Question

The tetrahedral complex which is diamagnetic but coloured :

• A. ${\left[NiC{l}_4\right]}^{2-}$
• B. ${\left[Cr{O}_4\right]}^{2-}$
• C. ${\left[Mn{O}_4\right]}^{2-}$
• D. ${\left[Cd{\left(CN\right)}_4\right]}^{2-}$

Answer 1

The correct option is B ${\left[Cr{O}_4\right]}^{2-}$

Color of transition metals are not only due to unpaired d-electrons, which cause d-d transition, but there is one more factor which makes colour in transition metals inspite of having $0$ unpaired electrons, which are as follows:

$LMCT$
$MLCT$

They are charge transfer phenomenon. $LMCT$(ligand to metal charge transfer) this occurs when metals are in higher oxidation charge from metal to ligand get transferred as a result of which it shows intense colour..

$MLCT$(Metal to ligand charge transfer) when metals are in lower oxidation state.

In case of $[Cr{O}_4{]}^{2-}$ $LMCT$ takes place because of which it shows intense colour, although it has no unpaired electrons for d-d transition, it is damagnetic.