Resonance Structures and Stabilization in XO4 Species: A Valence Bond Theory Approach, Study notes of Molecular Chemistry

Download Resonance Structures and Stabilization in XO4 Species: A Valence Bond Theory Approach and more Study notes Molecular Chemistry in PDF only on Docsity! Resonance Structures for Species of the Form XO4 q- Resonance stabilization is a major part of valence bond theory. This is how this particular method handles electron delocalization. In all these species, the X-O bonds are equivalent even though the individual Lewis structures contain both single and double bonds. When considering these systems, we need to do the following: 1) Write the possible Lewis structures and 2) Count the number of equivalent forms each one has. 3) Note that single-bonded oxygens have a formal charge of -1 and that double- bonded oxygens have a formal charge of 0. We start with the simplest case first. Here all the bonds are single bonds. Here is the structure. It should be obvious that it can have only one resonance form or, to put this another way, there is no resonance at all. Now, let us look at the case where there is a single double bond in the individual Lewis structures. We show this now. This has four equivalent resonance structures. This might be easier to visualize if we make a 3D rendering. If you rotate the top bond, you can see three equivalent structures. Then, placing the double bond at the top, you get one more. Thus—again--there are four equivalent resonance structures for this species. We now look at the next case, that with two double bonds. Here is the Lewis structure of one of the resonance forms. There are six possible resonance forms for this species. The 2D rendering above does not do it justice! Let us look at this species in three dimensions. This is rather hard to see but, the two double bonds are at the bottom and lower left, respectively. If you rotate around the single bond at the top, you see that there are three equivalent structures. Then, if you rotate around one of the double bonds, you see three more. Thus, as already mentioned, there are six equivalent structures. The last two cases correspond to the first two—in reverse order by symmetry. (Symmetry arguments are quite common in analyses such as these.) Here is the Lewis structure with three double bonds. You should be able to figure out why there are four equivalent structures. Finally, it is possible to have four double bonds (we shall show an example later). In this case—just as with having four single bonds—there is only one possible resonance structure. We draw this now.