Question

In water, the titanium (III) ion $\left[Ti\right(H_{2}O{)}_6{]}^{3+}$ has a broad absorption band at about $500nm$. Which colour light is absorbed by the ion?

• A. Red Orange
• B. Yellow
• C. Violet
• D. Blue green

Answer 1

Answer: (D)

Blue green

The colour in the coordination compounds can be readily explained in terms of the crystal field theory.

Consider, for example, the complex $\left[Ti\right(H_{2}O{)}_6{]}^{3+}$, which is violet in colour. This is an octahedral complex where the single electron ($T{i}^{3+}$ is a $3d^1$ system) in the metal d orbital is in the $t_{2g}$ level in the ground state of the complex. The next higher state available for the electron is the empty $e_g$ level.

If light corresponding to the energy of blue-green region is absorbed by the complex, it would excite the electron from $t_{2g}$ level to the $e_g$ level ($t_{2g}^1{e}_g^0\to t_{2g}^0{e}_g^1$ ).

Consequently, the complex appears violet in colour. The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron.