JEE Main 2023 · 24 January · morning · Single correct · +4 / −1
The primary and secondary valencies of cobalt respectively in [Co(NH₃)₅Cl]Cl₂ are :
- (a) 2 and 8
- (b) 2 and 6
- (c) 3 and 5
- (d) 3 and 6
Answer and solution
Answer (d) 3 and 6
Solution
Chemistry ma’am · 4 steps
Primary valency is cobalt's oxidation state, and secondary valency is how many ligands sit inside the bracket.
- Two kinds of valency. In Werner's theory, the primary valency is the oxidation state of the metal and is ionisable. The secondary valency is the coordination number, the number of ligand atoms bonded directly to the metal, and it is non-ionisable.
- Find oxidation state. Two Cl⁻ ions sit outside the bracket, so the complex ion is [Co(NH₃)₅Cl]²⁺. NH₃ is neutral and the coordinated chloride is -1.
x + 5(0) + (-1) = +2 ⇒ x = +3
- Count the ligands. Inside the bracket there are 5 NH₃ and 1 Cl⁻, each bonding through one atom. That makes a coordination number of 5 + 1 = 6.
- So. The primary valency is 3 and the secondary valency is 6, which is option (d).
The idea to keep. Primary valency means oxidation state, so work it out from the charges. Secondary valency means coordination number, so count every donor atom inside the square bracket, including anionic ones.
Answer from the NTA final answer key. Solution by PrepWiser.