Question

The tetrahedral complex which is diamagnetic but coloured :
A. \[{{\text{[NiC}}{{\text{l}}_{\text{4}}}{\text{]}}^{{\text{ - 2}}}}\]
B . \[{\left[ {{\text{Cr}}{{\text{0}}_{\text{4}}}} \right]^{{\text{ - 2}}}}\]
c . \[{\left[ {{\text{Mn}}{{\text{0}}_{\text{4}}}} \right]^{{\text{ - 2}}}}\]
D. \[{\left[ {{\text{Cd(CN}}{{\text{)}}_{\text{4}}}} \right]^{{\text{ - 2}}}}\]

Answer 1

Hint: Transition metals form complexes with different ligands using its d-orbital electrons. They can combine with different numbers of ligands and form different kinds of geometry. Colour of the complex of transition metals is found because of d-d transitions and charge transfer.

Complete step by step answer:
-When a metal forms complex, due to different extent of interaction with ligands the d orbitals are divided into several energy states. d-d transition of the transition metals is basically excitation of electrons by absorbing energy from lower energy level to higher energy level. After these transitions, electrons immediately come back with emission of some amount of radiation, for which the complex shows colour. This colour is the complementary colour of the light absorbed in excitation.
-Charge transfer transition is another reason behind the colour of the transition metal complex. There are two kinds of charge transfer.
-LMCT(ligand to metal charge transfer): when metals are in high oxidation states charge from metal to ligands transfers. As a result, it shows colour. For example, shows intense violet colour. Where oxygen to Mn+7 charge transfer takes place.
-MLCT(metal to ligand charge transfer): when metals are in low oxidation state, charge transfer from metal to ligand takes place.
 -Metal to metal charge transfer: Metal to metal charge transfer happens when metal is present in a complex with two different oxidation states. Transition of charge takes place from lower oxidation to higher oxidation state. In the case of Prussian blue , charge transfer takes place from Fe(II) to Fe(III).
In case of \[{\left[ {{\text{Cr}}{{\text{0}}_{\text{4}}}} \right]^{{\text{ - 2}}}}\], LMCT takes place. As a result, it shows intense colour.

So, the correct option is B.

Note:
Colour of the complex is not only because of d-d transitions, but also charge transfer transitions (LMCT or MLCT). LMCT happens when metal is in highest oxidation state and MLCT happens when metal is in low oxidation state. Metal to metal charge transfer happens when metal is present in a complex with two different oxidation states.