Do $\ce{PH3F2}$ and $\ce{XeS2}$ exist, in accordance with Valence Bond Theory?

According to Drago's Rule, if the central atom of a compound is an element of the third period, and the attached atom has an electronegativity lesser than 2.5, then hybridization does not occur, and bonding occurs solely in atomic orbitals.

In the case of a compound like $\ce{PH3F2}$, the fluorine atoms occupy the axial positions, while the hydrogen atoms occupy the equatorial positions. However, the electronegativity of fluorine is 4, hence the EN. difference with phosphorous is larger, making the axial bonds very strong, meanwhile the electronegativity of hydrogen is 2.1 (below 2.5), hence the equatorial bonds are very weak, and hybridization shouldn't occur ie. $\ce{PH3F2}$ shouldn't exist, according to Valence Bond Theory.

Searching for answers online just led me to this Quora post, contradicting the answer given, which I found inconclusive.

And I couldn't find any explanation for $\ce{XeS2}$ not existing, at all.

This doubt stemmed from the following JEE (competitive exam) question:

Identify the number of molecules which do not exist.

$\ce{SBr6}$, $\ce{PH5}$, $\ce{XeF^-_3}$, $\ce{PH4F}$, $\ce{PH3F2}$, $\ce{OF4}$, $\ce{XeF^-_5}$, $\ce{XeO4}$, $\ce{XeS2}$

The answer given was: (7)

$\ce{SBr6}$, $\ce{PH5}$, $\ce{XeF^-_3}$, $\ce{PH4F}$, $\ce{PH3F2}$, $\ce{OF4}$, $\ce{XeS2}$

I understood the rationale behind all, except for $\ce{PH3F2}$ and $\ce{XeS2}$.

Edit: If anyone could answer this question with respect to VBT, VSEPR and hybridization I'd gladly give you my upvote :)

  • $\begingroup$ A one minute Google search turns up pubs.acs.org/doi/pdf/10.1021/ic00322a013 $\endgroup$
    – Ian Bush
    Oct 31, 2023 at 13:59
  • 1
    $\begingroup$ I suggest you read one of the many answers here that explain why hybridisation should not be used here, or arguably anywhere. One example chemistry.stackexchange.com/questions/18427/… $\endgroup$
    – Ian Bush
    Oct 31, 2023 at 14:06
  • $\begingroup$ For future reference: for the body of questions, answers, and comments, chemistry.se offers to use mhchem as a comfortable method to add chemical equations. $\endgroup$
    – Buttonwood
    Oct 31, 2023 at 15:37
  • $\begingroup$ @Poutnik Considering that Chemistry is infamous for exceptions, I've gotten used to expecting the unexpected. Why don't you leave an answer explaining the rationale of your "chemist guts" (even if it doesn't incorporate theory)? $\endgroup$
    – Bongo Man
    Oct 31, 2023 at 15:45
  • 1
    $\begingroup$ The notion stated in the "Edit-statement" is simply not true and quite rude actually. Hybridisation is a necessary construct of Valence Bond Theory, which is an actual theory and not that handwavy stuff that is written in some textbooks. As such, hybridisation is not a concept that is frowned upon, at least when you try to really understand it. Sure, there are other ways of looking at things and you might not need the concept. Anyway, there is a time to use the concept of hybridisation and then there is a time where it simply is wrong. This has nothing to do with preference. $\endgroup$ Nov 7, 2023 at 17:25

2 Answers 2


In theory, all of the mentioned compounds in the answer don't exist except for $\ce{PH3F2}$. Difluorophosphorane has been characterized through spectroscopical methods a long time ago. Here is an excerpt from a paper1:

$\ce{PH3F2}$, first reported in 1971 by Seel and Velleman, is now accessible in pure form and has been unambiguously characterized by high resolution infrared spectroscopy. At present it marks the borderline between the well-established fluorophosphoranes $\ce{H_{n}PF_{5−n}, n = 2−5}$, and the unknown species $\ce{PH4F}$ and $\ce{PH5}$.

I am going to also shamelessly plug the paper2 mentioned by @IanBush in the comments.

Regarding $\ce{XeS2}$, it doesn't exist. However, oxosulfides of form $\ce{XeO_{n}S_{m}}$ are known3. Also, excimer $\ce{XeS}$* is known4.


  1. Gas-phase structure of difluorophosphorane determined by high resolution infrared spectroscopy, H. Beckers, H. Bürger, A. Rahner, Vol.54, 1991, DOI: 10.1016/S0022-1139(00)83877-4
  2. Preparation and spectroscopic characterization of difluorophosphorane, PH3F2. Phosphorus-31 NMR spectrum of protonated diphosphine, P2H5+, Rolf Minkwitz and Andreas Liedtke, Inorganic Chemistry 1989 28 (23), 4238-4242 DOI: 10.1021/ic00322a013
  3. Xenon oxides, sulfides, and oxysulfides. A theoretical ab initio investigation, Aristotle Papakondylis, Vol. 1015, 2013, DOI: 10.1016/j.comptc.2013.03.030
  4. Xenon oxide and xenon sulfide emission systems at 234 and 227 nm, J. Xu, D.W. Setser, J.K. Ku, Vol. 132, 1986, DOI: 10.1016/0009-2614(86)80640-6
  • $\begingroup$ Nice to have an Indian, who understands what a fellow JEE aspirant is going through, answer my question without relentlessly bashing me. +1 and marked as answer. $\endgroup$
    – Bongo Man
    Nov 17, 2023 at 6:19

As if to refute the hybridization theory once and for all, $\ce{PH3F2}$ (difluorophosphorane) exists! Andrews and Withnall[1] produced it and trapped it in an argon matrix by cocondensing phosphine and elemental fluorine into this matrix. The authors also mention that their result is the first experimental evidence for $\ce{PH2F}$, which "should be" more stable and easier to access according to the hybridization-based argument.


  1. Lester Andrews and Robert Withnall (1989). "Cocondensation reaction between phosphine and fluorine: matrix infrared spectra of difluorophosphorane, difluorophosphine and fluorophosphine". Inorg. Chem. 28, 3, 494–499. https://doi.org/10.1021/ic00302a023
  • 1
    $\begingroup$ Meh, that only proves they are actual intermediates in reaction between PH3 and F2. Many really surprising things can be trapped like that. $\endgroup$
    – Mithoron
    Oct 31, 2023 at 19:01
  • $\begingroup$ Still existence for me. $\endgroup$ Oct 31, 2023 at 19:02
  • $\begingroup$ Says guy who was doubting phosphonium fluoride ;p $\endgroup$
    – Mithoron
    Oct 31, 2023 at 22:07
  • $\begingroup$ But I did not see PH4F in the reference. Ionic PH4F would decompose and PH3F2 apparently did so partially in that reference. $\endgroup$ Oct 31, 2023 at 22:10
  • $\begingroup$ I'm saying it's a different league completely. In argon matrix you can have like anything, starting with single atoms of any element. You could probably even have a non-ionic PH4F. $\endgroup$
    – Mithoron
    Oct 31, 2023 at 22:19

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