Is the Nitrogen in NH3 sp3 or sp2 + 2p ?

  • #1

Summary:

I want to draw out the product of the reaction between NH3 and BF3.
In the reaction between NH3 and BF3, the boron will bond with the extra lone pair in NH3. This is only possible if we let Nitrogen stay unhybridized. However, the molecular structure for NH3 is tetrahedral (and is therefore hybridized as sp3) due to distancing the e- pairs.
If we go by this model, however, NH3 has no lone pair to donate to the empty orbital of BF3.

Also, how is it that NH3 contains a lone pair but does not show that in energy diagram. From what I know, if an element has a lone pair, it will be an unhybridized orbital in the energy diagram.

Thanks for any help.
 

Answers and Replies

  • #2
chemisttree
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It’s a distorted SP3. The lone pair occupies a large volume and the barrier to inversion with respect to nitrogen is fairly low so it behaves a lot like an isolated p suborbital.
 
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  • #3
It’s a distorted SP3. The lone pair occupies a large volume and the barrier to inversion with respect to nitrogen is fairly low so it behaves a lot like an isolated p suborbital.
Alright. So for the energy diagram, how would I draw this? Sp3 or Sp2 + 2p2 ?
 
  • #4
chemisttree
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Draw it like an SP3. Energy diagrams are not extremely useful for visualizing structures though. You do realize that the energy of an unshared electron pair is different than bonded electrons? So that the energy diagram of ammonia will look different than something like methane even though both are SP3?


 
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  • #5
Draw it like an SP3. Energy diagrams are not extremely useful for visualizing structures though. You do realize that the energy of an unshared electron pair is different than bonded electrons? So that the energy diagram of ammonia will look different than something like methane even though both are SP3?


Yes I know that. That's why I'm confused on why the energy diagram would have it as an sp3 hybridization rather than show the lone pair separately.
 
  • #6
chemisttree
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What are you calling an “Energy Diagram?” An MO diagram?

If you are referring to the MO diagram you must acknowledge that hybridization is used to explain what is wrong given a particular MO diagram interpretation. Without hybridization, carbon can only form two bonds rather than the four that we see in the real world. Likewise, a strict MO interpretation for ammonia gives us a localized electron pair in an s suborbital and three non-equivalent combinations of 1s and 2p suborbitals. This is not reality. The observed energies and bond angles don’t fit the MO handwaving description at all.
Something must be invoked to explain the H-N-H bond angles of 107 degrees (not 90!) and the observed dipole moment (not ~0!). So hybridization is invoked to explain these differences. Rather than a combination of atomic orbitals, a combination of hybridized atomic orbitals is used.

These diagrams are a bit like using a cartoon to explain real life. You get the gist of it but details are lacking. I will say it again... MO diagrams aren’t extremely useful for visualizing structures or predicting them.
 
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