What is Crystal Field Theory? Explain d-orbital splitting in an octahedral field.

CFT explains bonding in coordination compounds through electrostatic interactions between the metal ion and ligands. In an octahedral field, ligands approach along the x, y, z axes, raising the energy of dx²−y² and dz² orbitals (eg set) more than dxy, dxz, dyz (t₂g set). The energy gap between them is Δo (crystal field splitting energy).

Tamil Nadu Class 12 Chemistry — Coordination Compounds Notes, IUPAC Naming & Board Q&A

Q: What is the difference between primary and secondary valency in Werner's theory?

Primary valency (ionisable valency) corresponds to the oxidation state of the metal; it is satisfied by negative ions outside the coordination sphere. Secondary valency (non-ionisable, coordination number) is satisfied by ligands directly bonded to the metal inside the coordination sphere.

Exercise Solutions Available

Step-by-step answers for all textbook questions in this chapter

View Solutions →

Quick Summary

Coordination chemistry explains the bonding, structure, and properties of complex compounds. This chapter carries 8–10 marks in TN HSE. Werner's theory, IUPAC naming, isomerism types, and Crystal Field Theory (CFT) are the most important topics for board exams. Systematic practice of naming 5–6 complexes and identifying their isomers is essential.

1. Werner's Theory of Coordination Compounds

Alfred Werner proposed two types of valency for metal atoms in complexes:

Primary valency (oxidation state):

Ionisable — satisfied by negative ions that are outside the coordination sphere
Corresponds to the formal oxidation number of the metal

Secondary valency (coordination number):

Non-ionisable — satisfied by ligands directly bonded inside the coordination sphere (square brackets)
Determines the geometry (tetrahedral, square planar, octahedral)

Postulates: 1. Metal exhibits both primary and secondary valency in complexes. 2. Secondary valency is always satisfied and has a definite spatial arrangement. 3. Ligands within the coordination sphere do not ionise in solution.

Classic example: CoCl₃·6NH₃ rewritten as [Co(NH₃)₆]Cl₃

Primary valency = 3 (three Cl⁻ ions ionise, satisfying +3 oxidation state)
Secondary valency = 6 (six NH₃ molecules bonded directly to Co inside the sphere)
Three conductors of current → conductance confirms 3 ionic Cl⁻

Ionisation patterns of cobalt–ammonia–chloride series:

Formula	Modern notation	Ions in solution	CN
CoCl₃·6NH₃	[Co(NH₃)₆]Cl₃	3 Cl⁻	6
CoCl₃·5NH₃	[Co(NH₃)₅Cl]Cl₂	2 Cl⁻	6
CoCl₃·4NH₃	[Co(NH₃)₄Cl₂]Cl	1 Cl⁻	6
CoCl₃·3NH₃	[Co(NH₃)₃Cl₃]	0 Cl⁻	6

2. Key Terminology

Term	Definition
Coordination entity	The central metal and its ligands inside the square brackets
Central metal	Metal atom or ion that accepts electron pairs from ligands
Ligand	Ion or molecule that donates a lone pair to the metal (Lewis base); e.g., NH₃, Cl⁻, CN⁻
Coordination number (CN)	Number of bonds (donor atoms) directly bonded to the central metal
Coordination sphere	Central metal + all ligands; written inside [ ]
Counter ions	Ions outside the coordination sphere; balance the overall charge
Chelate	Complex with polydentate ligand forming a ring; extra stable (chelate effect)

3. Types of Ligands

Type	Donor atoms	Examples
Monodentate	1	Cl⁻ (chlorido), NH₃ (ammine), H₂O (aqua), CN⁻ (cyanido), OH⁻ (hydroxido), CO (carbonyl)
Bidentate	2	en (ethylenediamine, H₂N–CH₂–CH₂–NH₂), oxalate (ox, C₂O₄²⁻), 2,2'-bipyridyl
Polydentate	4–6	EDTA⁴⁻ (hexadentate, bonds through 2N and 4O atoms), DMGH
Ambidentate	2 possible sites	NO₂⁻ (bonds through N or O), SCN⁻ (bonds through S or N), CN⁻ (bonds through C or N)

Chelate effect: Polydentate ligands form multiple bonds to the same metal, creating a chelate ring. This is thermodynamically favoured due to the increase in entropy when a polydentate ligand replaces several monodentate ligands (more particles → higher entropy). EDTA forms extremely stable complexes with metal ions, used in titrations.

