Coordination Compounds (NCERT Class 12 Chemistry) - Chemca

Coordination Compounds

NCERT Class 12 Chemistry • Complete Study Guide & Practice Quiz

Class 12 JEE / NEET

List of Important Ligands

Ligand Formula	Common Name	IUPAC Name	Coordination Type	Denticity
NH₃	Ammonia	Ammine	Neutral ligand	Monodentate
H₂O	Water	Aqua	Neutral ligand	Monodentate
Cl⁻	Chloride ion	Chloro	Anionic ligand	Monodentate
OH⁻	Hydroxide ion	Hydroxo	Anionic ligand	Monodentate
CN⁻	Cyanide ion	Cyano	Anionic ligand	Monodentate
NO₂⁻ (via N)	Nitrito (N-bound)	Nitrito-N	Anionic ligand	Monodentate
NO₂⁻ (via O)	Nitrito (O-bound)	Nitrito-O	Anionic ligand	Monodentate
SCN⁻ (via S)	Thiocyanato (S-bound)	Thiocyanato-S	Anionic ligand	Monodentate
SCN⁻ (via N)	Thiocyanato (N-bound)	Thiocyanato-N	Anionic ligand	Monodentate
CO	Carbonyl	Carbonyl	Neutral ligand	Monodentate
C₂O₄²⁻	Oxalate ion	Oxalato	Anionic ligand	Bidentate (chelating)
en	Ethylenediamine	Ethane-1,2-diamine	Neutral ligand	Bidentate (chelating)
EDTA⁴⁻	Ethylenediaminetetraacetate	Ethane-1,2-diylbis(iminodiacetato)	Anionic ligand	Hexadentate (chelating)
NH₂CH₂COO⁻	Glycinate ion	Aminoacetato	Anionic ligand	Bidentate
SO₄²⁻	Sulfate ion	Sulfato	Anionic ligand	Usually bidentate
NO₃⁻	Nitrate ion	Nitrato	Anionic ligand	Monodentate / Bidentate
PPh₃	Triphenylphosphine	Triphenylphosphane	Neutral ligand	Monodentate

Comprehensive Study Guide Q&A

Basic Concepts

Q: What is a coordination compound?

A: A compound containing a central metal atom/ion bonded to ligands via coordinate bonds.
Q: What is a ligand?

A: An ion or molecule that donates a lone pair of electrons to the central metal atom/ion.
Q: What is coordination number?

A: The number of ligand donor atoms directly bonded to the central metal ion.
Q: What are monodentate ligands?

A: Ligands that donate only one pair of electrons (e.g., NH₃, H₂O, Cl⁻).
Q: Give an example of a polydentate ligand.

A: Ethylenediamine (en), Oxalate (C₂O₄²⁻).
Q: What is a chelating ligand?

A: A ligand that forms two or more coordinate bonds with the same central metal ion.
Q: What is the oxidation number of Co in [Co(NH₃)₆]Cl₃?

A: +3.
Q: What is the difference between inner and outer orbital complexes?

A: Inner orbital uses (n-1)d orbitals, outer orbital uses nd orbitals for hybridization.
Q: What is EAN rule?

A: Effective Atomic Number = Atomic number of metal + electrons donated by ligands – oxidation state.
Q: Which law explains isomerism in coordination compounds?

A: Werner’s coordination theory.

Nomenclature

Q: What is the name of [Cr(H₂O)₆]Cl₃?

A: Hexaaquachromium(III) chloride.
Q: Write the formula of potassium ferrocyanide.

A: K₄[Fe(CN)₆].
Q: Name [Ni(CO)₄].

A: Tetracarbonylnickel(0).
Q: Give the name of [Pt(NH₃)₂Cl₂].

A: Diamminedichloroplatinum(II).
Q: Name [Cu(NH₃)₄]SO₄.

A: Tetraamminecopper(II) sulfate.

Isomerism

Q: What type of isomerism is shown by [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅SO₄]Cl?

A: Ionization isomerism.
Q: Name the isomerism in [Co(en)₃]Cl₃.

A: Optical isomerism.
Q: [Pt(NH₃)₂Cl₂] shows which type of isomerism?

