Not Carbon s-p Hybridization, but Coordination Number Determines C-H and C-C Bond Length.

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C-H and C-C bonds as the carbon atoms involved vary, in s-p hybridization, along sp3 to sp2 to sp. Our quantum chemical bonding analyses based on Kohn-Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C-H and C-C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp3 to sp2 to sp.