CUET UG Chemistry Inorganic Chemistry Coordination Chemistry IUPAC Naming Isomerism Stability Constants

Must-Know (15–20 detailed bullets)

• The IUPAC name of [Co(NH₃)₆]Cl₃ is hexaamminecobalt(III) chloride; ligands are named before the metal in alphabetical order, with prefixes indicating number.
• In [CrCl₂(en)₂]Cl, "en" is ethane-1,2-diamine, a bidentate ligand; the correct name is dichloridobis(ethane-1,2-diamine)chromium(III) chloride.
• Anionic ligands end in "-o", e.g., Cl⁻ → chlorido, CN⁻ → cyanido, OH⁻ → hydroxido (NCERT uses "hydroxido", not "hydroxy").
• Neutral ligands use their molecular names, e.g., NH₃ → ammine, H₂O → aqua, CO → carbonyl.
• The oxidation state of the central metal is indicated in Roman numerals in parentheses, e.g., iron(II), iron(III).
• For coordination compounds with complex anions, the metal name ends in "-ate", e.g., [Fe(CN)₆]⁴⁻ is hexacyanidoferrate(II) ion.
• Linkage isomerism occurs due to ambidentate ligands like SCN⁻ (thiocyanato-S or thiocyanato-N) and NO₂⁻ (nitro-N or nitrito-O). Example: [Co(NH₃)₅(NO₂)]Cl₂ (yellow, nitro) vs [Co(NH₃)₅(ONO)]Cl₂ (red, nitrito).
• Coordination isomerism occurs in compounds with both cation and anion as complexes, e.g., [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].
• Ionization isomerism arises when counter ions exchange with ligands, e.g., [Co(NH₃)₅Br]SO₄ (gives white ppt with BaCl₂) vs [Co(NH₃)₅SO₄]Br (no ppt with BaCl₂).
• Hydrate (solvate) isomerism differs in whether water is inside or outside the coordination sphere, e.g., [Cr(H₂O)₆]Cl₃ (violet) vs [CrCl(H₂O)₅]Cl₂·H₂O (green).
• Geometrical isomerism is common in square planar (e.g., [Pt(NH₃)₂Cl₂]) and octahedral complexes (e.g., [Co(NH₃)₄Cl₂]⁺); cis and trans forms exist.
• Optical isomerism occurs in chiral complexes without plane of symmetry, e.g., [Co(en)₃]³⁺ has d and l forms; en = bidentate, leading to non-superimposable mirror images.
• The stability constant (K) for a complex [MLₙ] is defined as K = [MLₙ]/([M][L]ⁿ); higher K means greater stability.
• Stepwise stability constants (K₁, K₂, ..., Kₙ) decrease progressively; overall stability constant βₙ = K₁ × K₂ × ... × Kₙ.
• Chelate effect: complexes with chelating ligands (e.g., en, EDTA⁴⁻) are more stable than those with monodentate ligands due to increased entropy.
• EDTA⁴⁻ forms very stable complexes with metal ions like Ca²⁺ and Mg²⁺; used in estimation of hardness of water.
• The coordination number is the number of sigma bonds between ligands and central metal ion, e.g., 6 in [CoF₆]³⁻, 4 in [Ni(CN)₄]²⁻.
• Homoleptic complexes have only one type of ligand, e.g., [Co(NH₃)₆]³⁺; heteroleptic have more than one, e.g., [Co(NH₃)₄Cl₂]⁺.
• The IUPAC name of K₃[Fe(CN)₆] is potassium hexacyanidoferrate(III); Fe is in +3 oxidation state.
• The complex [NiCl₂(PPh₃)₂] shows geometrical isomerism (square planar Ni²⁺), but [Ni(CO)₄] does not (tetrahedral, symmetrical).

Difficulty Level

Intermediate — requires understanding of nomenclature rules, isomer types, and application of stability concepts, but no advanced theory beyond NCERT.

Common CUET Traps (3 bullets)

• Trap: Using outdated ligand names like "amino" instead of "ammine" for NH₃. Avoid: Always use "ammine" (with double m) for NH₃ ligand as per current IUPAC and NCERT.
• Trap: Confusing linkage isomerism with ionization isomerism; both involve NO₂⁻ but differ in exchange type. Avoid: Linkage isomerism keeps counter ion same but changes donor atom (N vs O); ionization changes which ion is inside/outside coordination sphere.
• Trap: Assuming all octahedral complexes with two different ligands show optical isomerism. Avoid: Only chiral complexes (e.g., with bidentate ligands like en in [M(en)₃]) show optical activity; [Co(NH₃)₄Cl₂]⁺ shows geometrical but not optical isomerism.