4. IUPAC Nomenclature Rules

1. Name the cation first (may be a simple ion or a complex ion), then the anion. 2. Inside the complex: a. Name ligands alphabetically (ignoring numerical prefixes like di-, tri-). b. Name the central metal last. c. Give the oxidation state of the metal in Roman numerals in parentheses immediately after the metal name. 3. Anionic ligands end in -o (Cl⁻ = chlorido, Br⁻ = bromido, CN⁻ = cyanido, OH⁻ = hydroxido, O²⁻ = oxido, SO₄²⁻ = sulphato, NO₂⁻ = nitrito) 4. Neutral ligands keep their molecular name with these exceptions: NH₃ = ammine, H₂O = aqua, CO = carbonyl, NO = nitrosyl 5. Numerical prefixes: di, tri, tetra, penta, hexa for simple ligands; bis, tris, tetrakis for complex ligands (those containing commas or multiplying prefixes themselves) 6. If the complex is an anion, the metal name gets the suffix -ate. (ferrate, cobaltate, platinate, cuprate, argentate, aurate)

Worked naming examples:

Formula	IUPAC Name
[Cu(NH₃)₄]SO₄	tetraamminecopper(II) sulphate
K₃[Fe(CN)₆]	potassium hexacyanidoferrate(III)
[Co(NH₃)₄Cl₂]Cl	tetraaminedichloridocobalt(III) chloride
[Pt(NH₃)₂Cl₂]	diamminedichloridoplatinum(II)
[Ni(CO)₄]	tetracarbonylnickel(0)
K₂[PtCl₄]	potassium tetrachloridoplatinate(II)
[Co(en)₃]³⁺	tris(ethylenediamine)cobalt(III) ion
[Fe(H₂O)₅(NCS)]²⁺	pentaaquathiocyanateiron(III) ion

5. Isomerism in Coordination Compounds

Structural Isomers

1. Ionisation isomers: Same molecular formula, but different ions in and outside the coordination sphere.

Example:

[Co(NH₃)₅Cl]SO₄ — chlorido in sphere, sulphate outside → gives SO₄²⁻ in solution
[Co(NH₃)₅SO₄]Cl — sulphate in sphere, chloride outside → gives Cl⁻ in solution

Test: React with AgNO₃ → first compound gives no precipitate; second gives white AgCl precipitate.

2. Linkage isomers: Ambidentate ligand bonds through different donor atoms.

Example:

[Co(NH₃)₅-NO₂]²⁺ (nitrito-N, nitrogen bonded) vs [Co(NH₃)₅-ONO]²⁺ (nitrito-O, oxygen bonded)
[Co(NH₃)₅-SCN]²⁺ (thiocyanato-S) vs [Co(NH₃)₅-NCS]²⁺ (isothiocyanato-N)

3. Hydrate (solvate) isomers: Water molecules are either inside (as ligands) or outside (as lattice water).

Example:

[Cr(H₂O)₆]Cl₃ — all six waters are ligands; green colour
[Cr(H₂O)₅Cl]Cl₂·H₂O — five waters as ligands, one free water, one Cl inside
[Cr(H₂O)₄Cl₂]Cl·2H₂O — four waters as ligands, two Cl inside, two free waters

4. Coordination isomers: Cationic and anionic complexes exchange ligands.

Example:

[Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]

Stereoisomers

1. Geometric (cis-trans) isomerism

Answer

Arises when identical ligands can be arranged differently relative to each other in space.

Square planar [MA₂B₂] complexes:

cis: identical ligands on the same side (90° apart)
trans: identical ligands on opposite sides (180° apart)

Example: [Pt(NH₃)₂Cl₂] (cisplatin = cis; transplatin = trans — cisplatin is an anti-cancer drug; transplatin is inactive)

Octahedral [MA₄B₂] complexes:

cis: two B ligands at 90° to each other
trans: two B ligands at 180° to each other

2. Optical isomerism

Answer

Non-superimposable mirror images of each other; termed d (dextrorotatory) and l (laevorotatory) forms.

