Question

A $MnO_4^ -$ is of intense pink colour, though $Mn$ is in $\left( { + 7} \right)$ oxidation state. It is due to
A.Oxygen gives colour to it
B.Charge transfer when $Mn$ gives its electron to oxygen
C.Charge transfer when oxygen gives its electron to $Mn$ making it $Mn\left( { + {\text{VI}}} \right)$ hence, coloured
D.None of the above is correct

Answer 1

To answer this question, you must recall the factors due to which coordination compounds show colours. The colours shown by coordinate compounds are explained by the crystal field theory and are either due to charge transfer or due to d- d transition.

Complete step by step answer:
Charge transfer in complexes occurs from the ligand to metal by the transfer of electrons from the molecular orbitals having ligand like character to the molecular orbitals having metal like character. The concept of charge transfer is mostly found dominantly in those complexes in which ligands have a relatively greater number of lone pairs as in the case of oxygen, sulphur or selenium. It is also common in compounds in which the metals have low energy empty orbitals.
Permanganate ion has four oxygen atoms as ligands and the oxidation state of manganese is $+ 7$ suggesting that the d- orbital of the central metal atom is empty. Thus, charge transfer takes place in permanganate ion and is responsible for its intense pink colour.
Due to the charge transfer in the molecule, an electron absorbs photons from the surroundings and gets promoted to the empty d orbital of manganese ion from the highest energy molecular orbital of the bond and is responsible for the colour.

Thus, the correct answer is C.

Note:
The nature of attraction between the metal and ligand during charge transfer is not a stable interaction and is very weak as compared to a covalent bond. Charge transfer is found in many different molecules whether they may be inorganic or organic.