Summary
NCERT Class 12 Chemistry Chapter 5, 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.
Coordination Compounds are central to modern inorganic and bio-inorganic chemistry. Alfred Werner proposed that metal ions exhibit primary (ionisable) and secondary (non-ionisable) valences, the latter equal to the coordination number and responsible for the geometry of the complex. Key concepts include ligand types (unidentate, didentate, polydentate, ambidentate, chelate), IUPAC nomenclature rules, and multiple forms of isomerism (geometric, optical, linkage, coordination, ionisation, solvate). Bonding is explained by Valence Bond Theory and Crystal Field Theory, the latter accounting for d-orbital splitting, colour, and magnetic properties. Coordination compounds have vital biological roles (chlorophyll, haemoglobin, vitamin B12) and industrial applications in metallurgy, catalysis, and medicine.
Key points & formulas
- 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).
- 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).
- 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.
- 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.
- 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.
- 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.
Frequently asked questions
01What are the main postulates of Werner's theory of coordination compounds?
Werner proposed that metal ions have two types of valences: primary valences (ionisable, satisfied by negative ions) and secondary valences (non-ionisable, satisfied by neutral molecules or negative ions). The secondary valence equals the coordination number, which is fixed for a metal, and determines the spatial (geometric) arrangement of ligands around the central atom, forming shapes such as octahedral, tetrahedral, or square planar.
02What is the spectrochemical series and how does it affect complex properties?
The spectrochemical series is an experimentally determined order of ligands based on their crystal field splitting ability: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO. Strong-field ligands (like CN⁻ and CO) produce large Δo, causing electron pairing and low-spin complexes, while weak-field ligands (like F⁻ and Cl⁻) result in small Δo and high-spin complexes.
03How does Crystal Field Theory explain the colour of coordination compounds?
In CFT, ligands split the d orbitals of the central metal ion into sets of different energy (t₂g and eɡ in octahedral complexes). When visible light of a specific wavelength is absorbed, an electron is promoted from the lower t₂g to the higher eɡ level (d-d transition). The colour observed is complementary to the absorbed wavelength. For example, [Ti(H₂O)₆]³⁺ absorbs blue-green light (~498 nm) and appears violet.
04Is the NCERT Class 12 Chemistry Chapter 5 PDF free to download?
Yes, the NCERT Class 12 Chemistry Part I Chapter 5 (Coordination Compounds) PDF is completely free to download on cbseprepmaster.com.
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