Dear Colorfully Confused,

As with many things in chemistry, this comes down to what is happening at the particle level.

Quick primer on coordination complexes: Simple, inorganic coordination complexes are comprised of: A central metal ion or atom acting as a Lewis acid (electron pair acceptor) A set of ligands, usually 2, 4, or 6, acting as Lewis bases (electron pair donors) By convention, the names for these complexes include the name and number of the ligand species in the complex, the name of the metal center, and the oxidation state of the metal. See the explanation below for examples.

Transition metals are common metal centers for coordination complexes, and it is the internal atomic structure that governs whether and what type of coordination complexes may be formed between the metal atom or ion and different types of ligands.

The characteristic blue color of aqueous copper(II) solutions comes from the hexaaquacopper(II) complex ion, [Cu(H₂O)₆]²⁺, which has an octahedral geometry. In dilute solutions, where there is a relatively large amount of water, this is the dominant species, and solutions containing the copper(II) ion appear blue.

The situation changes when there are relatively fewer water molecules compared to the copper cations and their counterions.

For copper(II) chloride, there is an equilibrium between two different complexes:

[CuCl₄]²⁻(aq) + 6H₂O(l) ⇌ [Cu(H₂O)₆]²⁺(aq) + 4Cl⁻(aq)

The tetrahedral [CuCl₄]²⁻(aq) is yellow while the octahedral [Cu(H₂O)₆]²⁺(aq) is blue. A mixture of the two appears green. As the concentration of water increases (when the CuCl₂ solution becomes more dilute), the equilibrium shifts toward the blue hexaaquacopper(II) complex ion.

A similar – though less dramatic – process occurs with copper(II) nitrate. Although nitrate (NO₃⁻) is a very weak ligand that doesn't compete strongly with ligand water molecules, the copper(II) ion is not fully solvated when this solution is concentrated. Anhydrous copper (II) nitrate is a deep green color (See ACS Stage Demo React with Copper for photos). When initially dissolved in a small amount of water, the nitrate ions can still coordinate the copper, making the solution appear more green until water molecules begin to replace the nitrate ion in the coordination sphere. This formation of the octahedral hexaaquacopper(II) ion gives the solution the characteristic blue color. (See Chemistry in Pictures: Nitric acid acts upon copper for a short video showing color changes as a copper(II) nitrate solution is created.)

Thanks for your great question!
Yours in Chemistry,
Labby