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Chelate, Sidgwick theory, IUPAC names

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.

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]⁺

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.

   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

✓ Answer checked by Claude Opus 5.5 (high effort)

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