Question

The following complexex are given:
(1) $trans-\left[Co\right(N{H}_3{)}_{4}C{l}_2{]}^{+}$
(2) $cis-\left[Co\right(N{H}_3{)}_2(en{)}_2{]}^{3+}$
(3) $trans-\left[Co\right(N{H}_3{)}_2(en{)}_2{]}^{3+}$
(4) $[NiC{l}_4{]}^{2-}$
(5) $[Ti{F}_6{]}^{2-}$
(6) $[Co{F}_6{]}^{3-}$
Choose the correct code:

• A. 1, 2 are optically active; 3 optically inactive
• B. 2 is optically active; 1, 3 are optically inactive
• C. 4, 5 are colored; 6 is colorless
• D. 4 is colored; 5, 6 are colorless

Answer 1

The correct option is B

2 is optically active; 1, 3 are optically inactive

As a thumb rule for ions being coloured, look for CFSE to be small; in other words, the ligand needs to be weak field. Plus, we need unpaired electrons that could transit between $e_g$ and $t_{2g}$.

$[NiC{l}_4{]}^{2-}$ as well as $[Co{F}_6{]}^{3-}$ have unpaired electrons with small CFSE. They are thus coloured. $[Ti{F}_6{]}^{2-}$ has a $d^0$ configuration thus can't be coloured.

The above two can't be optically active (due to the presence of plane(s) of symmetries).