---
title: "Chelate, Sidgwick theory, IUPAC names"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:chelate-sidgwick"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chelate-sidgwick
site: BCT Engineering
---

# Chelate, Sidgwick theory, IUPAC names

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2074 Bhadra, 2+2+1 marks
- Video: [9.8-Effective Atomic Number ( EAN ) / EAN Rule / Sidgwick Theory of coordination compounds](https://www.youtube.com/watch?v=QeDLXyXLZ2o) — IPL CHEMISTRY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chelate-sidgwick
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

What do you understand by a chelate and a chelating ligand? Describe Sidgwick theory of co-ordination compounds with an example. Write down the IUPAC names of the following compounds.
(i) Na₃[Al(C₂O₄)₃]  (ii) [Co(NH₃)₄Cl₂]⁺

## Short answer

A chelating ligand has multiple donor atoms binding to one metal ion forming a ring chelate. Sidgwick's EAN rule states the central metal accepts electron pairs until its effective atomic number equals the next noble gas, e.g., [Fe(CN)6]4- has EAN 36. IUPAC: sodium trioxalatoaluminate(III), tetraamminedichlorocobalt(III) ion.

## Answer

#### Chelate and chelating ligand
A **chelating ligand** (chelating agent) is a ligand that has two or more donor atoms and binds the same metal ion at two or more places, forming a ring. The ring-shaped complex so formed is a **chelate**. Example: ethylenediamine (en) and oxalate (C₂O₄²⁻) are bidentate; EDTA is hexadentate. In [Cu(en)₂]²⁺ each en forms a five-membered ring with Cu.
```text
   H₂C—NH₂╲
   |        Cu
   H₂C—NH₂╱
```
Chelates are more stable than comparable non-chelated complexes (chelate effect).

#### Sidgwick's theory (EAN rule)
Sidgwick proposed that a metal ion accepts electron pairs from ligands until its total number of electrons equals that of the next noble gas. This total is the **Effective Atomic Number (EAN)**:

EAN = (atomic number − electrons lost to make the ion) + 2 × (number of ligands)

Example: [Fe(CN)₆]⁴⁻ has Fe²⁺ (24 e⁻) + 6 × 2 = 36 = Kr, so it is stable.

Limitations: many stable complexes do not obey the rule, such as [Ni(NH₃)₆]²⁺ (38).

#### IUPAC names
(i) Na₃[Al(C₂O₄)₃]: **sodium trioxalatoaluminate(III)**
(ii) [Co(NH₃)₄Cl₂]⁺: **tetraamminedichlorocobalt(III) ion**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A chelating ligand
   A. has two or more donor atoms and forms a ring with the metal
   B. has one donor atom
   C. is neutral only
   D. never binds metals
   Answer: A. The correct option is: has two or more donor atoms and forms a ring with the metal. This follows from the answer on chelating ligand. (Key point: Chelating ligand)

2. According to Sidgwick, stable complexes tend to have an EAN equal to
   A. zero
   B. the next noble gas
   C. the metal's Z
   D. the same noble gas as before
   Answer: B. The correct option is: the next noble gas. This follows from the answer on ean rule. (Key point: EAN rule)

3. What is the EAN of [Fe(CN)6]4-?
   A. 35
   B. 34
   C. 36
   D. 38
   Answer: C. The correct option is: 36. This follows from the answer on ean of [fe(cn)6]4-. (Key point: EAN of [Fe(CN)6]4-)

4. What is the IUPAC name of Na3[Al(C2O4)3]?
   A. sodium tris(oxalate)aluminium(III)
   B. aluminium trioxalatosodium
   C. sodium trioxalatoaluminium(III)
   D. sodium trioxalatoaluminate(III)
   Answer: D. Anionic complex ends -ate. (Key point: Name)

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