Question

Atomic numbers of Cr and Fe are respectively 24 and 26. Which of the following is paramagnetic with spin of electron?

• A. $Cr(CO{)}_6$
• B. $\left[Fe\right(CO{)}_5]$
• C. $\left[Fe\right(CN{)}_6{]}^{4-}$
• D. $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$

Answer 1

The correct option is D $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$

As $CO$ and $CN$ are strong ligand, so, they will form low spin complexes pairing of electrons, which is possible in first three options. Only $\left[Cr\right(N{H}_3{)}_6{]}^{+3}$ ($d^3$configuration) have 3 unpaired electron. As ammonia is a weak field ligand, so, pairing is not possible. Therefore, it has three unpaired electrons, i.e., $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$ is paramagnetic.