Sciencemadness Discussion Board

Higher oxidation states of caesium.

Pyrovus - 24-5-2006 at 01:44

An alkali metal cation has the same electron configuration as the proceeding noble gas. The heavier noble gases can be oxidised to form compounds, and so in theory, an alkali metal cation should be able to be oxidised to form an analogous compound (although, with a charge of +1 they will be harder to oxidise than the corresponding noble gas). The question of course, is whether any of the stable alkali metals are sufficiently easily oxidised to form these higher compounds.

While viewing the wikipedia article on Caesium, I found this passage:

http://en.wikipedia.org/wiki/Cesium
Quote:

There is an account that caesium, reacting with fluorine, takes up more fluorine that it stoichometrically should. It is possible that, after the salt Cs+F− has formed, the Cs+ ion, which has the same electronic structure as elemental xenon, can like xenon be oxidised further by fluorine and form traces of a higher fluoride such as CsF3, analogous to XeF2.


So this would appear to suggest that caesium is capable of forming higher valent compounds. These compounds can be expected to be less stable, more difficult to form and more strongly oxidising than the analagous xenon compounds.

So, does anybody know anything about the stability of such compounds - they might provide an interesting use for caesium salts if they prove stable enough to produce in the home lab. Of course, formation of CsF3 would involve fluorine gas (eek!), but perhaps oxygen compounds could be formed instead eg (O=Cs=O)(+)F(-), analagous to XeO2. The caesium analog of perxenate, CsO6(3-), if it exists (which admittedly is doubtful) would be very interesting, as it would involve the unusually high +9 oxidation state.

[Edited on 24-5-2006 by Pyrovus]

Dr. Beaker - 25-5-2006 at 15:41

Interesting idea...
indeed Cs+ is isolobal with Xe and with the decrease in delta E between main quanton numbers doun the raw there should have been similar chemistry.
unfortunatly, ionization energy don't promise good news...
Cs+ 2421.8 kJ/mole
He 2372.3 kJ/mole

i.e it's easier to make He+ then Cs+2 ions!
as you know there is no He fluorides (or any other compound for that matter) known.
and I also remind you that the notion of noble gas compounds started when it was realized that Xenon has smaller ionization E then O2, while salts of O2+ where known.

Pyrovus - 27-5-2006 at 20:44

Well, we are talking about forming covalent compounds, as opposed to salts of Cs++, Cs+++ etc, so I'm not sure if much can be read into ionisation energies. If we take the second and third ionisation energies of nitrogen for instance, we get 2856 and 4578 kJ/mol (source www.webelements.com) respectively, more than the second and third ionisation energies of caesium (2234 and 3400 kJ/mol). Yet despite this, we are still able to obtain NO+ as a stable ion, and so if we go by ionisation energies alone, this suggests that it should be easier to form CsO+ than NO+.

[Edited on 28-5-2006 by Pyrovus]

Dr. Beaker - 29-5-2006 at 15:10

I dont really think you can compaire NO+ and the hypothetical CsO+.
1st: N and O has close electronegativities and therefore high covalent (and low ionic) character. the more reallistic model is described by MO theory. this is not the case with Cesium, which is the 2nd most electropositive element (CF theory).
2nd: in NO+ you have SP2 hybridization. 2S and 2P are small orbitals that have good overlap with each other. CSO+ need to use its 5p orbitals and will have weaker overlap. see for an example CO2 and silica. CO2 has pi bonds and SiO2 is all sigma bonded (Si=O bonds are too weak).

Dr. Beaker - 30-5-2006 at 11:20

quote from Machado, Francisco B. C.; Ghanty, Tapan K.; Chakravorty, Subhas; Davidson, Ernest R. Ab initio calculations on XFnq (X = I, Xe, Cs, and Ba; n = 1, 2, 4, and 6; q = -1, 0, +1, and +2) molecules. International Journal of Quantum Chemistry (2001), 81(3), 238-245:

"As expected by chemists, one unit of nuclear
charge from Xe (Z = 56) to Cs (Z = 57) completely
changes the chemistry and changes the 5s, 5p
electrons from valencelike to corelike in terms of
participating in covalent bonding. Similarly, reducing
the nuclear charge by one unit from Xe to I-
greatly increases the tendency to form polar covalent
bonds with fluorine atoms. As chemists always
expected, xenon itself is on the edge between the valencelike
behavior of the 5s, 5p electrons of I- and
the corelike behavior of Cs+."

halogen - 27-6-2006 at 21:22

Has the +9 ever been observed before?

12AX7 - 28-6-2006 at 13:50

I don't think so. You'd think some wild things could be done with those metals that give up a lot of electrons, like osmium or iridium. Curious that both only give up 8 electrons tops.

On the other hand, maybe nothing is capable of *keeping* that +9 charge. Fluorine would, but it would take nine crammed around a rather shrunken ion to do it. Oxygen is the best all around, allowing a positive heat of formation even for perchlorate, but five oxides around anything, hmm... Nothing else of course is suitable; nitride is too weak, and any atoms below these are too large to be as reactive.

Tim

Dr. Beaker - 29-6-2006 at 22:39

ioinic bonding for high oxides (and fluorides) of transition metals and f block elements is not a very good model. a more realistic one is a major covalent contribution, with some ioinic character. OsO4, TiCl4, UF6 are all volatile compounds, not what you'd exspect from an ionic compound like NaCl. so you can't quit say that UF6 has the U+6 ion.

