Bedlasky
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Colorful molybdate and tungstate complexes with Cu+ and VO2+
Hi.
I've recently done some experiments with molybdates and tungstates and I have some interesting results.
I was curious if exist insoluble vanadyl molybdate/tungstate. So I poured in to the test tube solution of Na3VO4, acidified it
and added some sodium metabisulfite. Result was beautiful blue solution containing vanadyl ions. Then I added it in to solutions of sodium
molybdate/tungstate. With molybdate very dark blue-purple solution was obtained. With tungstate dark brown solution was obtained. Upon further
acidification both solutions turned in to colourless.
I tried if molybdate/tungstate make complex with acidified vanadate but nothing happened. After additon metabisulfite in to the mix of
molybdate/tungstate and acidified vanadate nothing happend again. I obtained only green solutions, after another metabisulfite additon nothing
changed.
What are these complexes is I don't know. Maybe some kind of heteropolyacids where Mo/W atoms are displaced by V atoms. But heteropolyacids make
vanadium(V) - this is vanadium(IV). Another confusing thing is decolorization of both solutions after further acidification. Where is blue colour of
VO2+? Or are these solutions different complexes?
 
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Today I tried make Cu2MoO4 and Cu2WO4. I prepared solutions of sodium molybdate and sodium tungstate. In
to the both solutions I added some sodium metabisulfite. After that I added solution of copper sulfate. With molybdate I obtained brown precipitate
and with tungstate green-brown precipitate. But after half an minute both precipitates dissolved and formed solutions of the same colours. I compared
these solutions with green Cu(I)-sulfite complex which formed by action of sodium metabisulfite on copper sulfate.
Probably molybdate and tungstate are bonding to Cu(I) in Cu(I)-sulfite complex forming new Cu(I)-sulfite-molybdate/tungstate complex.
In the picture from left: Cu(I)-sulfite complex, Cu(I)-sulfite-tungstate complex, Cu(I)-sulfite-molybdate complex
8.12. - edit: After addition of thiosulfate into these solutions they turns in to colourless.
[Edited on 8-12-2019 by Bedlasky]
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Bezaleel
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Experiments with molybdate and tungstate are usually difficult. Many processes may take place, and there is a strong dependence on the pH of your
solution. I advise you to check Gmelins Handbuch on molybdates and tungstates.
The starting materials themselves also form a challenge. E.g. there is often confusion about ammonium molybdate and tungstate. Often these substances
are taken to be (NH4)2MoO4 or (NH4)2WO4, but usually they are a polymolybdate and tungstate.
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vano
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Beautiful colors. Do you store solutions of stable complexes?
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Bedlasky
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No, this was just test tube research. V(IV) heteropolymolybdate have very very intense purple colour, it can be used as very sensitive test for
vanadium. It have such a intense colour because it's an electron deficient compound - very similar to molybdenum blue. Practically it is reduced form
of vanadomolybdate [VMo12O40]3-. V(IV) can also replace Mo or W atoms in other heteropolyanions, like in [PVMo11O40]-.
Look here for more info.
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vano
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interesting. I liked the use of polysulfide. From which book did you learn similar reactions? Now i think aqueous inorganic chemistry is interesting.
[Edited on 21-1-2021 by vano]
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Bedlasky
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I have multiple sources, it is hard to find decent info about Mo chemistry (and with W chemistry it is even harder). I pracitcally start with random
set of reactions, this start my interest in molybdenum chemistry.
If you want, I can send to you all my online sources in U2U, but it is lot of reading.
I will update this article when I will have some time, because I tried to make some ferrocyanide and fluoride complexes in all oxidations states. I
also read something about periodate complexes recently and want to try NCS- complexes of Mo(VI). I have photos for two more articles about Mo
chemistry, but don't have much time. These Mo articles are complex and time consuming.
[Edited on 21-1-2021 by Bedlasky]
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vano
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Thank you so much. When you want then send it to me in U2U. I love reading, volume does not matter.
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