What is the formula for the Tetraammineaquachloridocobalt (III) chloride coordination compound?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \([Co(NH_3)_4(H_2O)Cl]Cl_2\)
To write a formula from the name of a coordination compound, read the ligands with their numerical prefixes, place them with the metal inside the square bracket, and then work out the charge of the complex ion to find how many counter ions are needed.
Tetraammine: four \( \ce{NH3} \) ligands (neutral).
Aqua: one \( \ce{H2O} \) ligand (neutral).
Chlorido: one \( \ce{Cl-} \) ligand.
Cobalt(III): oxidation state +3.
Charge on the complex ion:
\( \displaystyle +3 + 4(0) + 1(0) + 1(-1) = +2 \)
Two chloride counter ions balance this charge, so the formula is \( \ce{[Co(NH3)4(H2O)Cl]Cl2} \).
Why the other options are incorrect
Option (2) \([Co(NH_3)_4(H_2O)_2]Cl_3\) — Incorrect. This has two aqua ligands and no chlorido ligand; its name is tetraamminediaquacobalt(III) chloride.
Option (3) \([Co(NH_3)_4(H_2O)Cl_2]Cl\) — Incorrect. This has two chlorido ligands (dichlorido) and seven ligands in all, which does not fit the name or the usual coordination number 6 of cobalt(III).
Option (4) \([Co(NH_3)_4Cl_2]Cl.H_2O\) — Incorrect. Here the water is outside the coordination sphere as water of crystallisation; the name would be tetraamminedichloridocobalt(III) chloride monohydrate.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – …
