Coordination compounds are formulated and named according to the IUPAC system. A few rules for naming coordination compounds are:
(I) In an ionic complex, the cation is named first and then the anion.
(II) In the coordination entity, the ligands are named first and then the central metal atom/ion.
(III) When more than one type of ligand is present, the ligands are named in alphabetical order, without any consideration of charge.
As per IUPAC nomenclature, what is the name of the complex [Co(H2O)4(NH3)2]Cl3?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → diamminetetraaquacobalt(III) chloride
The cation is named first, then the anion. Inside the complex the ligands come first, in alphabetical order without regard to charge, then the metal with its oxidation state in Roman numerals.
Complex ion: three \( \ce{Cl^-} \) are outside the bracket, so \( \ce{[Co(H2O)4(NH3)2]^3+} \) is a cation, named before “chloride”.
Oxidation state of Co: \( \ce{H2O} \) and \( \ce{NH3} \) are neutral, so x + 4(0) + 2(0) = +3 and x = +3, written (III).
Ligand names: \( \ce{H2O} \) = aqua, four of them = tetraaqua; \( \ce{NH3} \) = ammine, two of them = diammine.
Order: the prefixes di and tetra are not counted; ammine (a-m) comes before aqua (a-q), so diammine is written first.
Metal in a cation: named as the element, cobalt.
Why the other options are incorrect
Option (1) tetraaquadiamminecobalt(II) chloride — Incorrect. Aqua is placed before ammine, against the alphabetical order, and cobalt is +3, not +2.
Option (2) tetraaquadiamminecobalt(III) chloride — Incorrect. The oxidation state is right, but aqua is placed before ammine.
Option (3) diamminetetraaquacobalt(II) chloride — Incorrect. The order is right, but three chloride ions balance a +3 complex ion, so the metal is cobalt(III).
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “1. [Cr(NH3)3(H2O)3]Cl3 is named as: triamminetriaquachromium(III) chloride
Explanation: The complex ion is inside the square bracket, which is a cation. The amine ligands are named before the aqua ligands according to alphabetical order. Since there are three chloride ions in the compound, the charge on the complex ion must be +3 (since the compound is electrically neutral).”