JEE Main · 2019 · 10 Apr 2019 · Shift 2 · Coordination Compounds

Ruby is corundum, $\mathrm{Al}{}_2\mathrm{O}{}_3$, in which a few $\mathrm{Cr}{}^{3+}$ ions slip into the octahedral sites normally held by $\mathrm{Al}{}^{3+}$, and that substitution is what colours it red; the statement placing those $\mathrm{Cr}{}^{3+}$ ions in beryl is the wrong one, since chromium in beryl gives green emerald instead. The other statements check out: the high-spin $d^6$ iron and $d^4$ chromium aqua ions both carry four unpaired electrons and so have near-equal moments, the $d^8$ nickel complex has two unpaired electrons for $2.83\text{BM}$, and the cobalt ammine's violet colour matches absorption of yellow light.