiple bonds. Many stable molecules with double or triple bonds between the lighter elements are well known, particularly compounds that have C=C, C=N, C=0,... 

So far in our treatment of the VSEPR model, we have not considered any molecules with multiple bonds. To see how these molecules are handled by this model, let s consider the NO3- ion, which requires three resonance structures to describe its electronic structure ... 

Desvergne, J.-P., Bouas-Laurent, H. (1990), Cycloaddition Reactions Involving 4n Electrons [2 + 2] Cycloadditions Molecules with Multiple Bonds Incorporated in or Linked to Aromatic Systems, in Photochromism, Molecules and Systems Durr, H., Bouas-Laurent, H., Eds. Elsevier Amsterdam. 

B-VWN and B-LYP, on the other hand, perform very well and give binding energies with very small mean error. For B-LYP, the mean error is 1.0 kcal/mol and the mean absolute error is 5.6 kcal/mol. The molecules which are underbound at this theoretical level are the simple hydrides with lone-pair electrons (H2O and NH3, for example). On the other hand, molecules with multiple bonds, as well as H2O2 and F2, tend to be overbound. The low mean error is partially due to the fairly small basis set used B-LYP theory leads to some overbinding with a large basis. 

Lewis Structures for Molecules with Multiple Bonds Sometimes, you ll find that, after steps 1 to 4, there are not enough electrons for the central atom (or one of the central atoms) to attain an octet. This usually means that a multiple bond is present, and the following additional step is needed ... 

N=0, C=C, and C=N bonds. Molecules with multiple bonds between heavier elements are much less stable and they can only become isolated when the multiple bonds are stabilized by protective substituents. The geometries of the latter... 

B. Lewis Structures of Molecules with Multiple Bonds... 

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes [3-52], The square prismatic [Re2Clg] ion [3-53], shown in Figure 3-36, played an important role in the history of the discovery of metal-metal multiple bonds. Figure 3-37a shows another molecular model with a metal-metal multiple bond. Its shape is similar to the paddles that propel riverboats. There is then a whole class of hydrocarbons called paddlanes [3-56], and one of their representatives is shown in Figure 3-37b. 

OBJECTIVE To learn how to write Lewis structures for molecules with multiple bonds. 

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