[Edited on 30-6-2006 by Dr. Beaker]

12AX7 - 30-6-2006 at 13:09

Yeah, I know. So call it "effective charge".

JohnWW - 30-6-2006 at 16:58

I think that plutonium also forms compounds in a +8 oxidation state, or at least a volatile tetroxide PuO4. +7 compounds of it and neptunium have also been isolated. But no +9 state of americium has been obtained, or anything hgher than +6. The only other element that just might have a +9 state is iridium, which would have to be as an oxide or oxyfluoride, and that would be extremely difficult. In the case of osmium, OsO4 can be made simply by heating the metal in air which produces its characteristic penetrating odor (do not try this at home - very volatile, poisonous, and reacts with organic matter to re-form the metal); but OsF8 apparently cannot be made because of excessive steric strain due to "crowding". It is no coincidence that Os and Ir are the densest of elements in the solid state, through having so many of their outer electrons in the conduction band as well as heavy nuclei close to the limit for stability.
As an alternative to trying to make compounds of Cs(III) or Fr(III), how about trying to make anions of helium and neon, He- and Ne-, isoelectronic with Li and Na metals but rather more reactive, by inducing them to take up an electron? I wonder if reacting He or Ne with Cs metal would do it, to give CsHe or CsNe.
Similarly, I wonder if BF3, a very strong Lewis acid, could take up a Ne atom to form BF3Ne, isoelectronic with the BF4- anion. A similar compound with Ar, BF3Ar, was at one time supposed to me made, but I am not sure what became of that claim.

[Edited on 1-7-2006 by JohnWW]

12AX7 - 30-6-2006 at 17:17

Meh.. it takes a lot of energy to make those accept electrons. I...can't seem to find a table in my textbook, it doesn't go up to noble gasses. :mad:

Wouldn't Xe or Kr be better anyway? He or Ne have smaller radii so the added electron would be closer to the nucleus, but it would also represent a much larger percentage of the cloud's charge. With Xe at least you stand a chance of a smaller percentage; I don't think size would matter much because an inert gas doesn't very much want an electron anyway!

Doesn't make sense to me at any rate, why attempt something isoelectronic with something that has a tenuous hold on its electron as-is?

Tim

guy - 30-6-2006 at 18:10

There's no way to form anions of noble gases because there is not enough affinity for the electron when because of shielding and repulsion. There also isn't enough positive charge to hold the electron. There is nothing favorable about it.
Making cations of noble gases however, is possible for the bigger ones.

neutrino - 30-6-2006 at 19:45

Quote:
Originally posted by JohnWW
how about trying to make anions of helium and neon, He- and Ne-, isoelectronic with Li and Na metals but rather more reactive, by inducing them to take up an electron? I wonder if reacting He or Ne with Cs metal would do it, to give CsHe or CsNe.


Shiny cesium metal samples are typically packed under argon. These samples stay shiny, so I doubt any reaction takes place. At least, none that would coat the metal in a dull salt layer. And if Ar doesn’t react visibly, I seriously doubt that He or Ne would either.

The only way I could see this working is by bubbling Kr or Xe through a solution of either Cs- anions or free electrons. (Both can be made with a suitable application of crown ethers.)

The_Davster - 30-6-2006 at 20:18

I've heard of hot cesium cesium(100C?) being worked with in an argon glovebox, no reaction occured.

At work I have a database for the crystalography of many many comounds, on monday Ill look up Cs and see what is in a higher oxidation state.


[Edited on 30-10-2006 by The_Davster]

Jdurg - 1-7-2006 at 15:27

Quote:
Originally posted by neutrino
Quote:
Originally posted by JohnWW
how about trying to make anions of helium and neon, He- and Ne-, isoelectronic with Li and Na metals but rather more reactive, by inducing them to take up an electron? I wonder if reacting He or Ne with Cs metal would do it, to give CsHe or CsNe.


Shiny cesium metal samples are typically packed under argon. These samples stay shiny, so I doubt any reaction takes place. At least, none that would coat the metal in a dull salt layer. And if Ar doesn’t react visibly, I seriously doubt that He or Ne would either.

The only way I could see this working is by bubbling Kr or Xe through a solution of either Cs- anions or free electrons. (Both can be made with a suitable application of crown ethers.)


Not always. Depending on how airtight your glass is, little bits of oxidizeable substances can slowly leach through the glass and give a bit of a discoloration to the cesium. I have a five-gram ampoule of cesium, and while I'm pretty sure that no oxygen has leached in, the ampoule itself must have been very dusty because there is a LOT of lint and dust on the inside of the glass. It appears as if the cesium has reacted with it because the glass itself got discolored in areas.

Cesium is indeed quite a nasty little bugger. I'm not worried at all about my sample getting ruined or reacting its way out of the glass. I'm just pissed that the glass is a bit hazy and golden colored now as the cesium has reacted a tiny bit with it and/or leached into it.

JohnWW - 2-7-2006 at 14:57

The last three posters talked about metallic caesium stored or handled under argon, which is a lot more electropositive than helium and neon. I wonder what would happen if Cs was introduced to He or Ne under substantial pressure, instead, which is what I am getting at.