Observed in octahedral complexes with polydentate ligands. Classic example: [Co(en)₃]³⁺ — has three ethylenediamine bidentate ligands creating a chiral structure with two non-superimposable forms.

Also observed in: [Cr(en)₃]³⁺, cis-[Co(en)₂Cl₂]⁺

6. Bonding Theories

Valence Bond Theory (VBT)

Metal atom uses hybrid orbitals to form coordinate bonds with ligands.

Strong field ligands (e.g., CN⁻, CO): force electron pairing in inner d-orbitals → inner orbital (low spin) complex → d²sp³ hybridisation for octahedral
Weak field ligands (e.g., F⁻, Cl⁻, H₂O): use outer d-orbitals → outer orbital (high spin) complex → sp³d² hybridisation for octahedral

Key VBT examples:

Complex	Hybridisation	Type	Geometry
[Fe(CN)₆]⁴⁻	d²sp³	Inner orbital, low spin	Octahedral
[FeF₆]³⁻	sp³d²	Outer orbital, high spin	Octahedral
[Ni(CN)₄]²⁻	dsp²	Inner orbital	Square planar
[NiCl₄]²⁻	sp³	Outer orbital	Tetrahedral
[Cu(NH₃)₄]²⁺	dsp²	Inner orbital	Square planar

Crystal Field Theory (CFT)

CFT treats the metal-ligand bond as purely electrostatic (ionic). Ligands are treated as point negative charges or dipoles.

d-orbital splitting in an octahedral field:

In a free metal ion, all five d-orbitals are degenerate (same energy). When six ligands approach along the x, y, z axes (octahedral arrangement):

dx²−y² and dz² orbitals (eg set) point directly toward ligands → repelled more → energy raised
dxy, dxz, dyz orbitals (t₂g set) point between ligands → repelled less → energy lowered

Energy level diagram:
             _____ _____
              eg (dx²-y², dz²)        +0.6Δo above average
 ---------- (average energy)
          ___ ___ ___
          t₂g (dxy, dxz, dyz)          -0.4Δo below average

Δo = crystal field splitting energy in octahedral field
CFSE = −0.4n(t₂g) + 0.6n(eg) Δo  (negative = stabilised)

High spin vs low spin (octahedral d⁶ example like Fe²⁺):

Weak field ligand (small Δo < pairing energy): electrons spread out (high spin, 4 unpaired, paramagnetic)
Strong field ligand (large Δo > pairing energy): electrons pair up in t₂g (low spin, 0 unpaired, diamagnetic)

Spectrochemical series (increasing field strength):

I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < en < CN⁻ < CO
   (weak field)                              (strong field)

7. Stability Constants and Applications

Stability constant (formation constant) K_f: Equilibrium constant for the formation of a complex from its components.

M + nL ⇌ [MLn],   Kf = [[MLn]] / [M][L]^n

Higher Kf = more stable complex. EDTA forms very stable complexes (log Kf ≈ 14–25) — used in EDTA titrations for metal ion estimation.

Applications of coordination compounds:

Application	Complex / Compound
Oxygen transport (biology)	Haemoglobin — Fe²⁺ with porphyrin ligands and O₂
Photosynthesis	Chlorophyll — Mg²⁺ complex
Analytical chemistry	[Fe(SCN)₆]³⁻ for detecting Fe³⁺ (blood-red colour)
Electroplating	Au is plated from [Au(CN)₂]⁻ solution
Cancer treatment	cis-[Pt(NH₃)₂Cl₂] (cisplatin) — anti-tumour drug
EDTA titrations	CaY²⁻ complex — hardness of water estimation
Extraction of metals	Silver and gold extracted using NaCN → [Ag(CN)₂]⁻

8. Worked Examples

Example 1 — IUPAC Naming

Question: Name the complex [Co(NH₃)₄Cl₂]Cl.

