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.
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 t2g set and a higher eg set, separated by Δo. The spectrochemical series ranks ligands by field strength (weak: I−, Br−, Cl−, F−, H2O … strong: NH3, en, CN−, CO); a large Δo (strong field) favours low spin, a small Δo (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.
Continue Revising — NCERT Solutions for Class 12 Chemistry:
Chapter 1: Solutions | Chapter 2: Electrochemistry | Chapter 3: Chemical Kinetics | Chapter 4: The d- and f-Block Elements | Chapter 5: Coordination Compounds
