In the modern age of accessible quantum mechanics (DFT and so on), teaching hybridization can be anticlimactic. However, simple concepts continue to be empowering. In the past, I mentioned the “rabbit ear” view of carbonyl oxygen’s lone pairs. Despite the documented existence of n-to-π* interactions that cannot be adequately described using hybridization models, there are many cases where the good old bonding arguments hold and help understand molecular structure.
Today I want to talk about the often overlooked Bent’s rule. By the way, there is a great review by Professor Igor Alabugin of Florida State University in the J. Phys. Org. Chem. on this subject (see the link below). Bent’s rule states that “s-character concentrates in orbitals directed toward electropositive substituents or, alternatively, that atoms direct hybrid orbitals with more p-character towards more electronegative elements”. Here is a practical demonstration of its utility from Igor’s paper. Consider the molecule of difluoromethane. Chemical intuition based on electron repulsion might suggest that the F-C-F angle should be larger than the H-C-H angle. In reality, the opposite is true, which is a direct consequence of Bent’s rule. This is a fantastic example.
Thanks for the link to the paper. I learned this rule in my introductory inorganic chemistry courses where we were able to compare molecular geometries and angles of molecules such as NR3, PR3, AsR3, OR2, SR2 etc… It’s nice to see there is more to that and the paper looks fantastic.
Thanks, glad you like it.
Great post, nice to see people out there enjoying this! 🙂
Nice to hear from one of the co-authors…
Co-authors are big fans of this blog!