A: Geometrical isomerism (cis/trans).
Q: What type of isomerism is shown when one ligand changes its point of attachment?

A: Linkage isomerism.
Q: Give an example of coordination isomerism.

A: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

Hybridization and Structures

Q: Hybridization of Ni in [Ni(CN)₄]²⁻?

A: dsp² (square planar).
Q: Hybridization in [NiCl₄]²⁻?

A: sp³ (tetrahedral).
Q: What is the geometry of [Fe(CN)₆]³⁻?

A: Octahedral.
Q: Magnetic moment of [Ti(H₂O)₆]³⁺? (d¹ system)

A: √1(1+2) = 1.73 BM.
Q: Which has more unpaired electrons: [Fe(CN)₆]³⁻ or [FeF₆]³⁻?

A: [FeF₆]³⁻ (weak ligand field).

Crystal Field Theory

Q: What is crystal field splitting?

A: Splitting of d-orbitals of a metal ion in presence of ligands.
Q: Why is [Fe(CN)₆]⁴⁻ diamagnetic?

A: CN⁻ is a strong field ligand causing pairing of electrons.
Q: Why is [FeF₆]³⁻ paramagnetic?

A: F⁻ is a weak field ligand, no pairing of electrons.
Q: In octahedral field, d-orbitals split into which sets?

A: t₂g (lower energy) and eg (higher energy).
Q: Write the electronic configuration of Co³⁺ in [CoF₆]³⁻.

A: t₂g⁴ eg² (high spin).

Color, Spectra, and Properties

Q: Why are transition metal complexes often colored?

A: Due to d-d transitions absorbing visible light.
Q: Why is [Ti(H₂O)₆]³⁺ purple?

A: d-d transition in d¹ system absorbs green light.
Q: Why is [Cu(H₂O)₆]²⁺ blue?

A: Absorption of red light causes complementary blue color.
Q: What is the magnetic nature of [Ni(CO)₄]?

A: Diamagnetic (all electrons paired).
Q: State the reason why [Mn(H₂O)₆]²⁺ is pale pink.

A: Weak d-d transitions (d⁵ high spin).

Important Compounds and Uses

Q: Formula of EDTA?

A: Ethylenediaminetetraacetic acid.
Q: Name one use of EDTA.

A: Used in complexometric titrations for hardness of water.
Q: What is cisplatin used for?

A: Anti-cancer drug.
Q: Name the active species in Vitamin B₁₂.

A: Co(III) coordination complex.
Q: Which coordination compound is known as Prussian Blue?

A: Fe₄[Fe(CN)₆]₃.

Advanced and Conceptual

Q: Define stability constant.

A: Equilibrium constant for the formation of a complex.
Q: Between [Fe(CN)₆]³⁻ and [FeCl₆]³⁻, which is more stable?

A: [Fe(CN)₆]³⁻ (due to strong field ligand CN⁻).
Q: Which follows chelate effect: EDTA or NH₃?

A: EDTA.
Q: Write IUPAC name of [Fe(H₂O)₅NO]SO₄.

A: Pentaaquanitrosyliron(II) sulfate.
Q: What is the charge on [MnO₄]⁻?

A: –1.
Q: Why are low spin complexes more stable than high spin sometimes?

A: Higher crystal field stabilization energy (CFSE).
Q: Which complex is low spin: [Fe(CN)₆]³⁻ or [Fe(H₂O)₆]³⁺?

A: [Fe(CN)₆]³⁻.
Q: What is the coordination number in [Cr(C₂O₄)₃]³⁻?

A: 6 (oxalate is bidentate).
Q: Write geometry of [Cu(NH₃)₄]²⁺.

A: Square planar or distorted tetrahedral.
Q: Which rule helps predict absorbance in coordination compounds?

A: Selection rules (Laporte and spin selection rules).

Structural and Conceptual

Q: Why does [Ni(CN)₄]²⁻ have square planar geometry while [NiCl₄]²⁻ has tetrahedral geometry?