Solution

Step 1: Identify the complex ion: [Co(NH₃)₄Cl₂]⁺ — cation
Step 2: Identify counter ion: Cl⁻ — chloride

Step 3: Name ligands alphabetically inside complex:
  - 4 NH₃ = tetraammine (NH₃ neutral → ammine; 4 = tetra)
  - 2 Cl⁻ = dichloro (anionic → chlorido; 2 = di)
  Alphabetical order: ammine comes before chlorido → tetraammine then dichloro

Step 4: Name metal with oxidation state:
  Charge on complex = +1, each NH₃ = 0, each Cl⁻ = −1
  Co + 0(4) + (−1)(2) = +1 → Co = +3 → cobalt(III)

Step 5: Combine:
  IUPAC name: tetraaminedichloridocobalt(III) chloride

Example 2 — Finding Oxidation State

Question: Find the oxidation state of platinum in K₂[PtCl₄].

Solution

K₂[PtCl₄] → 2K⁺ + [PtCl₄]²⁻

Charge on complex ion = −2
Each Cl⁻ contributes −1; four Cl⁻ = −4
Let Pt = x:  x + (−4) = −2
x = +2

Platinum is in +2 oxidation state (Pt(II)).

IUPAC name of the compound: potassium tetrachloridoplatinate(II)

Example 3 — Cis-Trans Isomers of [Pt(NH₃)₂Cl₂]

Question: Describe and distinguish the two geometric isomers of [Pt(NH₃)₂Cl₂].

Solution

Platinum(II) forms square planar complexes. For [Pt(NH₃)₂Cl₂]:

cis-isomer:
    Cl    NH₃
      \  /
  Pt
      /  \
    Cl    NH₃

The two Cl ligands are adjacent (90° apart).
Dipole moments of Pt-Cl bonds do not cancel → net dipole moment ≠ 0.
Melting point: 270°C
This is CISPLATIN — an anti-cancer drug.

trans-isomer:
    NH₃   Cl
       \  /
   Pt
       /  \
    Cl   NH₃

The two Cl ligands are opposite (180° apart).
Dipole moments of Pt-Cl bonds cancel → net dipole moment = 0.
Melting point: 270°C (different from cis)
This is TRANSPLATIN — biologically inactive.

Cisplatin binds to DNA strands at adjacent guanine bases (cis arrangement required for correct spacing), inhibiting DNA replication in cancer cells.

9. Important Board Questions with Model Answers

One-Mark Questions

Q1. Define coordination number. The number of ligand donor atoms directly bonded to the central metal atom or ion in a coordination compound.

Q2. What is a chelate compound? Give an example. A chelate is a coordination compound in which a polydentate ligand forms a ring (chelate ring) with the central metal. Example: [Cu(en)₂]²⁺ — each ethylenediamine forms a five-membered ring with Cu²⁺.

Q3. What is the hybridisation in [Ni(CN)₄]²⁻? dsp² hybridisation → square planar geometry. CN⁻ is a strong field ligand that forces electron pairing in the 3d orbital.

Q4. What are ambidentate ligands? Give an example. Ligands with two or more possible donor atoms, which can bond through different atoms in different complexes. Example: NO₂⁻ can bond through N (nitrito-N) or through O (nitrito-O).

Q5. Define crystal field splitting energy. The energy difference between the higher-energy eg set (dx²−y², dz²) and the lower-energy t₂g set (dxy, dxz, dyz) of d-orbitals when a metal ion is placed in an octahedral crystal field. Denoted Δo.

Short-Answer Questions (3 marks)

Q1. Explain Werner's postulates with an example.

Answer

Werner proposed: 1. Metals exhibit primary valency (ionisable = oxidation state) and secondary valency (non-ionisable = coordination number). 2. Secondary valency is always satisfied and is responsible for the spatial arrangement of ligands. 3. Primary valency is satisfied by anions outside the coordination sphere; secondary valency by ligands inside.

Example: [Co(NH₃)₄Cl₂]Cl

Primary valency = 3 (one Cl⁻ ionises in solution)
Secondary valency = 6 (4 NH₃ + 2 Cl⁻ inside sphere)
Octahedral geometry due to CN = 6

Q2. Describe geometric isomerism in square planar complexes with examples.

Answer

In square planar MA₂B₂ complexes, identical ligands (A and A) can be placed:

Adjacent (90°) → cis isomer
Opposite (180°) → trans isomer

Example: [Pt(NH₃)₂Cl₂]

cis: both Cl adjacent; has dipole moment; anti-cancer drug (cisplatin)
trans: Cl on opposite sides; net dipole = 0; biologically inactive

Note: Square planar complexes do not show optical isomerism with simple monodentate ligands.

