Chromium colours!

Hi

What is the colour of hexaamminechromium 3+?

I have 3 different books for Edexcel chemistry. One says VIOLET, another GREEN and another YELLOW...

??? Why are they different?
Also one book says when you react hexaaquachromium3+ with EXCESS OH- you get [Cr(OH)6]3- and another says [Cr(H2O)4(OH)2]-

The green one involved chloride ions which is understandable. So I have violet and yellow to decide now.... could the yellow one involve chloride too?

Books will have conflicting answers, but don't worry because if you look at the mark schemes for the exams they tend to allow like three different colours anyway.

yeah, look at a mark scheme!!

the colour of [Cr(NH3)6]3+ is apparently purple according to this: http://www.creative-chemistry.org.uk/alevel/module5/documents/N-ch5-07.pdf, which makes sense as the copper ammonia complex is a deep blue, whereas as hexaaquacopper(II) is a lighter blue and so purple would seem a logical colour for it to be.

As for the adding OH-, I guess it would depend on the extent of the ligand substitution. According to this http://en.wikipedia.org/wiki/Chromium , [Cr(OH)3] becomes [Cr(OH)6]3- in alkaline solution and [Cr(H2O)6]3+ in acidic solution, so I would guess that you get a mix of both H2O and OH in the complex. I'm not 100% though.

I think this whole situation is ridiculous... but thanks for looking!

No worries, sitting Salters F334 tomorrow so this was some good pseudo-revision. Complexes really are complex though (bad joke intended), like someone else said, use a recent mark scheme as that will give you the exam answer. Do you have to know all the colours and things for the exam, cus that sounds like massive hard work. We have to explain the colour in terms of electrons and d-subshell splitting etc, but it's unlikely we have to say what colour it is, unless it leads on to a question about colour.

AQA book says purple!

and it also says excess OH- gives you [Cr(OH)6]3-

hope this helps!

We have to know it for Cr, Mn, FeII, FeIII, Ni, Cu and Zn
Hexaaqua versions with OH, NH3 and excess of both. Quite annoying lol...

Yes we have to explain the colour too! Best of luck tomorrow

...

Hexaaquachromium(III) is a kind of violet-blue-grey colour. However, it is very common for chloride or sulfate ions to be present in a solution containing chromium(III), and these can undergo a ligand exchange reaction with the hexaaquachromium(III). As far as I know, multiple ions can be exchanged, but only one is required to give the green colour commonly associated with chromium(III). Chromium(III) might form yellow complexes with other ligands, but I've never come across one. Chromate(VI) is yellow, however.

As for your textbooks: both are correct. Remember that adding excess hydroxide is not a ligand exchange reaction. It's an acid-base reaction. As with most acid-base reactions (and for that matter, ligand exchange reactions also), there is an equilibrium set up. In this instance, there are several interacting equilibria between compounds in which varying numbers of water molecules have been deprotonated, and hence both the tetrahydroxo- and the hexahydroxo- compound will be present.

Hope this helps.