An ammoniacal metal salt solution gives a brilliant red precipitate on addition of dimethylglyoxime. The metal ion is :

We need to identify the metal ion that gives a brilliant red precipitate with dimethylglyoxime (DMG) in ammoniacal solution.

Recall the DMG test.

The dimethylglyoxime (DMG) test is a classic qualitative analytical test used specifically for the identification of nickel(II) ions ($$Ni^{2+}$$). When an ammoniacal solution containing $$Ni^{2+}$$ is treated with an alcoholic solution of DMG, a characteristic brilliant red (or rosy red) precipitate is formed.

Understand the chemistry.

The red precipitate is nickel dimethylglyoximate, $$[Ni(DMG)_2]$$. In this complex:

- $$Ni^{2+}$$ forms a square planar complex with two DMG molecules

- Each DMG acts as a bidentate ligand, coordinating through its two nitrogen atoms

- The complex is stabilised by intramolecular O-H...O hydrogen bonds between the two DMG ligands

- The ammonia provides the basic medium necessary for DMG to deprotonate and act as a ligand

Why not the other options?

- $$Cu^{2+}$$ does not give a red precipitate with DMG

- $$Fe^{2+}$$ gives a red colour with DMG but in different conditions

- $$Co^{2+}$$ gives a brown precipitate with DMG, not red

The correct answer is Option (2): $$Ni^{2+}$$.