JEE Main · 2023 · 10 Apr 2023 · Shift 1 · Coordination Compounds

A complex can be octahedral, low spin and diamagnetic only if it is six-coordinate and its ligand is strong enough to force all the d electrons to pair. The tetrachloronickelate(II) ion is four-coordinate and tetrahedral, so it is excluded at the outset. The hexachlorocobaltate(III) and hexafluorocobaltate(III) ions are both octahedral, but chloride and fluoride are weak-field ligands, so the $d^6$ cobalt(III) centre remains high spin with four unpaired electrons and is paramagnetic. In the hexaamminecobalt(III) ion ammonia is a strong-field ligand, and the six d electrons of cobalt(III) pair up completely as $t_{2g}^6{e}_g^0$, leaving no unpaired electron. The hexaamminecobalt(III) ion is the octahedral, low-spin, diamagnetic complex.