Write the IUPAC name for the coordination compound:
\( [Cr(NH_3)_3(H_2O)_3]Cl_3 \)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → triamminetriaquachromium (III) chloride
IUPAC names of coordination compounds follow fixed rules. The cation is named before the anion. Inside the coordination entity the ligands are named first, in alphabetical order, then the metal with its oxidation state in Roman numerals. The prefixes di-, tri- for the number of simple ligands are ignored in the alphabetical order, and bis-, tris- are used only for ligands whose names already contain a number or are complex.
Here the ligands are three \( \ce{NH3} \) (ammine) and three \( \ce{H2O} \) (aqua), both neutral. The oxidation state of chromium comes from the charge balance with three chloride ions:
\( \displaystyle x + 3(0) + 3(0) + 3(-1) = 0 \Rightarrow x = +3 \)
“ammine” comes before “aqua” alphabetically (m before q), so the name is triamminetriaquachromium(III) chloride.
Why the other options are incorrect
Option (2) triaquatriamminechromium (III) chloride — Incorrect. It names aqua before ammine, which breaks the alphabetical order of ligands.
Option (3) tris(amminetriaqua)chromium (III) chloride — Incorrect. It groups two different ligands inside one bracket as if they were a single ligand present three times; the formula has three ammine and three aqua ligands, each counted separately.
Option (4) triammine-tris(aqua)chromium (III) chloride — Incorrect. Aqua is a simple ligand, so its number is given by tri-, not tris-.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… (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. …
(iv) 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.”