CUET UG Chemistry: Inorganic Chemistry - Coordination Chemistry, IUPAC Naming, Isomerism, Stability Constants

Must-Know (15–20 detailed bullets)

• The IUPAC name of [Co(NH?)?]Cl? is hexaamminecobalt(III) chloride; ligands are named before the metal in alphabetical order, with prefixes indicating number.
• In [CrCl?(en)?]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., NH?-ammine, H?O-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)?] is hexacyanidoferrate(II) ion.
• Linkage isomerism occurs due to ambidentate ligands like SCN? (thiocyanato-S or thiocyanato-N) and NO (nitro-N or nitrito-O). Example: [Co(NH?)?(NO?)]Cl? (yellow, nitro) vs [Co(NH?)?(ONO)]Cl? (red, nitrito).
• Coordination isomerism occurs in compounds with both cation and anion as complexes, e.g., [Co(NH?)?][Cr(CN)?] vs [Cr(NH?)?][Co(CN)?].
• Ionization isomerism arises when counter ions exchange with ligands, e.g., [Co(NH?)?Br]SO? (gives white ppt with BaCl?) vs [Co(NH?)?SO?]Br (no ppt with BaCl?).
• Hydrate (solvate) isomerism differs in whether water is inside or outside the coordination sphere, e.g., [Cr(H?O)?]Cl? (violet) vs [CrCl(H?O)?]Cl?·H?O (green).
• Geometrical isomerism is common in square planar (e.g., [Pt(NH?)?Cl?]) and octahedral complexes (e.g., [Co(NH?)?Cl?]?); cis and trans forms exist.
• Optical isomerism occurs in chiral complexes without plane of symmetry, e.g., [Co(en)?]³? has d and l forms; en = bidentate, leading to non-superimposable mirror images.
• The stability constant (K) for a complex [ML?] is defined as K = [ML?]/([M][L]?); higher K means greater stability.
• Stepwise stability constants (K?, K?, ..., K?) decrease progressively; overall stability constant = K? × K? × ... × K?.
• Chelate effect: complexes with chelating ligands (e.g., en, EDTA) are more stable than those with monodentate ligands due to increased entropy.
• EDTA 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 [CoF?]³?, 4 in [Ni(CN)?]²?.
• Homoleptic complexes have only one type of ligand, e.g., [Co(NH?)?]³?; heteroleptic have more than one, e.g., [Co(NH?)?Cl?]?.
• The IUPAC name of K?[Fe(CN)?] is potassium hexacyanidoferrate(III); Fe is in +3 oxidation state.
• The complex [NiCl?(PPh?)?] shows geometrical isomerism (square planar Ni²?), but [Ni(CO)?] 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 NH?. Avoid: Always use "ammine" (with double m) for NH? ligand as per current IUPAC and NCERT.
• Trap: Confusing linkage isomerism with ionization isomerism; both involve NO 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)?]) show optical activity; [Co(NH?)?Cl?]? shows geometrical but not optical isomerism.

Practice MCQs (5 questions)

Q1. What is the correct IUPAC name of [Co(NH?)?Cl]Cl
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(H?O)?]Cl? and [CrCl(H?O)?]Cl?·H?O?
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 H?O, not counter ion exchange.

Q3. Which complex shows optical isomerism?
A. [Co(NH?)?Cl?]?
B. [Pt(NH?)?Cl?]
C. [Co(en)?]³?
D. [Cu(NH?)?]²?

Answer: C
Explanation: [Co(en)?]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 for [Cu(NH?)?]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 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;-refers to formation.

Q5. Which pair represents linkage isomers?
A. [Co(NH?)?NO?]²? and [Co(NH?)?ONO]²?
B. [Co(NH?)?][Cr(CN)?] and [Cr(NH?)?][Co(CN)?]
C. [Co(NH?)?Br]SO? and [Co(NH?)?SO?]Br
D. [Pt(NH?)?Cl?] (cis) and (trans)

Answer: A
Explanation: NO 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 NH?, "aqua" for H?O, "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)?]-hexacyanidoferrate(II); metal name ends with "-ate" in anionic complexes.
• Oxidation state in Roman numerals: e.g., cobalt(III), not Co³? in name.
• K?[Fe(CN)?] = potassium hexacyanidoferrate(III); CN? is cyanido, Fe is +3.
• Linkage isomerism: SCN?-thiocyanato-S or thiocyanato-N; NO-nitro (N) or nitrito (O).
• Ionization isomers give different ions in solution, e.g., SO?²? inside vs outside sphere.
• Hydrate isomers differ in H?O position: coordinated vs lattice water.
• [Co(NH?)?]Cl? (violet) vs [CoCl(NH?)?]Cl?·H?O (pale red) – hydrate isomers.
• Geometrical isomerism: cis/trans in square planar [Pt(NH?)?Cl?] and octahedral [Co(NH?)?Cl?]?.
• Optical isomerism requires absence of plane of symmetry; e.g., [M(en)?] shows d and l forms.
• Chelate effect: ?G more negative due to increased entropy when chelating ligands bind.
• Stability constant = [ML?]/([M][L]?); larger-= more stable complex.
• Stepwise constants: K? > K? > K? > K? due to statistical and electrostatic factors.
• EDTA forms 1:1 complexes with Ca²?, Mg²?; used in water hardness estimation.
• Homoleptic: [Fe(CO)?]; heteroleptic: [Co(NH?)?Cl?]?.
• Coordination number = number of sigma bonds from ligands to metal.
• [Cr(en)?]³? shows optical isomerism; [Ni(CO)?] does not show isomerism.
• Mnemonic: "A B C" for ligand naming order – Anionic, then Neutral, then Alphabetical.