Q3. Explain d-orbital splitting in an octahedral crystal field.

Answer

When six ligands (modelled as point charges) approach the metal along the x, y, z axes:

The d-orbitals that point along axes (dx²−y², dz² → eg set) experience greater repulsion and rise in energy by 0.6Δo.
The d-orbitals that point between axes (dxy, dxz, dyz → t₂g set) experience less repulsion and drop in energy by 0.4Δo.
The splitting energy Δo depends on the nature of the ligand (spectrochemical series): strong field ligands give large Δo; weak field ligands give small Δo.

Long-Answer Questions (5 marks)

Q1. Discuss IUPAC nomenclature rules for coordination compounds with five examples.

Answer

Rules: 1. Name cation before anion (complex may be either). 2. Inside complex: name ligands alphabetically, then metal with oxidation state in Roman numerals. 3. Anionic ligands end in -o. Neutral NH₃ = ammine, H₂O = aqua, CO = carbonyl, NO = nitrosyl. 4. Complex is anionic → metal name gets suffix -ate. 5. Use di/tri/tetra for simple ligands; bis/tris for complex ligands.

Examples: 1. [Cu(NH₃)₄]SO₄ → tetraamminecopper(II) sulphate 2. K₃[Fe(CN)₆] → potassium hexacyanidoferrate(III) 3. [Co(en)₃]Cl₃ → tris(ethylenediamine)cobalt(III) chloride 4. [Ni(CO)₄] → tetracarbonylnickel(0) 5. Na₂[Fe(CN)₅NO] → sodium pentacyanidoazanidoferrate(II) [sodium nitroprusside]

Q2. Explain the types of isomerism in coordination compounds with examples.

Answer

Structural isomers: 1. Ionisation: [Co(NH₃)₅Cl]SO₄ vs [Co(NH₃)₅SO₄]Cl — different ions inside and outside sphere. 2. Linkage: [Co(NH₃)₅NO₂]²⁺ (N-bonded) vs [Co(NH₃)₅ONO]²⁺ (O-bonded) — ambidentate ligand bonds through different atom. 3. Hydrate: [Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂·H₂O — water inside vs outside sphere. 4. Coordination: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆] — ligand exchange between cationic and anionic complex.

Stereoisomers: 5. Geometric: cis vs trans [Pt(NH₃)₂Cl₂] — identical ligands adjacent vs opposite. Cisplatin (cis) is active; transplatin (trans) is inactive. 6. Optical: [Co(en)₃]³⁺ — three bidentate ligands create a non-superimposable mirror image (d and l forms). Shown by rotation of plane-polarised light.

Common Mistakes

Writing metal before ligands in IUPAC name (ligands must come first, alphabetically)
Forgetting that complex anions get the suffix -ate on the metal name
Confusing ionisation isomers (different ions in/out sphere) with linkage isomers (different donor atom)
Thinking all square planar complexes show optical isomerism (they only show geometric isomerism with simple ligands)
Writing sp³d² and d²sp³ without specifying which is low spin and which is high spin

Revision Checklist

[ ] Werner's theory — primary vs secondary valency with numerical example
[ ] IUPAC naming rules practiced with at least 6 complexes including anion complexes
[ ] All four types of structural isomers — example for each
[ ] Geometric isomerism in square planar — cisplatin vs transplatin
[ ] Optical isomerism — [Co(en)₃]³⁺ example
[ ] VBT — d²sp³ vs sp³d² with specific examples [Fe(CN)₆]⁴⁻ and [FeF₆]³⁻
[ ] CFT — d-orbital splitting diagram, eg and t₂g labels, Δo
[ ] Spectrochemical series memorised
[ ] CFSE calculation concept
[ ] Applications — cisplatin, haemoglobin, EDTA

Get Samacheer Kalvi 12th Chemistry Guide on Amazon

Frequently Asked Questions

A coordination compound contains a central metal atom or ion bonded to a fixed number of ligands (ions or molecules that donate lone pairs). Example: [Cu(NH₃)₄]²⁺ — copper(II) ion bonded to four ammonia ligands. The entity inside square brackets is the coordination sphere.