Molecular Orbital Study Questions

The following questions are based on the molecular orbital results tabulated on the Molecular Structure Page. You will need to refer to these results to answer the questions.
A Best Lewis Structure and Donor Acceptor Interactions Tutorial is available to help you interpret those output sections. These Lewis structure calculations are done using NBO Analysis.

1. Bent's Rule

Bent's Rule states that an atom directs hybrids of greater p character toward more electronegative atoms (1,2). For example, look at the results for methanol. In the Best Lewis Structure section you will find the following:
Hybridization in the Best Lewis Structure
1. A bonding orbital for C1-O2 with 1.9982 electrons
__has 33.65% C 1 character in a s0.95 p3 hybrid
__has 66.35% O 2 character in a sp2.53 hybrid

The carbon atom uses an s0.95p3 hybrid orbital to bond to oxygen. On the other hand, the oxygen atom uses an sp2.53 hybrid to bond to the carbon. Therefore, the carbon uses more p character in its hybrid than the oxygen does.

Question: Find three more examples of molecules that follow Bent's rule. If you find any exceptions as you look around, note those also.

2. Expanded Octets

The influence of d orbitals is usually vastly overstated by simple hybridization arguments (look at H3PO2 for example). However, one would expect that third row elements with expanded octets would need to use the full expected contributions from d orbitals. Look at SF4 as an example. Remember that the Lewis Dot structure for SF4 would have one lone pair in addition to the four bonding pairs, which are shared with fluorines. One would then expect sp3d hybridization for sulfur, giving five equivalent hybrid atomic orbitals.

Question: Look at the results for SF4 and comment on the importance of d orbitals in this compound. Does the hybridization for each sulfur bond match the expected pattern?

3. Pi Bonding and d Orbitals in 3rd Row compounds

The influence of d orbitals is usually vastly overstated by simple hybridization arguments . The phosphorus oxyacids are often shown with a pi-bond between the phosphorus and the oxygen that is not bonded to a hydrogen. This pi-bond is usually described as using a d orbital on phosphorus. Or, alternatively, the sigma bonds are described as using sp2d hybrids on the central atom, which leaves a p-orbital left over for forming pi-bonds.

Question: Look at H3PO2 as an example. Look at the Bond Order section to determine if there is double bonding and then look at the Best Lewis Structure section to determine the contribution of d orbitals to the bonding. Also look at the phosphorus oxyanions to see if there is double bonding between phosphorus and oxygen and if there are large contributions from d orbitals.

4. Hybridization in 3rd Row compounds

Second period elements are very small compared to higher periods. Hybridization then plays an important role for second period elements, see H2O for example. Is hybridization as important for 3rd period (and higher) elements? Since 3rd period elements are larger, there is more room around the central atom and the absence of crowding decreases the effects of electron pair repulsion. For example, the bond angles for H2S, and H2Se are close to 90 degrees, not the expected value of 109.5 degrees from VSEPR and hybridization theory. To explore hybridization for the 3rd period, look at a series of sulfur compounds to see the importance of hybridization. In the absence of hybridization, we would expect "p3" hybridization, that is 90 degree bond angles with little or no s character. (The hybridization of several S containing compounds is the subject of ZumdahlChemistry Chapt. 9 problem 24.)

Question: Look at H2S, SF2, and SCl2 as examples. Look first at the bond angles. Then look at the Bond Order section to determine if there is double bonding and then look at the Best Lewis Structure section to determine the contribution of hybridization to the bonding. (a.) Is there a relationship between the bond angle and hybridization? Is this relationship what you would expect from VSEPR and hybridization theory? Now consider SO2, and SO3. (b.) Is there a correlation with the extent of double bonding and hybridization? Now look at SO4 2- The charge of the ion increases the total electron density in the substance. (c.) Speculate on the effects of the charge and the fact that there are four bonded atoms.

References

1. H. A. Bent, Chem. Revs.,1961, 61, 275-311.
2. J. P. Foster, F. Weinhold, J. Am.Chem. Soc.,1980,102, 7211-7218.
3. The tutorial from the NBO manual that discusses the Natural Bond Orbital Analysis will help in understanding the output data.

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