Question

The colour of a complex compound is due to –
A.Promotion of 3d-electrons of the central atom /ion to the 4s-orbital.
B.Promotion of 3d-electron of the central atom/ion to 4p orbital.
C.Promotion of 3d-electron of the central atom/ion to d-orbital
D.Promotion of 4s-electron of the central atom/ion to 4p-orbitals

Answer 1

Hint: The colour of any compound is because of excitation of electrons. Electrons emit light of certain wavelengths and it impart colour to them. Photons emitted from these spectrum provide colour to the compound.

Complete answer:
The colour of any compound is due to the fact that its electron absorbs energy and emits photons of certain wavelengths which impart colour to the compound. The colour of coordination compounds can be explained on the basis of crystal field theory.
In the presence of strong ligand , the five degenerate orbitals of \[3d\] ( ${d_{xy}},{d_{yz}},{d_{zx}},{d_{{x^2} - {y^2}}},{d_{{z^2}}}$ ) split into different group of orbitals( two with higher energy and three with lower energy). The orbitals with lower energy go into ${t_{2g}}$ and with higher energy are called ${e_g}$. The electrons after excitation make a jump from lower energy level to higher energy level. The energy corresponding to the wavelength will impart colour to that compound. And if the electron does not make transition , there will be no colour i.e. the substance will be colourless.
The colour of the compound is due to the $d - d$ transition of the electrons.
Hence, the correct option is (C).


Note:
The splitting of five degenerate orbitals of $3d$ into higher and lower energy orbitals is called crystal field splitting. The degenerate orbitals are the orbitals with the same energy level while the generated orbitals are those with different energy levels.