Coordination Compounds

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

Coordination compounds contain a central metal atom/ion bonded to surrounding ligands through coordinate (dative) bonds — the basis of Werner's theory of primary and secondary valence. NEET tests this unit densely: IUPAC nomenclature rules, the several flavours of isomerism, Valence Bond Theory's hybridisation-geometry-magnetism chain, and Crystal Field Theory's d-orbital splitting that explains both colour and magnetic behaviour covered under d-block chemistry.

Chemistry Syllabus Inorganic Chemistry ~25% weightage

Live Animation1 / 4

Central Metal Ion

1Unidentate (NH3)One donor atom per ligand

2Bidentate (en)Two donor N atoms, forms chelate ring

3Polydentate (EDTA)Six donor atoms, very stable chelate

4Ambidentate (SCN−)Binds via S or N

1Ligands & Coordination Number

A ligand donates an electron pair to the central metal through a coordinate bond; the coordination number counts how many such donor atoms surround it.

Learning Objectives

Define ligand, coordination number, and central metal atom/ion per Werner's theory of primary and secondary valence.
Classify ligands as unidentate, bidentate or polydentate (chelating), with examples (NH3, ethylenediamine, EDTA).
State the common coordination numbers (4 and 6) and their typical geometries.
Explain ambidentate ligands using SCN− as an example of a ligand that can bind through two different donor atoms.

Key Terms

LigandAn ion or neutral molecule that donates at least one electron pair to a central metal atom/ion, forming a coordinate (dative) bond.

Coordination NumberThe number of donor atoms directly bonded to the central metal atom/ion in a complex.

ChelateA cyclic complex formed when a polydentate ligand binds a metal at two or more points, e.g. ethylenediamine (en) forming a 5-membered ring with a metal.

Ambidentate LigandA ligand with two different donor atoms that can each coordinate to the metal, e.g. SCN− (via S) or NCS− (via N); NO2− (via N, nitro) or ONO− (via O, nitrito).

Alfred Werner's theory (1893) first explained coordination compounds by proposing two types of valence: primary valence (the metal's oxidation state, satisfied by anions) and secondary valence (the fixed coordination number, satisfied by ligands arranged in a definite geometry around the metal).

Classifying ligands by denticity

Unidentate: one donor atom, e.g. NH3, H2O, Cl−, CN−.
Bidentate: two donor atoms binding the same metal, e.g. ethylenediamine (en, via 2 N atoms), oxalate (C2O42−, via 2 O atoms).
Polydentate (chelating): many donor atoms, e.g. EDTA4− (hexadentate, 6 donor atoms — 4 O and 2 N), forming very stable chelate complexes used in water softening and as an antidote for heavy-metal poisoning.

Key PointChelate rings of 5 or 6 members are the most stable (the 'chelate effect') — this is why en and EDTA complexes are far more thermodynamically stable than the equivalent number of unidentate NH3 ligands.

Coordination number and geometry

Coordination number 4: tetrahedral (e.g. [Ni(CO)4]) or square planar (e.g. [Ni(CN)4]2−, [Pt(NH3)2Cl2]).
Coordination number 6: octahedral (e.g. [Co(NH3)6]3+), by far the most common coordination number for transition metal complexes.

Ambidentate ligands add a twist: SCN− can bond through its S atom (thiocyanato) or N atom (isothiocyanato), and NO2− can bond through N (nitro) or O (nitrito) — giving rise to linkage isomers of otherwise identical formula.