Coordination Compounds – Class 12 Chemistry Chapter 5 Notes & NCERT PDF

Introduction

Coordination compounds are the backbone of modern inorganic and bio-inorganic chemistry. In them a central metal atom or ion is bound to a number of anions or neutral molecules by sharing their electron pairs. Life depends on them: chlorophyll (Mg), haemoglobin (Fe) and vitamin B12 (Co) are all coordination compounds. They are also used in metallurgy, analytical chemistry, electroplating, dyeing and medicine. This unit asks four questions: how to describe a complex (definitions and nomenclature), in how many ways its atoms can be arranged (isomerism), why it forms and has its shape, colour and magnetism (VBT and CFT), and where it is used.

Key Concepts

5.1 Werner's Theory

Alfred Werner (1866-1919), a Swiss chemist and Nobel laureate of 1913, studied cobalt(III) chloride-ammonia compounds. With excess AgNO₃ in the cold, 1 mol of CoCl₃·6NH₃ (yellow) gave 3 mol AgCl, CoCl₃·5NH₃ (purple) gave 2 mol, and CoCl₃·4NH₃ (green and violet forms) gave 1 mol. He proposed:

The formulations are [Co(NH₃)₆]Cl₃, [CoCl(NH₃)₅]Cl₂ and [CoCl₂(NH₃)₄]Cl. A double salt such as Mohr's salt dissociates completely in water, whereas a complex ion keeps its identity.

5.2 Key Definitions

5.3 Nomenclature

Formulas (5.3.1): central atom first, then ligands in alphabetical order; polydentate ligand abbreviations in parentheses; the whole entity in square brackets with the charge outside.

Names (5.3.2): cation named before anion; ligands named alphabetically before the metal; anionic ligand names end in -o (chlorido, cyanido); neutral ligands keep their names except aqua, ammine, carbonyl and nitrosyl; prefixes di-, tri- (or bis-, tris- for complicated ligands); the oxidation state in Roman numerals; an anionic complex ends in -ate (ferrate, cuprate, argentate). Examples from the text: [Cr(NH₃)₃(H₂O)₃]Cl₃ is triamminetriaquachromium(III) chloride; [Co(H₂NCH₂CH₂NH₂)₃]₂(SO₄)₃ is tris(ethane-1,2-diamine)cobalt(III) sulphate; [Ag(NH₃)₂][Ag(CN)₂] is diamminesilver(I) dicyanidoargentate(I).

5.4 Isomerism

Stereoisomerism:

Structural isomerism:

5.5 Bonding in Coordination Compounds

VBT (5.5.1): the metal uses hybrid orbitals which accept ligand electron pairs. CN 4 gives sp³ (tetrahedral) or dsp² (square planar); CN 6 gives d²sp³ (inner orbital, low spin) or sp³d² (outer orbital, high spin).

Magnetic properties (5.5.2): [Mn(CN)₆]³⁻ has two unpaired electrons but [MnCl₆]³⁻ has four; [Fe(CN)₆]³⁻ has one but [FeF₆]³⁻ has five; [Co(C₂O₄)₃]³⁻ is diamagnetic while [CoF₆]³⁻ has four.

Limitations of VBT (5.5.3): it rests on assumptions, gives no quantitative magnetic interpretation, does not explain colour or stability, cannot reliably predict tetrahedral vs square planar geometry, and does not distinguish weak and strong ligands.

Crystal Field Theory (5.5.4): an electrostatic model with ligands as point charges or dipoles. In an octahedral field the d orbitals split into a lower t₂g set (dxy, dyz, dxz) and a higher e_g set (dx²-y², dz²); the gap is Δo. The e_g orbitals rise by (3/5)Δo and t₂g fall by (2/5)Δo. Spectrochemical series: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO. For d⁴: if Δo < P the configuration is t₂g³e_g¹ (weak field, high spin); if Δo > P it is t₂g⁴e_g⁰ (strong field, low spin). In a tetrahedral field the splitting is inverted and smaller, Δt = (4/9)Δo, so low-spin tetrahedral complexes are rare.

Colour (5.5.5): a d-d transition absorbs part of visible light and the complementary colour is seen. [Ti(H₂O)₆]³⁺ (d¹) absorbs blue-green and looks violet. Anhydrous CuSO₄ is white but hydrated is blue. Ruby is Al₂O₃ with 0.5-1% Cr³⁺; emerald also owes its colour to Cr³⁺.

Limitations of CFT (5.5.6): anionic ligands should split most but sit at the weak end of the series; covalent character is ignored.

5.6 Bonding in Metal Carbonyls

[Ni(CO)₄] is tetrahedral, [Fe(CO)₅] trigonal bipyramidal, [Cr(CO)₆] octahedral; [Mn₂(CO)₁₀] has an Mn-Mn bond; [Co₂(CO)₈] has a Co-Co bond bridged by two CO groups. The M-C bond has sigma character (CO lone pair donated to the metal) and pi character (back-donation from a filled metal d orbital into CO's empty π* orbital). This synergic bonding strengthens the metal-carbonyl bond.

5.7 Importance and Applications

Worked Examples

Key-Facts Sheet

Board Exam Focus

Q. [Fe(H₂O)₆]³⁺ is strongly paramagnetic whereas [Fe(CN)₆]³⁻ is weakly paramagnetic. Explain. Fe³⁺ is d⁵. H₂O is a weak-field ligand, so no pairing occurs and 5 unpaired electrons remain (sp³d², outer orbital). CN⁻ is strong field and forces pairing, leaving 1 unpaired electron (d²sp³, inner orbital).

Q. Give evidence that [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅(SO₄)]Cl are ionisation isomers. In solution the first gives SO₄²⁻ and a white BaSO₄ precipitate with BaCl₂. The second gives Cl⁻ and a white AgCl precipitate with AgNO₃. Same formula, different ions in solution.

Q. Explain why aqueous CuSO₄ with excess KCN gives no CuS with H₂S. Excess cyanide forms the very stable tetracyanidocuprate(I) ion, [Cu(CN)₄]³⁻ (copper as Cu(I), as noted in the unit). Free copper ions are practically absent, so CuS cannot precipitate.

Also practise writing IUPAC names and formulas, counting geometrical isomers, and drawing the d-orbital splitting diagram.

Key Terms

Common Mistakes