Talk:Rieke metal

More details on mechanism

The article should explain what is special about the Rieke process, e.g. why it ends up with a fine suspension of the metal rather than large grains or sponge. Also it should be specific about particle sizes.--Jorge Stolfi (talk) 17:26, 16 March 2019 (UTC)[reply]

We might take the alternative approach. There is no alternative to obtaining fine suspensions. If the process gave big chunky crystals or a sponge, then we'd have some explaining to do. We could nevertheless indicate the particle sizes because there must be some micrographs.

But maybe we should indicate how surface area is connected to reactivity in heterogeneous. --Smokefoot (talk) 17:39, 16 March 2019 (UTC)[reply]

I mean, one can obtain copper or gold metal by reacting a salt with some reducing agent; but the result is often largish particles that precipitate immediately. I suppose that they don qualify as "Rieke metals", right? One can obtain many metals from salts by electrolysis, but the result is often crystals or a sponge. So I wonder why the particle sizes are much smaller in that process... --Jorge Stolfi (talk) 09:16, 17 March 2019 (UTC)[reply]

I dont think that reduction of Cu or Au salts gives largish particles. Part of the issue is that not many metals can be reduced to the metal in solution. Reduction to the metal is something most easily done with late metals, but is the exception, rather than normal behavior. I mean, I have seen Ni crash out in a prep of Ni(C2H4)3, but that is an exotic process. You dont encounter anyone precipitating Ti⁰, Mo⁰, Nb⁰, etc. Electrolysis is (to me) a completely different game, since the deposition preferentially occurs layer by layer, since the first layer supplies the electrons. I have made Rieke Mg using potassium, and just remember it being black. The growth of Au crystals is an area of much study, but I am pretty sure that if one just adds borohydride to a gold salt you just get a black solid. Even "Pt black" is really complicated. I guess that we shuold look for sources. --Smokefoot (talk) 12:58, 17 March 2019 (UTC)[reply]

Well, here are some videos of metals being formed by reduction of salts:

silver from silver nitrate + copper
gold from HAuCl4 + Na2S2O5
copper from CuCl2 + iron
nickel from NiSO4 + zinc

Those experiments is why I found the Rieke process "not normal"... Is the use of the alkali metal as reduction agent and/or the aprotic solvent that makes the difference? --Jorge Stolfi (talk) 19:36, 17 March 2019 (UTC)[reply]

The videos conform with what is going on with making Rieke metals. One gets fine black or dark solids. The silver plating reaction is apparently slow, hence one gets some crystal growth. I guess if one did not stir the Rieke reaction and if there was a way to make sure that the KCl byproduct dissolved, it would be possible to grow crystals of Mg. But the reduction is rapid, and one gets a gemisch of KCl and fine Mg. The method uses excess oxidant (MgCl2, NiCl2, etc) to ensure that the alkali metal is fully consumed, since residual K would react with the down-stream organic substrates. --Smokefoot (talk) 21:16, 17 March 2019 (UTC)[reply]

This may no longer be relevant to the article, but I am still curious about the mechanism of the reaction.
Starting with the third method, that reacts MgCl₂ and pre-made lithium "naphtalide" (LiNp): I suppose that, since the solvents are polar, small amounts of the two salts will dissolve and dissociate to some extent. The Li⁺ and Cl⁻ then precipitate out as LiCl, which I guess is much less soluble than MgCl₂. Also two Np⁻ anions will donate their electrons to Mg²⁺ yielding Mg⁰ and radicals Np^•. These dimerize into "binaphthenyl" that remains in solution. (Maybe they will form MgNp₂ first, which then breaks apart?). Would this make sense?
--Jorge Stolfi (talk) 19:51, 19 March 2019 (UTC)[reply]

here are my views/comments, not that I really know how this all works:

MgCl₂ + 2 lithium "naphthalide" (LiNp) → Mg + 2 LiCl + 2 naphthalene

Its "just" electron transfer (ET). Naphthalene is regenerated.

Solids do not react near room temp, but solids do react with soluble species, especially when ET is involved since direct contact is not required.

In traditional Rieke chem, presumably MgCl2 dissolves via thf complex, and makes contact with solid K. KCl solid forms together with solid Mg. Its a mess, but a high surface area mess. --Smokefoot (talk) 02:31, 20 March 2019 (UTC)[reply]

Thanks! I thought that naphthalene lost one H⁺ to become the naphthalenide, but I see now that it just gains one electron instead. --Jorge Stolfi (talk) 06:10, 20 March 2019 (UTC)[reply]

Chemical Abstracts report

Interest in Rieke Mg and most metals is modest, judging from the citation data. It appear that Rieke Mg is used rarely. Of all the Rieke metals, the greatest interest has been in Rieke zinc, which is used to make polythiophenes. With about 1400 total citations, DOI:10.1021/ja00106a027 peaked in the year 2010 at about 95 citations per year. --Smokefoot (talk) 12:09, 20 March 2019 (UTC)[reply]