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sp2 and sp3 hybridization- the bond lengths

Posted: Tue Jun 05, 2018 2:51 pm
by Cindy Nguyen 1L
I would like some clarification on the bond lengths in sp2 and sp3 orbitals. Are the bond lengths in sp2 orbitals shorter because the orbitals have more s-orbital character (33% as opposed to 25% in sp3) and more s-orbital character means the electrons are closer to the nucleus? Also, in regards to hybrid orbitals and the previous question, how is a sigma bond + pi bond (a double bond) shorter than a sigma bond alone? In other words, what is it about the hybrid orbitals and sigma/pi bonds that makes the double/triple bond shorter/shortest respectively? Or maybe I'm thinking on the wrong track for bond lengths.

Re: sp2 and sp3 hybridization- the bond lengths

Posted: Wed Jun 06, 2018 6:21 pm
by Chem_Mod
Excellent question.

Yes, sp2 orbitals have more s character and are therefore concentrated closer to the nucleus.

However, sp2 orbitals still reach a maximum away from the nucleus, whereas the perpendicular p orbitals reach a maximum above and below the nucleus (compared to the bond vector). Therefore, maximum overlap for an sp2 orbital with the sigma-bonding orbital from another atom will happen when the atoms are still at a certain distance from each other, but maximum overlap for p orbitals perpendicular to the sigma bond will occur when the atoms are on top of each other. Therefore, a pi bond pulls the atoms closer than is optimal for a single sigma bond, which weakens the sigma bond slightly but strengthens the pi bond more, until the gain and loss cancel at the equilibrium bond length.