Werner was the first to describe the bonding features in coordination compounds. But his theory could not answer basic questions like :
(i) Why only certain elements possess the remarkable property of forming coordination compounds ?
(ii) Why the bonds in coordination compounds have directional properties ?
(iii) Why coordination compounds have characteristic magnetic and optical properties ?
Many approaches have been put forth to explain the nature of bonding in coordination compounds viz. Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT). We shall focus our attention on elementary treatment of the application of VBT and CFT to coordination compounds.
According to this theory, the metal atom or ion under the influence of ligands can use its (n−1)d, ns np or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.
CoCl3·6NH3 gives 3 moles of AgCl on reacting with AgNO3. The correct formula for this compound is :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → [Co(NH3)6]Cl3
By Werner’s theory, only the chloride ions that lie outside the coordination sphere (the primary, ionisable valences) are free in solution and are precipitated by \( \ce{AgNO3} \) as AgCl. Chloride bonded to the metal inside the square brackets is not precipitated. Cobalt(III) has a coordination number (secondary valence) of 6.
1 mol of \( \ce{CoCl3} \)·\( \ce{6NH3} \) gives 3 mol of AgCl, so all three \( \ce{Cl^{-}} \) are ionisable. The six positions around \( \ce{Co^{3+}} \) are then filled by the six \( \ce{NH3} \) molecules:
\( \displaystyle \ce{[Co(NH3)6]Cl3 + 3AgNO3 -> [Co(NH3)6](NO3)3 + 3AgCl v} \)
The compound is a 1:3 electrolyte, [Co(NH3)6]Cl3.
Why the other options are incorrect
Option (2) [CoCl(NH3)5]Cl2·NH3 — Incorrect. Only two \( \ce{Cl^{-}} \) are outside the sphere, so it would give 2 mol of AgCl; also an \( \ce{NH3} \) molecule outside the sphere is not part of a stable formula, since Co(III) binds six groups.
Option (3) [CoCl2(NH3)4]Cl·2NH3 — Incorrect. Only one ionisable \( \ce{Cl^{-}} \), so only 1 mol of AgCl would form.
Option (4) [CoCl3(NH3)3]·3NH3 — Incorrect. All three Cl are bonded to cobalt, so the compound is a non-electrolyte and gives no AgCl.
NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – “… In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution.
1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (Violet) gave 1 mol AgCl”
| Colour | Formula | Solution conductivity corresponds to |
|---|---|---|
| Yellow | [Co(NH3)6]3+3Cl− | 1:3 electrolyte |
| Purple | [CoCl(NH3)5]2+2Cl− | 1:2 electrolyte |
| Green | [CoCl2(NH3)4]+Cl− | 1:1 electrolyte |
| Violet | [CoCl2(NH3)4]+Cl− | 1:1 electrolyte |