[Co(Cl)6]3- is blue in solution and reacts with sodium hydroxide to form [Co(OH)6]3-. What is the likely color

Question

It's violet, but I need to know why. Thanks.

Answer 1

The colour is due to ligand field splitting, which depends on the bonding between the central ion and the groups complexed to it.

OH- has a stronger ligand field (is higher in the spectrochemical series) than Cl-.

So the absorption will shift to higher energy.

Actually, it is very much easier to predict which way the absorption bands will move (in this case, to higher energy) than to predict the colour, which depends on how the eye reacts to the gaps between them. There is not much difference spectroscopically between solutions that look purple and solutions that look green.

Environment can cause fairly small changes in the absorption spectrum which nonetheless cause large changes in the subjectively observed colour. You might like to find out what causes the red colour of rubies, and compare that colour with the green colour of Cr2O3.

I hope this is not too much more than you wanted to know.