1 markMCQSection A

The secondary valency of Co in the complex [Co(NH₃)₅(NO₂)]²⁺ is

Q: The secondary valency of Co in the complex [Co(NH₃)₅(NO₂)]²⁺ is

**Correct option: (D) 6** Secondary valency = **coordination number** = number of ligand donor atoms directly bonded to the central metal ion. - 5 × NH₃ (monodentate) = 5 bonds - 1 × NO₂ (monodentate, nitrito/nitro) = 1 bond - **Total = 6** Hence the secondary valency (coordination number) of Co is **6**.

Coordination Compounds

Secondary valency (coordination number)

Options

(A)5

(B)1

(D)6

Official Answer

Correct option: (D) 6

Secondary valency = coordination number = number of ligand donor atoms directly bonded to the central metal ion.

5 × NH₃ (monodentate) = 5 bonds
1 × NO₂ (monodentate, nitrito/nitro) = 1 bond
Total = 6

Hence the secondary valency (coordination number) of Co is 6.

secondary valencycoordination numberWerner's theorymonodentate ligandoctahedralprimary valencycobalt complexligand donor atom

Marking Scheme

11 mark: correct option (D) 6.
2Full mark awarded only for 6; reasoning (count of all six monodentate ligands) supports but is not separately required in an MCQ.

Hint

Secondary valency = coordination number = total number of ligand donor atoms bonded to the metal.

Quick Oral Answer

Secondary valency is the coordination number; here five ammonia molecules plus one nitro group give six ligand bonds, so it is six and the geometry is octahedral.

Analysis & Explanation

Concept

In Werner's theory, every metal shows two kinds of valency:

Primary valency — ionisable, satisfied by negative ions, equals the oxidation state (here Co is +3, since 5 NH₃ are neutral, NO₂⁻ is −1, and overall charge is +2).
Secondary valency — non-ionisable, satisfied by ligands, equals the coordination number and fixes the geometry.

Counting the donor atoms

All six ligands (5 NH₃ + 1 NO₂) are monodentate, so each contributes one bond → secondary valency = 6 (octahedral).

Exam trap

Students confuse secondary valency with the overall charge (+2) or with the number of only one type of ligand (5). Secondary valency counts all coordinated donor atoms, regardless of charge.

Why the distractors are wrong

(A) 5 counts only the NH₃ ligands and ignores NO₂.
(B) 1 counts only the NO₂ ligand.
(C) 4 would suit a tetragonal/tetrahedral count, not this complex.

Common Mistakes

1Equating secondary valency with the overall ionic charge (+2) instead of counting ligands.
2Counting only the NH₃ groups (5) and forgetting the NO₂ ligand.
3Confusing secondary valency with primary valency (oxidation state = +3).

Interesting Facts

Alfred Werner received the 1913 Nobel Prize in Chemistry for this bonding theory — the first ever awarded to an inorganic chemist.

The NO₂ ligand is ambidentate: bonded through N it is 'nitro', through O it is 'nitrito', giving linkage isomers of this very cobalt complex.

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Frequently Asked Questions

What is the difference between primary and secondary valency?

Primary valency is ionisable and equals the metal's oxidation state (here +3), satisfied by anions. Secondary valency is non-ionisable, equals the coordination number (here 6), and is satisfied by ligands. Secondary valency also decides the geometry of the complex.

What is the oxidation state of Co in this complex?

NH₃ is neutral and NO₂ carries −1. If overall charge is +2, then $x + 0 + (-1) = +2$ , giving $x = +3$ . So Co is in the +3 oxidation state, while its coordination number (secondary valency) remains 6.