Coordination Compounds

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

Coordination compounds are built around a central metal ion or atom bonded to surrounding ligands through coordinate (dative) bonds. From Werner's foundational theory of primary and secondary valency, through systematic nomenclature and the rich variety of isomerism these compounds display, to the bonding theories (VBT, CFT) that explain their colour and magnetism, this topic ties directly into real applications in medicine and industry that JEE loves to test.

Chemistry Syllabus Structure Inorganic Chemistry ~33% weightage

Live Animation1 / 4

Coordination Sphere

1Central Metal IonLewis acid, accepts electron pairs

2LigandsDonor atoms bond directly

3Coordination NumberFixed count, sets geometry

4Counter IonsOutside sphere, ionisable

1Werner's Theory & Ligands

Alfred Werner's 1893 coordination theory explained compounds like CoCl3·6NH3 by proposing metals carry two distinct valencies — an ionizable primary valency and a fixed, directional secondary valency satisfied by ligands.

Learning Objectives

State Werner's postulates: primary valency (ionizable, equals oxidation state) and secondary valency (non-ionizable, equals coordination number, fixes geometry).
Define ligand, donor atom and coordination number with examples.
Classify ligands as unidentate, bidentate, polydentate (chelating) or ambidentate, with an example of each.
Distinguish the coordination sphere (written in square brackets) from ionisable counter ions outside it.

Key Terms

Coordination numberThe number of ligand donor atoms directly bonded to the central metal ion.

Chelate/chelating ligandA polydentate ligand that binds through two or more donor atoms to the same metal, forming a ring; increases complex stability (the chelate effect).

Ambidentate ligandA ligand with two different donor atoms that can bind through either one, e.g. NO2-/ONO- or SCN-/NCS-.

Coordination sphereThe central metal plus its directly bonded ligands, written inside square brackets; any counter ions outside the brackets remain ionisable.

Alfred Werner (1893) explained puzzling 'double salts' like CoCl3·6NH3 by proposing that metal ions possess two distinct types of valency.

Primary and secondary valency

Primary valency — ionizable, satisfied by negative ions, numerically equal to the oxidation state, and non-directional.
Secondary valency — non-ionizable, satisfied by ligands (neutral or negative), fixed in number as the coordination number, and directional — it fixes the geometry of the complex.

In [Co(NH3)6]Cl3, cobalt's secondary valency is 6 (bonded to 6 NH3 ligands) and its primary valency is 3 (the 3 Cl- ions ionise freely in solution).

Ligands and donor atoms

A **ligand** is an ion or molecule that donates an electron pair to the metal (a Lewis base) through its donor atom.

Unidentate — one donor atom (NH3, Cl-, H2O).
Bidentate — two donor atoms (ethylenediamine 'en', oxalate C2O4^2-).
Polydentate/chelating — many donor atoms (EDTA4- is hexadentate, binding through 6 sites).
Ambidentate — two different donor atoms, used one at a time (NO2-/ONO-, SCN-/NCS-).

Exam TipChelating ligands generally form more stable complexes than the equivalent number of unidentate ligands — the 'chelate effect' — largely due to the favourable entropy of ring formation.