Which of the following compounds is non-conducting in aqueous solution? (The coordination number of cobalt is 6 in each.)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → CoCl3·3NH3
With coordination number 6, every \( \ce{NH3} \) and as many \( \ce{Cl^-} \) as needed fill the six places around cobalt. Only the \( \ce{Cl^-} \) ions left outside the bracket are set free in water and carry current; the bracketed entity does not dissociate.
CoCl3·6NH3 — \( \ce{[Co(NH3)6]Cl3 -> [Co(NH3)6]^3+ + 3Cl^-} \): 4 ions, conducting.
CoCl3·5NH3 — \( \ce{[CoCl(NH3)5]Cl2 -> [CoCl(NH3)5]^2+ + 2Cl^-} \): 3 ions, conducting.
CoCl3·4NH3 — \( \ce{[CoCl2(NH3)4]Cl -> [CoCl2(NH3)4]^+ + Cl^-} \): 2 ions, conducting.
CoCl3·3NH3 — \( \ce{[CoCl3(NH3)3]} \): three \( \ce{NH3} \) and all three \( \ce{Cl^-} \) are inside the bracket, so no ions are set free.
Only \( \ce{[CoCl3(NH3)3]} \) is a neutral complex that gives no ions, so it is non-conducting.
NCERT (2026-27 edition):
5.2 Definitions of Some Important Terms Pertaining to Coordination Compounds – “A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl3(NH3)3] is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions.”
5.1 Werner’s Theory of Coordination Compounds – “… the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions.”