JEE Advanced 2020 · Paper 1, Chemistry question 10

(A) $\ce{[FeCl4]-}$: $\ce{Fe^{3+}}$ with four weak-field $\ce{Cl-}$ ligands, $sp^3$, tetrahedral. True.

(B) $\ce{[Co(en)(NH3)2Cl2]+}$ is of the type M(AA)B₂C₂. It has three geometrical isomers:

• NH₃ groups trans to each other;
• Cl groups trans to each other;
• both pairs cis (this one is chiral).

So not 2. False.

(C) $\ce{Fe^{3+}}$ ($d^5$, high spin) has 5 unpaired electrons, $\mu=5.92$ BM. $\ce{Co^{3+}}$ ($d^6$) with en and $\ce{NH3}$ is low spin, with 0 unpaired electrons. True.

(D) Low-spin $\ce{Co^{3+}}$ uses inner $d$ orbitals: $d^2sp^3$, not $sp^3d^2$. False.

(A), (C)