Retire hybrid atomic orbitals? Not yet!
19 June 2011 - Conceptual tools
In the latest edition of the Journal of Chemical Education, Alexander Grushow bans orbital hybridisation to the scrapheap that is obsolete theories (DOI). Students have to get used to MO theory and hybridisation can join the ranks of phlogiston theory. Hybridisation proponents argue that the theory is very useful in explaining things like molecular geometry and bond energies but according to Grushow MO theory can do the job as well. Take for example methane. In orbital hybridisation 2 2s electrons and 2 2p electrons on carbon mix to form 4 identical sp3 atomic orbitals available for bonding to 4 hydrogen atomic orbitals. This explains why in methane the 4 C-H bonds are identical.
The assumption that the theory also makes sure a tetrahedral orientation of the 4 sp3 orbitals is incorrect according to Grushow because that can already be derived from VSEPR theory. According to this theory the 4 valence electrons on carbon simply stay out of each others way as far as possible in a tetrahedral configuration but Grushow is quick to remark this is another localized electron model albeit less harmful.
In MO theory Grushow continues, the C-H bonds may be identical but with different bonding energies. This is a difficult concept to grasp but Grushow thinks this matter should rest until students get to somewhat more advanced quantum mechanics which is much simpler if students do not have incorrect notions of localized bonding orbitals already stuck in their heads.
Does this blog agree? For starters, MO theory does not have a qualitative and didactic recipe for the explanation of relative MO energies. Calculations are laborious and require extensive mathematical background. Computer models are black boxes with no didactic value at all. Grushow proposes to ban orbital hybridization in university textbooks when the chemical literature is rife with it. Perhaps as a more ambitious plan the science journal editors and referees should be persuaded to take the first step.