Hybridisation is the mixing of orbitals of similar energies, so when we perform hybridisation, why don't we prefer $4\mathrm s$ in place of the $3\mathrm d$ orbital for hybridisation, even though $4\mathrm s$ has less and more comparable energy to $3\mathrm p$ and $3\mathrm s$? Why can't we make an outer orbital complex like $\mathrm{sp^3s}$ instead of $\mathrm{sp^3d}$?

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    $\begingroup$ chemistry.stackexchange.com/a/76740/16683 This sp3d hybridisation thing is complete bogus anyway, so you're on the right track. I'd suggest unlearning these ideas of "outer orbital complex" etc. it is a very poor description of bonding in transition metal complexes. See linked answer for more info. $\endgroup$ Commented Aug 18, 2020 at 2:34
  • $\begingroup$ i didnt get my answer there $\endgroup$ Commented Aug 18, 2020 at 4:00
  • $\begingroup$ i need to know why cant we have an hybridisation of sp3s $\endgroup$ Commented Aug 18, 2020 at 4:01
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    $\begingroup$ There simply isn't a good rule to decide which orbital mixture is a "good" one. The concept works rather perfectly for $2s2p^{1,2,3}$ (covering all possible permutations there), and that has seduced some people to extend the concept into higher orbitals. Many other chemists think they fell into a little trap set up by nature with this extension. ;-) $\endgroup$
    – Karl
    Commented Aug 18, 2020 at 5:51
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    $\begingroup$ Does this answer your question? Sulfur trioxide - vacant d-orbitals $\endgroup$
    – Mithoron
    Commented Aug 18, 2020 at 16:41

2 Answers 2


I would suggest that this entire factory of bogus hybridization stories should be eliminated from general and organic chemistry curricula. The word mixing is worse, as it is a jug where one can add different orbitals and run a blender and here is the mixture. This stuff is good for rote memorization only.

Only after a thorough QM course and mathematical foundation, these ideas should be introduced with an example along with historical development. Neither the textbook author, nor the teacher know what it is. The only real experimental observable is only electron densities via X-ray diffraction or other techniques.


M. Farooq is correct that hybridization is a concept that seems to lead students astray more often than not, but your intuition within this simple concept is entirely correct.

When s, p and d orbitals are all involved in bonding (I'll avoid the word "hybridization"), it is most often the orbitals that are close in energy, for example, 4s, 3d and 4p rather than 4d with 4s and 4p. This is why d orbital involvement with bonding is most often discussed with respect to transition metals (d-block elements).

For p-block elements whose only empty d orbitals are from the same shell as the p orbitals (eg 4d with 4p), d orbital participation in bonding is now known to be very limited because those d orbitals are so high in energy relative to the p. The same applies to the s orbitals that you propose to involve in bonding. Unfortunately, many textbooks and teachers still refer to this outdated model of hypervalent sulfur and phosphorus compounds, for example, as using those higher energy d orbitals for bonding, which is likely the source of your confusion.

Once you realize that d orbitals from the same shell that are much higher in energy are not generally involved in bonding, then your question becomes moot.


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