Why are Axial Bonds in PCl5 Longer? The NEET explanation you actually need

Mistakes/Tricks

1. TITLE

Ambidentate Ligands: $-NO_2$ vs $-ONO$ Naming Confusion — The difference 90% students get wrong

2. THE MISTAKE STATEMENT

THE COMMON MISTAKE:
Students confuse $-NO_2$ with $-ONO$ because they both have the identical chemical formula ($NO_2^-$) and represent the exact same nitrite ion. Most students think that since the ion is the same, the IUPAC naming is exactly the same, completely ignoring which atom is actually binding to the central metal.

3. WHY THIS MISTAKE HAPPENS

• What's the source of confusion? Both ligands involve the exact same atoms and have the same overall charge, acting as an ambidentate ligand. This leads to linkage isomerism, where the formula looks identical but the bonding is fundamentally different.
• What similar-sounding concept causes this? Students lazily use old trivial names like "nitro" or "nitrito" interchangeably without specifying the donor atom, which violates modern IUPAC nomenclature rules.
• How do coaching centers sometimes teach this wrong? They sometimes teach the formula as just "$NO_2^-$" in all contexts, failing to emphasize that the way the formula is written ($-NO_2$ vs $-ONO$) is a deliberate clue telling you which atom is acting as the electron donor.

4. THE CORRECT UNDERSTANDING

THE TRUTH:
The $NO_2^-$ ion is an ambidentate ligand, meaning it has two different donor atoms available, but only one links to the central metal ion in a specific complex.

$-NO_2$ is: The nitrite ion coordinating to the metal through the Nitrogen (N) atom. In IUPAC nomenclature, this must be explicitly named nitrito-N. In Jørgensen's classic $[Co(NH_3)_5(NO_2)]Cl_2$ complex, this linkage forms the yellow isomer.

$-ONO$ is: The nitrite ion coordinating to the metal through the Oxygen (O) atom. In IUPAC nomenclature, this must be explicitly named nitrito-O. In the same cobalt complex, this linkage forms the red isomer.

Key difference: The element symbol written closest to the metal or written first in the ligand formula is the one actively donating its electron pair.

5. COMPARISON TABLE

6. MCQ (Tests the exact confusion point)

What is the correct IUPAC name for the coordination complex $[Co(NH_3)_5(ONO)]Cl_2$?

(a) pentaamminenitrito-N-cobalt(III) chloride
(b) pentaamminenitrito-O-cobalt(III) chloride
(c) pentaamminenitrocobalt(III) chloride
(d) pentaammineoxygennitrocobalt(II) chloride

7. ANSWER + WHY THE TRAP WORKS

Correct: (b)

If you chose (a), you made the classic mistake of ignoring the specific arrangement of atoms in the ligand's formula! The formula explicitly writes O first ($-ONO$), which means oxygen is the donor atom. Therefore, it must be named nitrito-O. The question specifically asked about the $-ONO$ linkage, not the $-NO_2$ linkage, so "nitrito-N" is incorrect.

8. THE TRICK TO NEVER FORGET

MEMORY TRICK:
"The FIRST letter is the FIRST to donate."

• If you see $-NO_2$ $\rightarrow$ N is written first $\rightarrow$ Nitrogen is the donor $\rightarrow$ nitrito-N.
• If you see $-ONO$ $\rightarrow$ O is written first $\rightarrow$ Oxygen is the donor $\rightarrow$ nitrito-O.

9. RAPID FIRE — Test Yourself

Quick check — Which one?

1. Ligand is written as $M-NO_2$ → [nitrito-N] or [nitrito-O]?
2. The ligand causes Jørgensen's complex to turn red → [nitrito-N] or [nitrito-O]?
3. The ambidentate ligand uses Oxygen as the donor atom → [nitrito-N] or [nitrito-O]?

Answers: 1-[nitrito-N], 2-[nitrito-O], 3-[nitrito-O]

10. NCERT REFERENCE

$-NO_2$ and $-ONO$ are defined in: NCERT Class 12 Chemistry, Chapter 5 (Coordination Compounds) under the sections "Important Terms" (Ambidentate Ligands) and "Structural Isomerism" (Linkage Isomerism).

11. CTA

More comparisons from this chapter → Chemical Bonding & Molecular Structure Guide