IUPAC name of [Co(NH3)4(H2O)Cl]Cl2

The correct answer is Option (2) → Tetraammineaquachloridocobalt(III) chloride

A coordination compound is named by these IUPAC rules:

• The cation is named first, then the anion, as in simple salts.

• Inside the complex, the ligands are named first, in alphabetical order, and then the metal. Prefixes such as di, tri, tetra are not counted when alphabetising.

• Anionic ligands end in -o (\( \ce{Cl^-} \) is chlorido); neutral ligands keep their names, except \( \ce{NH3} \) = ammine and \( \ce{H2O} \) = aqua.

• The oxidation state of the metal follows in Roman numerals in brackets.

In \( \ce{[Co(NH3)4(H2O)Cl]Cl2} \), the complex is the cation. The two \( \ce{Cl^-} \) ions outside the bracket show that it carries a charge of +2. Inside, \( \ce{NH3} \) and \( \ce{H2O} \) are neutral and the chlorido ligand is −1, so the oxidation state x of cobalt is:

    \( \displaystyle x+4(0)+(0)+(-1)=+2 \)

    \( \displaystyle x=+3 \)

The +2 is the charge of the whole complex ion; cobalt itself is +3, because it also balances the −1 of the chlorido ligand.

In alphabetical order the ligands are ammine, aqua (a-m comes before a-q) and chlorido. Four ammine ligands take the prefix tetra. The counter ions are named last as chloride.

So the IUPAC name is tetraammineaquachloridocobalt(III) chloride.

NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “The names of coordination compounds are derived by following the principles of additive nomenclature. Thus, the groups that surround the central atom must be identified in the name. They are listed as prefixes to the name of the central atom along with any appropriate multipliers. The following rules are used when naming coordination compounds:

(i) The cation is named first in both positively and negatively charged coordination entities.

(ii) The ligands are named in an alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula).

(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H₂O, ammine for NH₃, carbonyl for CO and nitrosyl for NO. While writing the formula of coordination entity, these are enclosed in brackets ( ).

…

(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.”