A: CN⁻ is a strong field ligand (dsp² hybridization), while Cl⁻ is weak field (sp³ hybridization).
Q: Which complex is diamagnetic: [CoF₆]³⁻ or [Co(NH₃)₆]³⁺?

A: [Co(NH₃)₆]³⁺, because NH₃ induces pairing (low spin).
Q: Why does [Ti(H₂O)₆]³⁺ show color, but [Sc(H₂O)₆]³⁺ is colorless?

A: Ti³⁺ has d¹ electron (d-d transition possible), Sc³⁺ is d⁰ (no d-d transition).
Q: Between [Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻, which one is more stable and why?

A: [Fe(CN)₆]³⁻ is more stable due to stronger CFSE of d⁵ low spin.
Q: Why is tetraamminecopper(II) sulfate solution blue?

A: It absorbs orange-red light due to d-d transitions, reflecting blue.

Further Isomerism

Q: Can a tetrahedral complex show geometrical isomerism?

A: No, only square planar and octahedral complexes show it.
Q: Which complex shows optical isomerism: [Co(en)₃]³⁺ or [Co(NH₃)₆]³⁺?

A: [Co(en)₃]³⁺ (chiral due to bidentate ligands).
Q: Why does [PtCl₂(NH₃)₂] have two isomers but [NiCl₄]²⁻ does not?

A: Square planar geometry of Pt complex allows cis/trans; Ni complex is tetrahedral.
Q: Which type of isomerism is shown by [Co(NO₂)(NH₃)₅]Cl₂ and [Co(ONO)(NH₃)₅]Cl₂?

A: Linkage isomerism (NO₂⁻ vs ONO⁻).
Q: Differentiate coordination and ionization isomers with one example.

A: [Cu(NH₃)₄][PtCl₄] vs [Pt(NH₃)₄][CuCl₄]; [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br.

Further Crystal Field Theory

Q: Why are d⁰ and d¹⁰ complexes usually colorless?

A: No d-d transitions possible.
Q: Arrange [Ti(H₂O)₆]³⁺, [V(H₂O)₆]³⁺, [Cr(H₂O)₆]³⁺ in increasing CFSE.

A: Ti³⁺ < V³⁺ < Cr³⁺ (increasing d-electrons).
Q: Why is [Fe(H₂O)₆]²⁺ strongly paramagnetic, but [Fe(CN)₆]⁴⁻ is diamagnetic?

A: H₂O is weak ligand (high spin d⁶), CN⁻ is strong ligand (low spin d⁶).
Q: Explain Jahn–Teller distortion in [Cu(H₂O)₆]²⁺.

A: Unequal occupancy of eg orbitals (d⁹ system) leads to elongation along z-axis.
Q: Which is the stronger ligand, en or NH₃?

A: en, due to chelate effect and stronger field strength.

Stability and Reactions

Q: Between [Ag(NH₃)₂]⁺ and [Ag(CN)₂]⁻, which is more stable?

A: [Ag(CN)₂]⁻ (cyanide forms very stable complexes).
Q: Why is EDTA a better ligand than NH₃?

A: It is hexadentate (chelating effect gives higher stability).
Q: Why does [Fe(CO)₅] have strong metal-ligand bonds?

A: CO shows synergic bonding (σ donation + π backbonding).
Q: Why is [Cr(NH₃)₆]³⁺ violet, but [Cr(H₂O)₆]³⁺ green?

A: Different ligand field strengths cause different absorption frequencies.
Q: Which is thermodynamically more stable: [Co(en)₃]³⁺ or [Co(NH₃)₆]³⁺?

A: [Co(en)₃]³⁺ due to chelation.

Conceptual Traps

Q: Why does [Zn(NH₃)₄]²⁺ not show color?

A: Zn²⁺ is d¹⁰, no d-d transitions.
Q: In [Mn(H₂O)₆]²⁺, why is CFSE = 0?

A: High spin d⁵, equal filling of t₂g and eg
Q: Why is [Fe(CN)₆]³⁻ less stable than [Fe(CN)₆]⁴⁻?

A: Fe³⁺ has higher charge density than Fe²⁺, so faces more repulsion with 6 CN⁻ ligands.
Q: Why do d-d transitions violate Laporte’s rule in [Ti(H₂O)₆]³⁺?

