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While studying the topic of chemical bonding, I came across a statement given by my teacher: Elements of the third period and above do not show hybridisation. An example, in the case of $\ce{PH_3}$, $99.9$% s-character exists in the lone pair due to $\ce{P-H}$ bonds being weaker.

Is this the exact statement of Drago's rule? On various other sources, I have found conflicting views, some say that hybridization actually exists and the actual statement is different, the one being taught to students is wrong.

What is Drago's rule? Does it really exist?

I had gone through this answer, it basically explains a quantum mechanical reason for explaining the observation. What I'm seeking is the exact statement of Drago's rule.

For example, in $\ce{PH3}$ does the lone pair exist in pure s-orbital or in a hybrid orbital that is not $\mathrm{sp^3}$ but some other orbital whose nature can be explained and generalized by Drago's rule?

Safdar Faisal
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  • Unfortunately, the idea that the lone pair on PH3 is nearly a pure s orbital has long ago been debunked. Linus Pauling popularized the idea in the 1930's, but modern molecular orbital theory showed it to be false. – Andrew Jul 24 '20 at 12:35
  • @Andrew Can you please shed some more light on that? The pure s-character of the lone pair was told to me by a professor on another forum and he had made this statement with quite a conviction. – sfsg Jul 24 '20 at 13:44
  • @Aniruddha Deb I had gone through this answer, it basically explains a quantum mechanical reason for explaining the observation. What I'm seeking is the exact statement of Drago's rule. For eg. In PH3 does the lone pair exist in pure s-orbital or in a hybrid orbital that is not sp3 but some other orbital whose nature can be explained and generalized by Drago's rule? – sfsg Jul 24 '20 at 13:48
  • Drago's rule is :For any Group in general,the formula for calculating the number of hybrid orbitals is group number of central element + no.of atoms surrounding the central element(except oxygen)-charge on the central element(consider the charge including its sign) whole divided by 2. I pulled it from here. It seems to be in only Indian Textbooks and is an extension(albeit weak) using the assumptions of Bent's rule – Safdar Faisal Jul 24 '20 at 14:26
  • A weird coincidence? Search "Drago's rule made by" on google (forgive grammar). – Safdar Faisal Jul 24 '20 at 14:27
  • @Andrew I was not aware of that actually. Could you please post an answer with more details? – Aniruddha Deb Jul 24 '20 at 14:54
  • @AniruddhaDeb There are many discussions of this in other questions on this forum about PH3 and SH2 bond angles as well as about Bent's rule. I don't think another answer is needed. – Andrew Jul 24 '20 at 16:56
  • Here's a quotation from Albright et al. Orbital Interactions in Chemistry 2nd Edition (2013) p. 144. "Occasionally somewhat peculiar hybridization arguments have been constructed for molecules like H2S and H2Se which have rather acute bond angles. Since they are close to 90, p atomic orbitals are used to form the A—H bonds leaving the lone pair in an unhybridized s orbital on A. It is clear from the shape of 1a1 and 2a1 that this is far removed from reality." – Andrew Jul 24 '20 at 16:57
  • Thx for all the help, just a small [email protected] to conclude, what percent s-character would actually be present in PH3 lone pair, if not pure s. – sfsg Jul 24 '20 at 18:16
  • Any "rule" that involves d-orbital hybridisation should be deprecated. There is a reason why you can't find a statement of Dragon's rule: it is not rooted in a solid enough theory. – Martin - マーチン Jul 24 '20 at 19:34
  • @Martin-マーチン "There is a reason why you can't find a statement of Dragon's rule" Of course, we won't find a rule based on a mythical creature ;) – Nilay Ghosh Nov 05 '22 at 09:44
  • @NilayGhosh But... But... But... maybe we should though. It'll be an interesting one, I'm sure. And then we possibly can use Dragon's rule to burninate Drago's rule. – Martin - マーチン Nov 05 '22 at 18:48

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