In the class, I was told that $\ce{H3C^.}$ has a trigonal planar structure with the unpaired electron in $\mathrm{2p_z}$ orbital. But $\ce{H3C -}$ has a trigonal pyramidal structure.


But why does this happen? Why does $\ce{F3C^.}$ also doesn't have a trigonal planar?

Edit : I have seen one possible answer in a different question. But is there any answer that is explained with the help of VSEPR or MOT or VBT?

  • $\begingroup$ Please do not use MathJax formatting in titles. $\endgroup$ – Mathew Mahindaratne Jun 3 at 17:42
  • $\begingroup$ How could it be trigonal planar if it has an electron in a p orbital? $\endgroup$ – Josh Mitchell Jun 3 at 17:48
  • $\begingroup$ @Mithoron , Is there any other explanation from VSEPR theory or VBT or MOT? $\endgroup$ – Noah J. Standerson Jun 4 at 5:47
  • $\begingroup$ @JoshMitchell , Carbon in $H_3C•$ is $sp^2 $ hybridised . All the unpaired electrons present in $sp^2$ hybrid orbitals form bonds with Hydrogen. The remaining unpaired electron is in $p$ orbital $\endgroup$ – Noah J. Standerson Jun 4 at 5:52
  • $\begingroup$ chemistry.stackexchange.com/questions/34993/… $\endgroup$ – Mithoron Jun 4 at 16:35

CH3 radical is Sp2 hybridised carrying the lone electron in a pure p orbital. This is so because Sp2 hybridised orbitals are lower in energy than Sp3 hybrid orbitals (The more the S character the lesser is the energy of the orbital). When there is a lone electron present, it is more efficient to keep the bond pair electrons in lesser energetic Sp2 hybrid orbitals and lone electron in higher energy pure P orbital than keeping all bonding pairs and the lone electron in Sp3 hybrid orbitals. So most of the radicals are Sp2 hybridised.

But in situations like •CF3 where the atoms/groups attached to C are of high electronegativity things may change. According to Bent's rule more electronegative atoms/groups will favour less electro negative orbitals i.e. orbital s with lesser s character(Sp3 has lesser S character compared to Sp2). This is quite obvious because the more the S character of the C center the more is the tendency of the C center to attract the bond pair towards itself and in cases like electronegative atom, it will also try to attract the bond pair towards itself. This makes some unhappy situation. This is reflected by the Bent's rule. That is why •CF3 radical favours Sp3 hybridisation.

It's not that only such radicals like •CF3 can acquire Sp3 hybridisation. In many other cases like bridge head of adamentum also acquire Sp3 hybridisation becauseof opposition by its rigid structure to acquire planarity on generation of radical though it's not that much easy process to form radical here.

In conclusion, a radical will favour that hybridisation which will be energetically favourable for it.

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    $\begingroup$ Sp2 hybridised orbitals are lower in energy than Sp3 hybrid orbitals - this is not true. Hybrid orbitals are no eigenfunctions of the Hamiltonian, thus they cannot be assigned an eigenvalue. It is also not true to say 'electronegative orbitals', because electronegativity is the property of an element. I also cannot get behind your description of Bent's rule. It is very, very important to state that hybridisation is a model, which results from a particular structure. Obviously the structure that is most common will have the lowest possible energy. $\endgroup$ – Martin - マーチン Jun 5 at 14:08
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    $\begingroup$ What I meant about energy of hybrid orbital is- S orbital is much lower in energy than P orbital and this is because the electrons in S orbital are much closer to nucleus than p orbital. And also if you consider the hybridization, the more the S character in hybrid orbital, the more are the electrons closer to the nucleus and so if you put them in more S charactered hybrid orbital then being much more closer to nucleus it will have lesser energy. Bent's rule is just a rule to deal with elements having different electronegativities while putting them in hybrid orbital model. $\endgroup$ – Nilarun Koley Jun 5 at 14:50
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    $\begingroup$ I would like to say that electronegativity is not property of an element or anything. It's rather a relative concept. What ever I meant here by electronegativity of hybrid orbital is the attraction force on electrons when they are kept in that hybrid orbital. Please feel free to review my comment. I am even in my undergrad and wrote this answer with the articles or books I ever read. $\endgroup$ – Nilarun Koley Jun 5 at 14:57
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    $\begingroup$ I'm sticking with my initial assessment: Your answer is wrong. Electronegativity is the power of an atom to attract electrons to itself (doi.org/10.1351/goldbook.E01990); it is therefore the property of an atom (or element). It is a relative concept, but not in the way you are describing it. It is still wrong to say that an hybrid orbital can be assigned an energy eigenvalue. Hybridisation is a descriptive concept. Bent's rule, while derived with hybridisation in mind, is independent of it. It is an empirical observation, which remains valid for any wavefunction. $\endgroup$ – Martin - マーチン Jun 5 at 15:39
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    $\begingroup$ Please answer the question in your view point. I appreciate your knowledge. I wanna know answer to this question too. Yes you are correct that we can't assign eigenvalue to hybrid orbital. I also mentioned in the last comment what I meant to say in my answer. Neither do I deny your logic nor I am obstinate to my answer. I am still a learner. So please feel free to answer this question. (Please don't say again and again 'your answer is wrong'). What happens in chemistry is just because it's energy favourable. So this is not a perfect answer. Please answer the question. $\endgroup$ – Nilarun Koley Jun 5 at 15:56

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