The IUPAC name of [CoCl_2(en)_2]Cl complex is:

The correct answer is Option (1) → Dichloridobis(ethane-1,2-diamine)cobalt(III) chloride

A coordination compound is named by these IUPAC rules: the cation is named before the anion; inside the complex the ligands are named first, in alphabetical order, then the metal with its oxidation state in Roman numerals. Multiplying prefixes (di, tri, bis, tris) are ignored when alphabetising; bis is used for ligands whose names already contain a number, such as ethane-1,2-diamine.

First find the oxidation state of cobalt. The outer \( \ce{Cl-} \) shows the complex ion is \( \ce{[CoCl2(en)2]+} \); chloride is −1 and en is neutral:

    \( \displaystyle x + 2(-1) + 2(0) = +1 \Rightarrow x = +3 \)

Now order the ligands: chlorido (c) comes before ethane-1,2-diamine (e). Adding the prefixes gives dichlorido and bis(ethane-1,2-diamine). So the name is dichloridobis(ethane-1,2-diamine)cobalt(III) chloride.

Why the other options are incorrect

• Option (2) Dichloridobis(ethane-1,2-diamine) cobalt(II) chloride — Incorrect. The ligand order is right, but cobalt is in the +3 state, as the charge balance shows.

• Option (3) Bis(ethane-1,2-diamine)dichloridocobalt(II) chloride — Incorrect. Both the ligand order and the oxidation state are wrong.

• Option (4) Bis(ethane-1,2-diamine)dichloridocobalt(III) chloride — Incorrect. The oxidation state is right, but this order alphabetises by the prefix “bis”; prefixes are ignored, so chlorido comes first.

NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… The ligands are named in an alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula). … Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses. … Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: