The IUPAC name for the complex Hg[Co(SCN)_4]

The correct answer is Option (1) → Mercury tetrathiocyanatocobaltate (III)

In naming a coordination compound the cation is named first; in the complex ion the ligands come before the metal, and a complex anion takes the ending -ate (cobalt → cobaltate) followed by the oxidation state of the metal in Roman numerals.

Here the cation is mercury and the complex anion is \( \ce{[Co(SCN)4]^{-}} \) with four thiocyanato (\( \ce{SCN^{-}} \)) ligands. Its full name is mercury(I) tetrathiocyanato-S-cobaltate(III): with \( \ce{Hg^{+}} \) as the cation,
x + 4(−1) = −1 ⇒ x = +3

So the name is mercury tetrathiocyanatocobaltate(III).

Why the other options are incorrect

• Option (2) Mercury tetrathiocyanatocobaltate (IV) — Incorrect. The form of the name is right, but cobalt is +3 here, not +4.

• Option (3) Tetracyanocobalt mercuric (III) — Incorrect. The ligand is thiocyanato (\( \ce{SCN^{-}} \)), not cyano (\( \ce{CN^{-}} \)); the cation must be named first; and the anionic complex must end in -ate.

• Option (4) Tetrathiocyanato cobalt mercuric (IV) — Incorrect. The cation is named last, the -ate ending is missing, and the oxidation state of cobalt is wrong.

Note: The NCERT book names Hg[Co(SCN)₄] as mercury (I) tetrathiocyanato-S-cobaltate(III), and the answer follows it. The compound actually contains Hg²⁺ and the [Co(SCN)₄]²⁻ ion (mercury(I) exists as Hg₂²⁺, not as Hg⁺), so its name should read mercury(II) tetrathiocyanato-S-cobaltate(II).

NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… Write the IUPAC names of the following coordination compounds: … (e) Hg[Co(SCN)₄]
Solution … (e) mercury (I) tetrathiocyanato-S-cobaltate(III)”