Practice MCQs (5 questions)

Q1. What is the correct IUPAC name of [Co(NH₃)₅Cl]Cl₂?
A. Pentamminechlorocobalt(III) chloride
B. Chloropentamminecobalt(III) chloride
C. Pentamminechloridocobalt(III) chloride
D. Pentamminecobalt(III) chloride chloride

Answer: C
Explanation: Ligands are named alphabetically as "chlorido" and "ammine", with "penta" prefix; "chlorido" comes before "ammine" alphabetically.
Why others fail: Option A uses outdated "chloro" instead of "chlorido" and incorrect order.

Q2. Which type of isomerism is shown by [Cr(H₂O)₆]Cl₃ and [CrCl(H₂O)₅]Cl₂·H₂O?
A. Ionization isomerism
B. Linkage isomerism
C. Hydrate isomerism
D. Coordination isomerism

Answer: C
Explanation: These are hydrate isomers because water molecules are either inside or outside the coordination sphere.
Why others fail: Option A is tempting because ions differ, but the key difference is position of H₂O, not counter ion exchange.

Q3. Which complex shows optical isomerism?
A. [Co(NH₃)₄Cl₂]⁺
B. [Pt(NH₃)₂Cl₂]
C. [Co(en)₃]³⁺
D. [Cu(NH₃)₄]²⁺

Answer: C
Explanation: [Co(en)₃]³⁺ has no plane of symmetry and exists as non-superimposable mirror images due to bidentate en ligands.
Why others fail: Option A has geometrical isomers but is not chiral; students often confuse geometrical with optical isomerism.

Q4. The stability constant β₄ for [Cu(NH₃)₄]²⁺ is 2.1 × 10¹³. What does this imply?
A. The complex is highly unstable
B. The complex formation is negligible
C. The complex is very stable
D. The value refers to dissociation

Answer: C
Explanation: A high β₄ value (2.1 × 10¹³) indicates the complex is very stable in solution.
Why others fail: Option D is tempting because students confuse stability constant with dissociation constant; β refers to formation.

Q5. Which pair represents linkage isomers?
A. [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺
B. [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]
C. [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br
D. [Pt(NH₃)₂Cl₂] (cis) and (trans)

Answer: A
Explanation: NO₂⁻ can bind via nitrogen (nitro) or oxygen (nitrito), forming linkage isomers.
Why others fail: Option C shows ionization isomerism, which is structurally similar but involves different ions, not donor atoms.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ Use "ammine" (not amino) for NH₃, "aqua" for H₂O, "carbonyl" for CO.
• ⚠️ Anionic ligands: Cl⁻ → chlorido, CN⁻ → cyanido, OH⁻ → hydroxido, F⁻ → fluorido.
• ⚠️ Alphabetical order in naming: ammine before chlorido (a before c), not by prefix.
• ⚠️ [Fe(CN)₆]⁴⁻ → hexacyanidoferrate(II); metal name ends with "-ate" in anionic complexes.
• ⚠️ Oxidation state in Roman numerals: e.g., cobalt(III), not Co³⁺ in name.
• ⚠️ K₃[Fe(CN)₆] = potassium hexacyanidoferrate(III); CN⁻ is cyanido, Fe is +3.
• ⚠️ Linkage isomerism: SCN⁻ → thiocyanato-S or thiocyanato-N; NO₂⁻ → nitro (N) or nitrito (O).
• ⚠️ Ionization isomers give different ions in solution, e.g., SO₄²⁻ inside vs outside sphere.
• ⚠️ Hydrate isomers differ in H₂O position: coordinated vs lattice water.
• ⚠️ [Co(NH₃)₆]Cl₃ (violet) vs [CoCl(NH₃)₅]Cl₂·H₂O (pale red) – hydrate isomers.
• ⚠️ Geometrical isomerism: cis/trans in square planar [Pt(NH₃)₂Cl₂] and octahedral [Co(NH₃)₄Cl₂]⁺.
• ⚠️ Optical isomerism requires absence of plane of symmetry; e.g., [M(en)₃]ⁿ⁺ shows d and l forms.
• ⚠️ Chelate effect: ΔG more negative due to increased entropy when chelating ligands bind.
• ⚠️ Stability constant βₙ = [MLₙ]/([M][L]ⁿ); larger β = more stable complex.
• ⚠️ Stepwise constants: K₁ > K₂ > K₃ > K₄ due to statistical and electrostatic factors.
• ⚠️ EDTA⁴⁻ forms 1:1 complexes with Ca²⁺, Mg²⁺; used in water hardness estimation.
• ⚠️ Homoleptic: [Fe(CO)₅]; heteroleptic: [Co(NH₃)₄Cl₂]⁺.
• ⚠️ Coordination number = number of sigma bonds from ligands to metal.
• ⚠️ [Cr(en)₃]³⁺ shows optical isomerism; [Ni(CO)₄] does not show isomerism.
• ⚠️ Mnemonic: "A B C" for ligand naming order – Anionic, then Neutral, then Alphabetical.