|Last year Alexander Grushow commented in The Journal Of Chemical Education that if it were up to him the hybrid atomic orbital (HAO) model would be abolished in favour of the molecular orbital model (MO) (see blog here) just as phlogiston theory some centuries ago. A long time nothing happened but in this month's issue we have no less that 5 replies and of course a Grushow rebuttal.|
Nivaldo Tro disagrees (DOI) and argues that valence bond theory must then be on the chopping board as well because it goes hand in hand with HAO theory. He appeals to the educational value: VB theory is halfway between a simple Lewis structure and complex MO theory. VB theory provides the first introduction for students to the notion that overlapping orbitals lower the overall energy of the electrons involved. As an aside Tro also notes that VSEPR theory (also favored by Grushow) cannot explain the rigidity of a double bond.
Landis and Weinhold also disagree (DOI). Since when does VB theory not accurately predict the structure of methane? Natural bond orbital (NBO) hybrid orbital theory, according to Landis and Weinhold, can calculate the electron distribution (4 equal tetrahedral bonds) like the best of them. It can even tackle the methane photoelectron spectrum with two peaks in a 3:1 ratio if you remember that the spectrum is the result of electron excitation between methane and the excited states of the ionized molecule.
More disagreement from Hiberty, Volatron and Shaik (DOI) who point out that mathematically any MO can be converted to a localised MO (LMO) describing a localised bond between two atoms. These LMO's look exactly like hybrid orbitals. Hiberty et al. also remark that only the hybrid model can explain the so-called bond transferability which explains why all C-H bonds in alkanes have the same bond length and why all acetylenic protons are acidic.
Donald Truhlar also disagrees (DOI) and is making the same point as Hiberty: the total electron density picture can be build from molecular orbitals or from localised molecular orbitals. These orbitals do not reflect true wave functions and you need a bunch of them to create one.
And finally DeKock and Strikwerda (DOI) note that in molecular mechanics molecules are also treated as a collection of localised bonds with atoms joined by springs.
And where does this leave Grushow?. Apparently no one came to his rescue. In his rebuttal (DOI) Grushow argues that it was never his intention to abandon the hybrid orbital but that he merely proposed to remove it from entry level chemistry classes. It would inject the wrong ideas in students heads that lack the proper mathematical background anyway and after all only 5% of these students will continue in a chemical career.
Well if that is the line of reasoning why advance to the level of VSEPR theory? Why not resurrect phlogiston theory for the other 95% and leave it at that? After all, this theory accurately describes phlogiston as a substance released during combustion. At some level around a stone-age camp-fire that is all you need to know about chemistry.