A: Vibronic coupling relaxes selection rule.
Q: Why is [Ni(CO)₄] diamagnetic despite Ni(0) being d⁸?

A: Strong CO field causes pairing → dsp² hybridization, no unpaired e⁻.

Further Applications

Q: Why is K₂[HgI₄] used in Nessler’s reagent?

A: It detects NH₃ by forming a brown complex.
Q: How does cisplatin act as an anticancer drug?

A: It binds to DNA and prevents replication.
Q: Why is EDTA used in water softening?

A: It binds strongly to Ca²⁺ and Mg²⁺ ions.
Q: Which coordination compound is used in photography fixing?

A: Sodium thiosulfate (Na₂S₂O₃).
Q: What gives the deep blue color in the Cu²⁺–NH₃ complex test?

A: Formation of [Cu(NH₃)₄]²⁺.

Advanced/Competitive Edge

Q: Which complex shows both linkage and geometrical isomerism?

A: [Co(NO₂)₂(NH₃)₄]⁺.
Q: Out of [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺, which is more easily oxidized?

A: Fe²⁺ → Fe³⁺, because Fe³⁺ is more stable in water.
Q: Write CFSE expression for high spin octahedral complexes.

A: CFSE = –0.4xΔ₀ + 0.6yΔ₀, where x = no. of e⁻ in t₂g, y = e⁻ in eg.
Q: Which shows greater magnetic moment: [CoF₆]³⁻ or [Co(NH₃)₆]³⁺?

A: [CoF₆]³⁻ (weak field, high spin, more unpaired e⁻).
Q: Why is [Cr(en)₃]³⁺ optically active?

A: It exists as two enantiomers (d and l forms).

Quantitative and CFSE Tricks

Q: Calculate CFSE of [Fe(CN)₆]⁴⁻. (Fe²⁺, d⁶, low spin)

A: CFSE = (–0.4×6Δ₀) = –2.4Δ₀ + pairing energy.
Q: Spin only magnetic moment of [MnF₆]³⁻ (d⁴ high spin)?

A: √4(4+2) = √24 = 4.9 BM.
Q: Why does NH₃ act as a ligand but not CH₄?

A: NH₃ has a lone pair on N, CH₄ has no lone pairs.
Q: In [Cr(H₂O)₆]³⁺, what is the color?

A: Violet-green (depends on light) due to d³ transitions.
Q: Which is stronger ligand according to spectrochemical series: H₂O or NH₃?

A: NH₃.

Edge Case Problems

Q: Why does Co²⁺ prefer tetrahedral complexes while Co³⁺ prefers octahedral?

A: Higher charge stabilizes more coordination.
Q: What is the CFSE of d³ octahedral high spin complex?

A: –1.2Δ₀.
Q: In [Ni(CN)₄]²⁻, why is Ni²⁺ using inner d-orbitals?

A: Strong field CN⁻ causes pairing into 3d.
Q: Is [Cu(NH₃)₄]²⁺ square planar or tetrahedral?

A: Distorted square planar (Jahn–Teller distortion).
Q: Why is NO considered an ambidentate ligand?

A: It can bind via N or O atom.

Miscellaneous Challenge

Q: Can [Zn(NH₃)₄]²⁺ be isostructural with [Cu(NH₃)₄]²⁺?

A: No, Zn²⁺ complex is perfect tetrahedral; Cu²⁺ shows distortion.
Q: Which has higher CFSE: strong field low spin or weak field high spin complexes?

A: Strong field low spin.
Q: Why do tetrahedral complexes rarely show low spin?

A: Splitting energy (Δt) is too small to cause pairing.
Q: Why does [Fe(CO)₅] show synergic bonding but [Fe(H₂O)₆]²⁺ does not?

A: H₂O cannot accept back-donation, CO can.
Q: Which complex has greater stability: [Cu(NH₃)₄]²⁺ or [Cu(H₂O)₄]²⁺?

A: [Cu(NH₃)₄]²⁺ (ligand field stabilization+stronger donor ability).