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Bedlasky

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posted on 28-3-2020 at 17:12

Metal heteropolymolybdates

Hi.

I did few experiments with heteropolymolybdates in last days. Most of you probably read about heteropolymolybdates of metalloids and nonmetals - like B, Si, Ge, P, As, Sb, Te etc.

But there are also metal heteropolymolybdates which aren't that often mentioned. They are formed in acidic solution (but they are destroyed by higher concentration of acids, best resistivity to acids have V^V heteropolymolybdate). I often prepared them by mixing of metal salt with excess of ammonium heptamolybdate. This solution have optimal pH for formation of all metal heteropolymolybdates, so no acid is needed.

For V^IV and Cu^ICu^II I used sodium metabisulfite as reductor.

Colours of solutions:

V^IV: Very dark purple

V^V: Orange to yellow-orange

Cr^III: Pink in acidic/green in neutral solution

Mn^II: Yellow

Fe^II: Dark brown

Fe^III: Colourless in acidic/brown in neutral solution

Co^II: Red

Ni^II: Firstly pale green, than turquoise and finally blue (similar to CuSO4 solution). Complex formation is slow, it take some time (I let it stand for a day).

Cu^ICu^II: Dark brown

Cu^II: Grennish blue

Zn^II: Colourless

Al^III: Colourless

Ammonium salts of these heteropolymolybdates have quite low solubility in cold water (but in hot water are soluble - at least trivalent heteropolymolybdates are, from divalent once I tried heat only Co, Ni and Mn). Co and Ni heteropolymolybdates are unstable in hot solution - after heating you obtain precipitates of Co and Ni "molybdates".

Fe^II-polymolybdate is unstable and it decomposes in to molybdenum blue and ferric ions.

Ammonium salt of V^V-polymolybdate is insoluble. If you add some NH4Cl and heat the solution, you obtain orange precipitate. Potassium and rubidium salts are soluble - so there isn't similarity with phosphomolybdates.

Other ammonium salts slowly crystallized out. This is slow process - I must let it stand for a day, but two days will be better.

Colours of solids:

V^IV: ? (I have probably too low concentration of V in solution, so there wasn't any crystal formation)

V^V: Orange

Cr^III: Pink

Mn^II: Yellow

Fe^III: White

Co^II: Beige

Ni^II: Light blue

Cu^ICu^II: Mix of light grey and dark brown

Cu^II: Pale grennish blue

Zn^II: White

Al^III: White

If you add metal salt in excess in to the molybdate solution, you obtain "metal molybdate" precipitates (probably with V exception).

Cr^III: Green

Mn^II: White (sometimes with very pale yellow tint)

Fe^II: Dark brown

Fe^III: Brown

Co^II: Violet

Ni^II: Pale green

Cu^ICu^II: Dark brown

Cu^II: Turquoise

Zn^II: White

Al^III: White

I'll add later some pictures.

Btw. Formation of Sn^II-polymolybdate is responsible for quick reduction of molybdate by Sn^II.

Some literature: https://open.bu.edu/ds2/stream/?#/documents/223173/page/1

I tried preparation of Co^III-polymolybdate but without success.

[Edited on 29-3-2020 by Bedlasky]

Bezaleel

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posted on 2-4-2020 at 15:00

Hi, That's a great overview of the colours these compound take. And a lot of work in testing all of this. Thanks for sharing.

I wonder, have you considered to add an oxidiser to the Co(II) and Ni(II) solutions? It seems that the higher oxidation state contributes to the stability of the molybdate complexes, so that might work either way for stabilising Co(III) and Ni(III) or Ni(IV) polymolybdates.

I have tested Pr(III) and Er(III) with ammonium heptamolybdate, but nothing spectacular happened. I think they just precipitate the regular molybdates Er2(MoO4)3 and Pr2(MoO4)3. It could be that these f-block elements are just a size to big for forming a heteropolymolybdate.

Bedlasky

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posted on 2-4-2020 at 19:32

Thanks.

I tried to oxidized Co(II) and Mn(II) with persulfate on hot water bath, but without succes.

When I'll do experiments with cerium, I'll try reactions of Ce(III) and Ce(IV) with molybdate and tungstate.

I yesterday uploaded photos in to the computer. I'll upload it at evening on SM.