Question

What makes tetrahedral and octahedral transition metal complexes coloured?

Answer 1

Whenever an electron absorbs light of a particular wavelength then it emits color. When tetrahedral and octahedral complexes are formed then the electrons absorb some amount of energy which makes them colored in nature. Also the concept of crystal field splitting field energy comes here.

Complete Step By Step Answer:
When a complex is formed then the ligand approaches the central metal atom which will split the orbital of the central metal atom. The orbital will be $d -$ orbital and hence splitting of $d -$ orbital takes place. The energy which is released during this splitting of the orbital is known as crystal field splitting energy and is denoted by $\Delta$ . Thus for tetrahedral complexes it is denoted by ${\Delta _t}$ and for octahedral complexes it is denoted by ${\Delta _o}$ . The relation between the tetrahedral splitting energy and octahedral splitting energy is given by:
$\Rightarrow {\Delta _t} = \dfrac{4}{9}{\Delta _o}$
Whenever the value of both these energies are in the range of energy of visible wavelength which is $\left( {400 - 700} \right){\text{ nm}}$ , then these complexes show colour. When a strong ligand approaches the central metal atom then it absorbs energy of higher wavelength and when a weak ligand approaches the central metal atom then it absorbs energy of lower wavelength. But the energy which is absorbed while splitting must be the range of visible wavelength to observe coloured complexes.

Note:
If the absorbed energy is more than energy of wavelength of visible range then we do not see any colour of the complex. It must be in between the visible range of the human eye. The colour of the complex is mainly related to absorption and release of energy by atomic orbital while making transactions from lower orbit to higher or vice-versa.