Write the formula for tetraamminediaquacobalt(III) chloride

The correct answer is Option (4) → \([Co(NH_3)_4(H_2O)_2]Cl_3\)

The formula of a complex is read off its name part by part:

• tetraammine — four \( \ce{NH3} \) ligands.

• diaqua — two \( \ce{H2O} \) ligands.

• cobalt(III) — the central metal, \( \ce{Co^3+} \). Both ligands are neutral, so the complex ion is \( \ce{[Co(NH3)4(H2O)2]^3+} \).

• chloride — the counter ion; three \( \ce{Cl-} \) are needed to balance +3.

The metal symbol comes first, then the ligands, all inside square brackets, with polyatomic ligands in parentheses and the counter ions outside: \( \ce{[Co(NH3)4(H2O)2]Cl3} \).

Why the other options are incorrect

• Option (1) \([Co(NH_3)_4 H_2O] Cl_3\) — Incorrect. Only one water ligand; “diaqua” needs two, and Co(III) would then be five-coordinate.

• Option (2) \([Co(NH_3)_5 Cl] Cl_2\) — Incorrect. This is pentaamminechloridocobalt(III) chloride; it has no water ligand.

• Option (3) \([Co(NH_3)_4 Cl_2]^+[OH]^-\) — Incorrect. This has chlorido ligands and a hydroxide counter ion, not aqua ligands and chloride ions.

NCERT (2026-27 edition):
Formulas of Mononuclear Coordination Entities – “The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom. The following rules are applied while writing the formulas:
(i) The central atom is listed first.
(ii) The ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge. …
(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.”