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Author: Subject: Ammonium salts preparation....
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[*] posted on 1-5-2007 at 03:23

On the subject of exotic ammonium salts, I wonder if it might be possible to create ammonium hydride, NH5. Ordinarily you would expect the hydride anion to act as a base, and deprotonate the ammonium ion to give ammonia gas and hydrogen. However, if you could trap the ammonium ion inside a crown ether or something similar, you could prevent it from interacting with the hydride ions and render the ammonium hydride stable. It's basically the same idea as was used to prepare stable alkali metal anion salts - trap the alkali metal cation in a crown ether to prevent it reacting with the anion. And on the note of alkali metal anions, ammonium natride would be pretty nice :).

[Edited on 1-5-2007 by Pyrovus]

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[*] posted on 11-5-2007 at 16:24

what ammonium salt and what reagent is it going to be if I want to pour the reagent onto a bucket of the amm salt and get huge clouds of ammonia?
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[*] posted on 11-5-2007 at 18:56

The NH4+ cation is smaller than, and has less available orbitals than, the Na+, K+, and other alkali metal cations which have been successfully chelated in hexadentate crown-ether ligands, primarily for the purpose rendering them soluble in aprotic organic solvents. In fact, NH4+ has no spare 2s and 2p orbitals available with which to accept electron pairs from crown-ether molecules, unline Na+, K+, and even Li+. Even if it could somehow, steric factors would allow only a tetradentate ligand to chelate it; but there is no known case of any second-row element, after filling its 2s and 2p orbitals with 8 electrons, then engaging in further bond-formation by utilizing empty 3s and 3p (and 3d) orbitals. By contrast, third-row elements can expand beyond 8 valence electrons (up to 12, with more than 6 ligands being sterically precluded) by utilizing empty 3d orbitals.

However, it may just be possible to make He, Ne, and possibly Ar atoms accept electrons to become anions which are isoelectronic with monatomic Li, Na, and K, but even more reactive, if they could be stabilized somehow. Stabilization with a hexadentate crown-ether ligand in the case of Ne- and Ar-, and tetradentate crown-ether ligand in the case of He-, might just do it. If, as Pyrovus says, crown ether-chelated chelated alkali metal anions have been obtained by inducing them to accept electrons, it just might be possible with inert gas atoms, although the spin-pairing of the two valence electrons in an alkali metal anion may make them more stable than an inert-gas anion..

Even if NH5, as the salt NH4+H-, could be somehow obtained by mixing a NH4+ salt and an alkali metal hydride salt (spontaneously flammable in air) in a non-oxidizing polar aprotic solvent, it is likely to be a fairly ephemeral species, because of the driving-force of the heat of formation of H2 from H+ and H-.

It may be for the same reason that all attempts to prepare NF5, as the salt NF4+F-, have been unsuccessful, in spite of the existance as very stable compounds of NF4BF4, NF4PF6, NF4SbF6, and NF4ClO4,. However, even these latter compounds are not easy to prepare, as they require either the addition of F+ to NF3, or loss of an electron by NF3 with subsequent bonding by an F atom. For example, NF4SbF6 can be prepared by reacting NF3 with the salt KrFSbF6, one of the few isolatable Kr compounds (obtained by reaction of KrF2 with SbF5); the KrF+ cation decomposes to Kr and F+, then the latter reacts with NF3. ClF6+ has been obtained in the same manner. Methods that have been used to try to prepare NF5 have been primarily heating NF3 with F2 under high pressure. Similarly, although the OF3+ cation, containing tetravalent oxygen, is theoretically thermodynamically stable, all attempts to prepare it, as the salt of either F- or BF4- or SbF6- or other anions, have been unsuccessful, even though of course H3O+ exists in all aqueous solutions of acids.
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