I Diatomic molecules containing potassium

[BillKet]

Hello! Are there any diatomic molecules containing potassium that can be produced and are stable (in terms of the chemical bonding) both as positive ions and as neutral molecules? Thank you!

 

[Baluncore]

KCl
https://en.wikipedia.org/wiki/Potassium_chloride
KI
https://en.wikipedia.org/wiki/Potassium_iodide

 

[TeethWhitener]

You’d have to define stable. At STP, no. But potassium halides and KH (and probably many others) are stable as diatomics in the vapor phase. As for the ions, I’m not sure off the top of my head, but I can’t imagine KF+ falls apart whereas KF is stable in the vapor phase. I’ll do some more digging; a quick perusal of Herzberg didn’t give me quick answers.

 

[BillKet]

KCl
https://en.wikipedia.org/wiki/Potassium_chloride
KI
https://en.wikipedia.org/wiki/Potassium_iodide

I am a bit confused. If you remove one electron from them, won't the bond break? Hence you can't have them as positive ions.

 

[BillKet]

You’d have to define stable. At STP, no. But potassium halides and KH (and probably many others) are stable as diatomics in the vapor phase. As for the ions, I’m not sure off the top of my head, but I can’t imagine KF+ falls apart whereas KF is stable in the vapor phase. I’ll do some more digging; a quick perusal of Herzberg didn’t give me quick answers.

Thank you for the reply and sorry for the loose post. It doesn't have to be STP. I am interested mainly in producing them for spectroscopic studies, so if they are produced by (say) laser ablation, hence at quite high temperatures, that is totally fine. However, in the case of KF+, I thought that the removed electron is the bonding one, which would make the molecule break apart.

 

[Twigg]

KRb, NaK come to mind, but in the linked experiments they're only "stable" because their mean free path is ridiculously low high (near vacuum, low temperature).

Edit: I had it backwards before. The mean free path was very long, so the collision rate (and by extension the reaction rate) is negligible).

 

[TeethWhitener]

Thank you for the reply and sorry for the loose post. It doesn't have to be STP. I am interested mainly in producing them for spectroscopic studies, so if they are produced by (say) laser ablation, hence at quite high temperatures, that is totally fine. However, in the case of KF+, I thought that the removed electron is the bonding one, which would make the molecule break apart.

It helps to work out the MO diagram. The MOs will be (approximately) linear combinations of the 4s valence subshell of K and the 2p valence subshell of F. The bonding orbital will be a sigma orbital that is actually lower in energy than the HOMO, which will be non-bonding p-type orbitals on F. So ionization of KF will remove an electron from a non-bonding orbital. Again, I haven’t found any experimental results, and I haven’t run an ab initio calculation on the system, but that’s what my gut says.

 

[sysprog]

All of the halogens form stable compunds with potassium (depending on what is meant by 'stable', as @TeethWhitener said)

Does potassium bromide look stable?

 

[TeethWhitener]

I believe @BillKet is specifically referring to diatomic molecular potassium species (analogous to HF, for example), which the alkali halides clearly do not form at STP.

 

[Baluncore]

Are there any diatomic molecules containing potassium that can be produced and are stable (in terms of the chemical bonding) both as positive ions and as neutral molecules?

I am a bit confused. If you remove one electron from them, won't the bond break? Hence you can't have them as positive ions.

And that is why I thought the OP contained a contradiction. It seems to me you have defined the species you are looking for out of existence.

 

[BillKet]

And that is why I thought the OP contained a contradiction. It seems to me you have defined the species you are looking for out of existence.

Why would that be the case? For example, BaF exists both as a neutral molecule and as BaF+. As long as the removed electron is not bonding, the positive ion can still be stable.

 

[TeethWhitener]

Why would that be the case? For example, BaF exists both as a neutral molecule and as BaF+. As long as the removed electron is not bonding, the positive ion can still be stable.

It can still be stable even if the electron is removed from a bonding orbital. The simplest case is H₂⁺.

 

[BillKet]

It can still be stable even if the electron is removed from a bonding orbital. The simplest case is H₂⁺.

I agree, but this was my original question. Are there any diatomic molecules containing potassium that we know about (either theoretically or experimentally) to be stable both as neutral and ions?

 

[TeethWhitener]

I agree, but this was my original question. Are there any diatomic molecules containing potassium that we know about (either theoretically or experimentally) to be stable both as neutral and ions?

I was banging my head against a wall with how ungoogleable your request for (e.g.,) the KCl cation was, when it occurred to me that it's completely obvious that KCl⁺ exists and can be easily detected: just look up mass spectrometry on KCl. MS only detects ions, and a quick google search pulls up scads of results on the mass spectrum of KCl⁺ (in its various isotopic forms).

As for optical spectroscopy, NIST's webbook doesn't give any info for the cations of KH, KF, KCl, or KBr--only a reference to Herzberg. I would bet a decent amount of money that someone back in the 50s or 60s looked at them (possibly earlier), but again, if there's any insight buried in the "KCl cation" google search, it's among an ocean of irrelevant basic high school chemistry articles.

 

[Twigg]

On second thought, it seems you're only interested in whether or not the electronic structure of the molecule is stable (not whether it is chemically inert). In that case, KRb absolutely fits the bill.

I know KRb+ is electronically stable because the experiments (for example) that magnetoassociate KRb molecules detect the molecules by photo-ionization followed by time-of-flight detection. (The time-of-flight detection allows them to determine the mass of the molecule and be sure that they're detecting KRb+ and not K+ or Rb+). If KRb+ wasn't electronically stable, it would dissociate virtually instantly, and would be detected after a different time-of-flight.

As I mentioned above, I know people have looked at NaK. I know of one group (skip to page 100/199 of the pdf) that has prepared Feshbach molecules of $K_2$ just below the dissociation threshold. I don't know if anyone has done ionization to $K_2^+$.

I know alkali-alkali dimers are pretty niche, but I thought I'd throw it out there.