Why is Chlorine not hybridised in BCL3 using hybridisation concept in VB Theory

[Nicholas Kang]

I have been offline for almost 2 years and now I am back. I hope you guys still remember me...

 

Here is a question on physical chemistry.

 

Simple question. It is about Valance Bond theory and the use of hybridisation/hybridization (depending on whether your English is American/British variant) concept.

My teacher taught me how to explain Boron trichloride's structure using VB theory. Boron atom undergoes sp2 hybridisation. Each sp2 hybrid orbitals will have an unpaired electron that will be shared with the unpaired electron in Chlorine's 3p orbital. So, the orbital overlap diagram looks like this. (Check the attached file.)

The 3 red orbitals are the 3 sp2 hybrid orbitals while the 3 green orbitals are from Chlorine's 3p orbital.

My question is why Chlorine does not undergo hybridis(z)ation as well? Supposedly, each Chlorine atom should undergo sp3 hybridis(z)ation to produce 4 sp3 hybrid orbitals, one of which is shared with the sp2 hybrid orbital of Boron atom. So, 3 sp3 hybrid orbitals from 3 Chlorine atoms share 6 electrons with 3 sp2 hybrid orbitals from Boron atom.

Thanks for answering. Glad to see you guys again.

Regards,

Nicholas.

 

Edited February 17, 2017 by Nicholas Kang

[fiveworlds]

My question is why Chlorine does not undergo hybridis(z)ation as well? Supposedly, each Chlorine atom should undergo sp3 hybridis(z)ation to produce 4 sp3 hybrid orbitals, one of which is shared with the sp2 hybrid orbital of Boron atom. So, 3 sp3 hybrid orbitals from 3 Chlorine atoms share 6 electrons with 3 sp2 hybrid orbitals from Boron atom. ​

 

 

Why would it? Do you believe this configuration would be more stable?? If chlorine shares an electron with boron then it has a full sublevel. Regardless the other the accepted theory.

[Nicholas Kang]

Ok, finally someone replied. Thanks fiveworlds. Personally, I think this configuration is more stable, since hybridisation stabilizes a molecule. Apparently, if achieving octet or duplet is the only thing required for the formation of a particular molecule, why would you hybridise the central atom as well?

 

So, it is clear that you hybridise atoms of molecules to make them stable and hybridise all atoms would definitely make the whole molecule stable. Am I wrong?

 

Nicholas.

Edited February 26, 2017 by Nicholas Kang

[OldChemE]

It may also be that since Chlorine is much more electronegative (being in group 7) than Boron the most energy efficient configuration favors hybridization of the Boron.

[Nicholas Kang]

It may also be that since Chlorine is much more electronegative (being in group 7) than Boron the most energy efficient configuration favors hybridization of the Boron.

 

Can you explain it in detail? How does electronegativity affects hybridization?

[BabcockHall]

Chlorine is much larger than boron. I am not sure that its orbitals will overlap effectively with the orbitals of boron.

[Nicholas Kang]

Chlorine is much larger than boron. I am not sure that its orbitals will overlap effectively with the orbitals of boron.

Are you sure? I thought Boron is larger than Chlorine. The atomic size of elements decreases across a period. And I have checked the atomic radius for both Boron and Chlorine.

 

Boron: 87 pm

 

Chlorine: 79 pm

 

So, apparently, Boron is larger than Chlorine.

Edited March 2, 2017 by Nicholas Kang

[Sensei]

Read this

http://www.chemguide.co.uk/atoms/properties/atradius.html

It describes differences between different radii calculation methods.

[Nicholas Kang]

Ok, Sensei. In the diagram, Cl atom is larger than B atom. Now, BabcockHall says he thinks Chlorine's orbitals will not (unsure) overlap effectively with the orbitals of boron.

 

So, what do you think, Sensei?

 

What is your opinion? (Simply put, what is your answer to my question? )

Edited March 2, 2017 by Nicholas Kang

[OldChemE]

In my earlier comment about electronegativity of Chlorine-- I was postulating that since Chlorine is more electronegative than Boron, it holds the electrons shared with Boron more tightly than Boron can. In hybridization, the orbitals of an atom shift to accomodate the bond-- so I'm wondering if the energy efficient approach is for the atom that holds the electrons less tightly to hybridize. You would have to dig into this more to check ti, as I am not an expert on hybridization.