Sciencemadness Discussion Board

Complex of copper(II)

woelen - 21-9-2008 at 11:56

This experiment is quite interesting, showing that copper(II) can achieve red colors, even without any oxidation to copper(III). Simply mixing with concentrated hydrobromic acid and concentrated hydrochloric acid allows formation of blood-red copper(II) compounds. Copper(II) also can be deep purple. Just a matter of the right ligand. Remarkably, even simple bromide and chloride can do the trick:

http://woelen.homescience.net/science/chem/exps/copper_halog...



EDIT by woelen: Changed link, so that it works again.

[Edited on 12-6-12 by woelen]

Jor - 21-9-2008 at 14:18

Wow I never realised Cl or Br complexes of copper had those colors! Always thought they were green, however I never made such a concetrated solution.
I'm wanting to make ammonium tetrabromocuprate(II) dihydrate. What would you recommend? Just reacting 1 mol of copper(II) oxide with 4 mols HBr (about 40% concetration), then add 2 moles of ammonia (25%), and evaporate?

ScienceSquirrel - 22-9-2008 at 03:14

Quote:
Originally posted by Jor
Wow I never realised Cl or Br complexes of copper had those colors! Always thought they were green, however I never made such a concetrated solution.
I'm wanting to make ammonium tetrabromocuprate(II) dihydrate. What would you recommend? Just reacting 1 mol of copper(II) oxide with 4 mols HBr (about 40% concetration), then add 2 moles of ammonia (25%), and evaporate?


That will not work as ammonia is a stronger base and ligand than bromide.

To make acompound like that you would need to use something like tetrabutylammonium as the cation.

You could try making the potassium salt but quite a few of these species are only known in solution or are very difficult to separate and purify.

Jor - 22-9-2008 at 03:24

Yes I understand ammonia is a stronger ligand than bromide, but ammonium is not.

1 Mol copper oxide with 4 mols HBr:

CuO + 2 HBr --> CuBr2 + H2O
CuBr2 + 2 HBr --> 2 H+ + CuBr4 2-

the ammonia added will react with the excess H+.

So overall:

CuO + 4 HBr + 2 NH3 --> 2 NH4+ + CuBr4 2-

Or just add ammonium bromide to a copper(II)bromide solution.

Ammonium tetrabromocuprate(II) dihydrate does excist, just type into google.

For example:
http://cat.inist.fr/?aModele=afficheN&cpsidt=18250074

It is sad that the complex salt can be obtained by slow evaporation. But AFAIK no ther ions may be present, e.g. sulphate, or it will not be a pure salt, so therefore I will leave from copper(II) oxide or maybe carbonate.

[Edited on 22-9-2008 by Jor]

ScienceSquirrel - 22-9-2008 at 03:35

I would get hold of the paper and try their method.

Getting the concentrations just right can be critical.

If you can get hold of this paper you will find an easy preparation of the corresponding chloro compound and an investigation of its thermochromic properties.

Choi, S.; Larrabee, J. A. "Thermochromic Tetrachlorocuprate(II): An Advanced Integrated Laboratory Experiment" J. Chem. Ed., 1989, 66, 774.

[Edited on 22-9-2008 by ScienceSquirrel]

Jor - 22-9-2008 at 06:21

Quote:
Originally posted by ScienceSquirrel
I would get hold of the paper and try their method.

Getting the concentrations just right can be critical.

If you can get hold of this paper you will find an easy preparation of the corresponding chloro compound and an investigation of its thermochromic properties.

Choi, S.; Larrabee, J. A. "Thermochromic Tetrachlorocuprate(II): An Advanced Integrated Laboratory Experiment" J. Chem. Ed., 1989, 66, 774.

[Edited on 22-9-2008 by ScienceSquirrel]


I do not have access to the papers unfortunately. I will try making it on test-tube scale first. Just dripping conc. HBr on CuO (just made a few grams, don't have it :P), and when it is had dissolved, add the same volume HBr as needed to dissolve the CuO. Then add conc. ammonia, such that H+ is in slight excess compared to NH3, so hardly any NH3 will compete with Br- as a ligand. Then evaporate slowly. Will do it in the following week.

Ok I tried it today. I took a small spatula of black copper(II) oxide, and added a total of 11 drops 40% HBr (it's very hard to see if all CuO has dissolved, probably 3-4 drops dissolved it so I needed only 8 drops 40% HBr). The HBr has a very slight yellow color,at the time I made it 2-3 months ago it was colorless. A deep dark red liquid resulted. To this I added 4 drops of 25% reagent grade ammonia. Some fumes were observed, these were ofcourse ammonium bromide. The liquid is still dark red, and has a pH of about 3 (I used pH-paper). It has some green solid floating in it, after the ammonia has been added. Is this the desired complex salt? I will evaporate to see what I get. Because I did so small scale molar ratios were hard to do right, and thus there probably is excess ammonium bromide in solution, but this is no problem, as it is white.



[Edited on 22-9-2008 by Jor]

woelen - 22-9-2008 at 09:27

I can imagine that making the dry salt is very difficult, because these complexes are EXTREMELY hygroscopic. You could try evaporating in air, until no further drying occurs and then dry over concentrated H2SO4 in a small closed vessel.

I don't know the color of solid (NH4)2CuBr4, but I can perfectly imagine that it is dark green. There are quite a few compounds, whose solid color is quite different from the color in solution (I even have pale orange/pink bromothymol blue, which when added to water gives a dark blue solution).

I would dissolve CuO in conc. HBr, using excess CuO and then decant the dark solution from the solid CuO and in a separate test tube make NH4Br and add this to the solution of CuBr2. Adding ammonia directly to the solution of CuBr2 in excess HBr does not seem a good idea to me. This allows formation of ammine complexes, which may interfere. Ammine complexes can be quite stable. It is known that ammine-complexes of cobalt(III) or chromium(III) are so stable that the ammonia is not converted to ammonium, not even in conc. HCl. I do not think the copper complex is that stable, but I can imagine that at low to medium concentration of free acid some of the ammine complex can survive.

ScienceSquirrel - 22-9-2008 at 09:39

This paper shows the thermochromic transition of the chlorocuprate in aqueous solution, the bromocompound behaves similarly

http://khimiya.org/pdfs/Experime1.pdf

Baphomet - 24-9-2008 at 20:22

Last night I made an orange suspension:

Ascorbic Acid + NaHCO3 -> Sodium Ascorbate + H2O + CO2
CuCl2 + Sodium Ascorbate -> Cu(?) + CuCl + Copper Ascorbate(?) + NaCl

I'm not sure exactly how much of what was present, it was all eyeballed rather than measured.

It produced a very bright orange colloid, that took forever to settle. Appears dark brown after settling. Cool :)

woelen - 25-9-2008 at 11:55

The orange material is not copper metal, but Cu2O. The color of Cu2O ranges from bright yellow when very finely divided, to brick red, when in the form if crystals.

Baphomet - 25-9-2008 at 19:28

how would ascorbic form Cu2O? it's a reducing agent. does this mean it can only reduce CuO -> Cu2O, but not Cu2O -> Cu?

gsd - 25-9-2008 at 20:00

Quote:
Originally posted by ScienceSquirrel
If you can get hold of this paper you will find an easy preparation of the corresponding chloro compound and an investigation of its thermochromic properties.

Choi, S.; Larrabee, J. A. "Thermochromic Tetrachlorocuprate(II): An Advanced Integrated Laboratory Experiment" J. Chem. Ed., 1989, 66, 774.


http://jchemed.chem.wisc.edu/Journal/Issues/1989/Sep/jceSubs...

gsd