Re: Why do copper compounds possess two different colors in the same +2 state?

A simple question which needs some background before a simple answer can be 
provided.  I would first refer you to a link entitled "Electronic 
Configuration of the Atom" provides a series of 
links to understanding the electronic configuration of the atom.  One of 
those links, entitled the   "Electromagnetic 
Spectrum" , provides an overview of 
the electromagnetic spectrum and how various types of radiation produce 
different types of transitions in atoms and molecules.  Radiation in the 
visible spectrum largely interacts with the outer electrons on an atom.  
Because these outer electrons interact with light of specific frequencies, 
it is transitions of these electrons which cause some frequencies of light 
to be adsorbed, while other frequencies are not adsorbed.  If you pass 
white light through a sample of material, if some frequencies are adsorbed, 
the light which is transmitted through the sample is now colored, since 
some of the components of white light are now missing.  So color is 
produced by certain frequencies of light being selectively adsorbed by 
electron transitions.  Analysis of the characteristic adsorption of light 
is a very reliable method of establishing the presence or absence of 
specific atoms.
One way to change what frequencies are adsorbed is by changing the 
electronic configuration of an atom or ion, for example by changing its 
oxidation state.  A permanganate ion is a good example of such a 
transition.  When in a higher oxidation state, you can observe lovely dark 
purple solutions.  After it is reduced (for example by oxidizing some 
organic matter), the solution is brown.
Starting from the first web link, I found a link entitled "Interaction of 
Light with Matter", 
which attempts to address the issue of why electrons adsorb certain 
frequencies, while ignoring others.  If you think of an electron as 
mathematically analogous to a weight on a spring, the weight/spring 
combination will oscillate at a specific frequency, depending upon the 
spring constant and the amount of mass attached to it.  Radiation which is 
of the same frequency as the harmonic frequency of the weight spring 
combination will be adsorbed, while all else will be ignored.  
The amount of force on the outer electron can be changed depending on the 
presence or absence of other surrounding atoms.  One good way to change the 
environment surrounding an atom or ion is to dissolve it into a solvent.  
If you want to change the type of surrounding atoms to produce a color 
change once in solution, changing the pH of the solution will frequently do 
the trick.  PH indicators and organic dyes will often undergo significant 
color changes as you change the neighboring atoms from H+ to OH-.  Since 
the H+ and OH- ions are much different in size and polarity, perhaps you 
can imagine that they are changing the spring constant acting on the 
electrons, thus changing the frequency of light that is adsorbed.
Atoms with lower atomic weight are seldom colored, while those with higher 
weight are frequently colored.  Copper compounds are frequently colored, 
just like its neighbors iron and cobalt.  In the case of copper, acid 
environments produce a light blue solution.  When base is added, 
particularly if you use ammonia to raise the pH, the copper will complex 
with the ammonia to form a deep blue solution of copper amine.  No change 
is oxidation state but very dramatic changes in light adsorption by the 
copper ions.
When an ionic compound is dried, the color of the resulting solid will be 
influenced by its counter ion (copper sulfate versus copper chloride, for 
example) and whether or not there are water molecules included in the 
resulting crystal structure.  Some compounds might appear dark blue, some 
light blue or green, and others almost white.   Again, the forces between 
adjacent atoms will change the relative forces acting on the outer 
electrons, influencing which radiation will be adsorbed and which will be 
ignored.  Hope this helps.