Class 12 Chemistry Chapter 5 Coordination Compounds – Revision Notes

Quick revision notes for Class 12 Chemistry Chapter 5 – Coordination Compounds, covering Werner’s theory, nomenclature, isomerism, bonding theories, and applications. Ideal for last-minute board exam revision.

Last Updated: September 23, 2026

Werner’s Theory and Basic Definitions

Werner’s postulates distinguish a metal’s primary valence (ionisable, = oxidation state) from its secondary valence (non-ionisable, = coordination number, fixed spatial geometry). A ligand is bonded to the central metal via a donor atom; ligands can be unidentate (one donor atom, e.g. NH₃), didentate (two, e.g. ethane-1,2-diamine), polydentate/chelating (several, e.g. EDTA), or ambidentate (two possible donor atoms, only one used at a time, e.g. NO₂⁻, SCN⁻). Complexes with only one type of ligand are homoleptic; those with more than one type are heteroleptic.

IUPAC Nomenclature

Key rules: cation is named before anion; ligands are named alphabetically (ignoring multiplying prefixes like di-, tri-, bis-, tris-); anionic ligand names end in “-o” (e.g. chlorido, cyanido, oxalato); the metal’s oxidation state is written in Roman numerals in parentheses immediately after its name; and if the complex ion is an anion, the metal name takes the suffix “-ate” (e.g. ferrate, cobaltate, nickelate).

Isomerism

Structural isomerism: linkage (ambidentate ligand bonds via different atoms), ionisation (interchange of ligand and counter-ion), coordination (interchange of ligands between cationic and anionic complex parts), and solvate/hydrate isomerism. Stereoisomerism: geometrical (cis-trans, fac-mer, in square-planar and octahedral complexes) and optical (non-superimposable mirror images, common in octahedral chelate complexes like [Cr(C₂O₄)₃]³⁻).

Bonding Theories: VBT and CFT

Valence Bond Theory (VBT) assigns hybridisation (d²sp³ or sp³d² for octahedral, dsp² for square planar) based on whether the ligand forces electron pairing (strong field, low spin) or not (weak field, high spin). Crystal Field Theory (CFT) explains this via d-orbital splitting: in an octahedral field, the d-orbitals split into a lower t_2g set and a higher e_g set, separated by Δ_o. The spectrochemical series ranks ligands by field strength (weak: I⁻, Br⁻, Cl⁻, F⁻, H₂O … strong: NH₃, en, CN⁻, CO); a large Δ_o (strong field) favours low spin, a small Δ_o (weak field) favours high spin. Colour arises from d-d transitions across this gap; magnetic behaviour follows from the number of unpaired electrons via μ=√n(n+2) BM.

Metal Carbonyls and Stability

Bonding in metal carbonyls is synergic: σ-donation from CO to the metal, plus π-backdonation from the metal’s filled d-orbitals into CO’s empty π* orbital. Complex stability is enhanced by the chelate effect — polydentate ligands (like en, oxalate, EDTA) form more stable complexes than equivalent unidentate ligands.

Applications

Coordination compounds are vital biologically (chlorophyll: Mg; haemoglobin: Fe; vitamin B₁₂: Co), in medicine (cisplatin as an anticancer drug, EDTA for heavy-metal poisoning), analytically (EDTA titrations for water hardness), and metallurgically (extraction of Ag/Au as cyanide complexes, Ni purification via the Mond process).

One-Line Summary

Chapter 5 builds from Werner’s foundational theory through modern IUPAC nomenclature, the several types of isomerism unique to coordination compounds, and the VBT/CFT bonding theories that together explain their colour, magnetism, and structure, closing with the wide biological, medicinal and industrial importance of these compounds.

Written by Satish

NCERTBooks.org is an independent educational resource run by a small team focused on making official NCERT textbooks easy to find, read, and download for students, parents, and teachers across India. We are not affiliated with NCERT or the Ministry of Education -- we organise publicly available NCERT content by class and subject, verify links against official sources, and build tools (like our in-browser reader) that make studying more convenient. Every guide we publish is written and reviewed by our team based on the actual NCERT curriculum and syllabus.