Chapter 5 — Coordination Compounds

Chapter 5 of the Class 12 Chemistry NCERT textbook, "Coordination Compounds", covers Werner's theory, nomenclature, isomerism, and bonding theories (VBT and CFT) for complexes where a central metal ion is bound to ligands in a definite geometric arrangement.

• Werner's picture of coordination — The chapter builds on Werner's insight that metal ions carry both primary (ionisable) and secondary (non-ionisable) valences, with the secondary valence fixing the coordination number and hence the octahedral, tetrahedral or square-planar shape of a complex.
• Ligands, naming and isomerism — It organises ligands by how many donor atoms they use and lays out IUPAC naming rules, then surveys the many ways complexes can differ in structure — geometric, optical, linkage, coordination, ionisation and solvate isomerism.
• Bonding theories that explain colour and magnetism — Valence Bond Theory and Crystal Field Theory are used to account for shape, colour and magnetic behaviour, with CFT's d-orbital splitting explaining low- versus high-spin complexes and the colours seen in transition-metal solutions.
• Biological and practical importance — The chapter connects the theory to life and industry — chlorophyll, haemoglobin and vitamin B12 are all coordination compounds, and complexes serve in metallurgy, catalysis and medicine such as the anticancer drug cisplatin.

1. 01Werner's theory distinguishes primary (ionisable) valence and secondary (non-ionisable) valence; the secondary valence equals the coordination number and determines geometry (octahedral, tetrahedral, square planar).
2. 02Ligands are classified by denticity: unidentate (Cl⁻, NH₃), didentate (ethane-1,2-diamine, C₂O₄²⁻), polydentate (EDTA⁴⁻ is hexadentate), and ambidentate (NO₂⁻, SCN⁻ can bind through either of two atoms).
3. 03Isomerism types include geometric (cis/trans, fac/mer), optical (non-superimposable mirror images in octahedral complexes with didentate ligands), linkage, coordination, ionisation, and solvate isomerism.
4. 04Crystal Field Theory explains colour via d-d electronic transitions and magnetic behaviour via d-orbital splitting (Δo for octahedral, Δt = 4/9 Δo for tetrahedral); strong-field ligands cause low-spin and weak-field ligands cause high-spin configurations.
5. 05Metal carbonyls exhibit synergic bonding: ligand-to-metal σ donation plus metal-to-ligand π back-donation into the CO antibonding π* orbital, stabilising the M–C bond.
6. 06Biological significance: chlorophyll (Mg), haemoglobin (Fe), and vitamin B12 (Co) are coordination compounds; medicinal uses include cisplatin (anticancer), EDTA (lead poisoning), and chelate therapy for toxic metal removal.