I've been asking myself for a while now how counter-ions sometimes change the colors of coordination compounds. Like when I use d-metals as counter-ions to precipitate Cyanoferrates.
I looked at some crystal structures and saw that in the case of a cyanoferrate the terminal cyanide-N coordinate in an octahedral-prismatic geometry around the counter-ion. And I know that the cyano-ligand is a bridging ligand like in for example Prussian Blue, where the electron between Fe(II) and Fe(III) is moved across the cyano-ligand, probably through the π-systems. So I expect two ligand field splittings to happen, the one around the iron and the one around the counter-ion. These colors overlap to create a new color. Also depending on how lewis-acidic the counter-ion is it might pull electron density across the cyano-ligand lowering the gap and causing a red-shift.
I couldn't find much about that topic. It might be related to how colors derive in solid state compounds where I have no idea how this works. I know Lithium metal can be formulated as a huge molecule with many Li-Atoms, overlapping their atomic orbitals forming two big bands as the molecular orbitals create overlap so much. And usually salts crystallize in metal crystal structures so for example every second element is exchanged for the counter ion. If this was true then a colored solid state compound (salt) is a huge molecular orbital scheme, too creating two bands and depending on the size of their band gap the color is created (as I said I can't find anything on this topic). Sometimes the thermochromism in ZnO is explained by band gaps so I assume this concept of metals applies for salts as well. Then or course the counter-ion would have a much more complicated role here.
If anyone knows an answer to any of these questions I'd be really thankful.