Coordination compounds are formulated and named according to the system set up by the Inorganic Nomenclature Committee of the International Union of Pure and Applied Chemistry (IUPAC). It is called the IUPAC System of Nomenclature.
The following rules are applied while writing the formulas:
(i) The formula of the cation whether simple or complex is written first followed by that of the anion.
(ii) The coordination entity is written in square brackets.
(iii) The sequence of symbols within the coordination entity is: first the symbol of the central metal atom followed by ligands in alphabetical order. According to the latest IUPAC recommendations, the placement of a ligand in the list does not depend on its charge. The ligands in the coordination entity are arranged as:
(a) The different ligands are arranged alphabetically according to the first symbol of their formulae. For example, H2O, NH3, NO3–, SO42– and OH–, etc. are cited at H, N, N, S, and O. The ligands containing carbon and hydrogen are cited only under C.
The IUPAC name of \([Cr(NH_3)_4Cl_2]NO_3\) is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Tetraamminedichloridochromium(III) nitrate
In \( \ce{[Cr(NH3)4Cl2]NO3} \) the complex ion \( \ce{[Cr(NH3)4Cl2]^+} \) is the cation and \( \ce{NO3^-} \) is the anion.
Oxidation state of Cr: x + 4(0) + 2(−1) = +1, so x = +3, written (III).
Ligands: \( \ce{NH3} \) is named ammine (double m); four of them give tetraammine. \( \ce{Cl^-} \) is an anionic ligand: chlorido; two give dichlorido. In alphabetical order, ammine comes before chlorido.
Metal in a complex cation: named as the element, chromium.
The cation is named first, then the anion: tetraamminedichloridochromium(III) nitrate.
Why the other options are incorrect
Option (1) Tetraaminodichlorochromium nitrate — Incorrect. “Amino” is not the name for an \( \ce{NH3} \) ligand (it must be ammine), “chloro” is the older form of chlorido, and the oxidation state (III) is missing.
Option (2) Tetraminodichloridochromium(II) nitrate — Incorrect. The ligand name is misspelt (tetraammine is needed), and chromium is +3, not +2.
Option (3) Tetraaminedichloridochromium(II) nitrate — Incorrect. “Amine” with one m is not the ligand name for \( \ce{NH3} \), and the oxidation state (II) is wrong; the charge balance gives +3.
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear 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. …”
“Note: The 2004 IUPAC draft recommends that anionic ligands will end with–ido so that chloro would become chlorido, etc.”
“(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. …”