Hi All - I am currently using Housecroft and Sharp's 4th edition with supplements from Meissler and Tarr's 3rd ed.. Both of these texts discuss [FHF]- in the early MO chapter(s) as an example of 3c-2e bonding. I'm okay with both text's approach to this ion by using LGO's on the F atoms that combine with the central H atom to illustrate formation of a bonding, non-bonding, and antibonding level for the F 2pz ortbitals with the H 1s. It's easy to "see" a 3c-2e interaction from that approach.
For an upcoming unit on acids and bases, I was going to use Meissler and Tarr's chapter since they stress donor-acceptor interactions rather than just review a lot or GenChem/Analytical as does the Housecroft Text. In the Meissler Acid/base chapter, they approach the issue of the hydrogen bond by using [FHF]- again. this time, they start with the MO's for the F molecule and combine them with the pz of the incoming F- ion. After long discussion and many, many diagrams, they then state that the interaction is best thought of as a 3c-4e interaction and their diagram clearly shows this even though the middle MO is essentially non-bonding.
Myself - I am more comfortable thinking about H-bonding as 3c-4e and the distinction between the two for [FHF]- seems to be more a matter of perspective than anything else. What I am worried about is the direct contradiction that Meissler and Tarr have in their text and the potential to really confuse the students. For most of them - this is their first substantial treatment of MO theory other than HOMO-LUMO exposure in second semester Organic.
Does anyone have ideas on a student-friendly way to resolve this? Currently - I am considering skipping the coverage of 3c-2e in the earlier material, even though that rules out discussion of boranes, in favor of giving them a solid appreciation for hydrogen bonding and hydrogen transfer from a MO perspective.
Thanks,
Dave.
The good news is that Paul Fischer has completely revamped the hydrogen bondng section in the 5th Ed of Miessler, Fischer and Tarr. But I think this issue still sneaks into the acid-base chapter with [F-H-F]- being viewed as "a three-center-four-electron covalent hydrogen bond". I'll admit to not having noticed this before. but I agree that it is a bit confusing. I don't think there is anything wrong with showing students both. You can then explain how our simple approach (3c-4e-) isn't quite right when you examine the MO diagram. Think about CO2. A Lewis structure suggests two different pi bonds (2c-2e, one to each O atom) and yet the MO diagram shows two degenerate pi bonds (that you could call 3c-2e bonds).
Chip is right about the new edition clearing up some of the confusion. They still show the HF + F- sigma bonding diagram, they still state that the sigma molecular orbitals are bonding, nonbonding, and antibonding, but the MO discussion basically stops there (thank goodness). There are no more confusing diagrams. And then the author heads into a fairly detailed discussion about current IUPAC criteria for hydrogen bonding.
More importantly (and correctly, I think), the new edition emphasizes that the hydrogen bond can be ascribed to "varying relative contributions of" electrostatic interaction (because of the very polar X-H bond), covalent character (as shown in the MO diagram), and dispersion forces. I would emphasize the electrostatic part.
If I understsnd the discussion correctly, we are talking about situations in which you bring together three orbitals with four electrons, generating one bonding orbital, one non-bonding orbital, and one antibonding orbital, in which the bonding and non-bonding orbitals are filled. Examples include:
[FHF]-
[(H2O)H(H2O)]+
The pi system of allyl anion or a carboxylate anion, such as acetate
The three linear F-S-F interactions of SF6 (ignoring contributions from sulfur's s-orbital)
The transition state of the SN2 reaction
The pi (and sigma) bonding in CO2.
In all of these cases, we can describe these as a 3c-2e bond OR a 3c-4e bond. Technically, only two electrons are in a *bonding* orbital, but 4e- are smeared around between the three atoms in an overall bonding interaction.
In all of these cases, delocalizing the electrons over three atoms relative to two increases the bond strength by about 40% (Hueckel theory), so there is a "bonus" stabilization that ultimately comes from lowering the kinetic energy of the bonding electrons (bigger 'box' for your 'particles').
This is a really importing bonding motif, and I always try to teach it in my courses.
If memory serves, the hydrogen bonds in water are ca. 50% covalent, 50% Coulombic (electrostatic dipole interaction).