Question

Why are amine complexes of Cu(II) more intensely blue than the aqua complexes?

Answer 1

The color of the complex that we see is the complimentary color of the light being absorbed.

For example, if the complex absorbs yellow light, then it will appear to us violet in color.

Aqua complexes of copper appear aqua in color because they must be absorbing the complimentary light of reddish orange color

On the other hand amine complexes are intense blue in color because they absorb the complimentary light of yellowish color

Now if we take a look at the wavelengths of these lights , we see that wavelength for reddish orange color is approximately 650 nm and that for yellowish color is about 570 nm

The relationship between energy and wavelength is inverse. That is larger the wavelength, lower is the energy.

Therefore we can say that amine complexes absorb light of higher energy than aqua complexes.

This can be explained using Crystal Field Splitting theory. According to this theory, as the ligand approaches the metal ion, it causes splitting of "d" orbitals. The amount of splitting depends upon the strength of the ligand. Larger the splitting, more is the energy absorbed and therefore the complex would have more intense color.

Between H2O (aqua) and NH3 (amine) , NH3 is the stronger ligand and therefore causes more splitting of the orbital. As a result of this, amine complexes absorb light of higher energy which is yellow light and appear intense blue in color.