Chapter 5: Coordination Compounds

Syllabus: NCERT Rationalised 2025–26  •  Branch: Inorganic Chemistry  •  Board: CBSE / RBSE

1. Introduction

Coordination compounds constitute an important class of inorganic compounds in which a central metal atom or ion is surrounded by a number of ions or neutral molecules called ligands.

This chapter discusses the fundamental concepts of coordination chemistry including Werner’s theory, nomenclature, isomerism and modern theories of bonding like Valence Bond Theory (VBT) and Crystal Field Theory (CFT).

2. Werner’s Theory of Coordination Compounds

Alfred Werner (1898), the father of coordination chemistry, proposed the first successful theory to explain the nature of bonding in coordination compounds. He postulated that metals exhibit two types of valencies or linkages.

2.1 Postulates of Werner's Theory

• 1. Two types of Valencies: The central metal atom possesses:
  • Primary Valency: Corresponds to the Oxidation State.
  • Secondary Valency: Corresponds to the Coordination Number.
• 2. Nature of Linkages:
  • Primary valencies are ionisable and are satisfied by negative ions.
  • Secondary valencies are non-ionisable and are satisfied by neutral molecules or negative ions.
• 3. Spatial Arrangement: The secondary valencies are directed towards fixed positions in space, giving a definite geometry to the complex (e.g., Octahedral, Tetrahedral, Square planar).

2.2 Difference Between Primary and Secondary Valency

Basis	Primary Valency	Secondary Valency
Nature	Ionisable	Non-ionisable
Corresponds to	Oxidation State	Coordination Number
Satisfied by	Anions only	Neutral molecules or Anions
Directional Character	Non-directional	Directional (Decides Geometry)

2.3 Experimental Verification (Precipitation Data)

Werner verified his theory by reacting cobalt-ammonia complexes with excess Silver Nitrate ($AgNO_3$). The moles of AgCl precipitate confirm the number of ionisable chloride ions (outside the coordination sphere).

Old Formula (Colour)	Moles of AgCl	Modern Formula	Electrolyte Type
$CoCl_3 \cdot 6NH_3$ (Yellow)	3	$[Co(NH_3)_6]Cl_3$	1:3
$CoCl_3 \cdot 5NH_3$ (Purple)	2	$[Co(NH_3)_5Cl]Cl_2$	1:2
$CoCl_3 \cdot 4NH_3$ (Green)	1	$[Co(NH_3)_4Cl_2]Cl$	1:1

3. Definitions of Important Terms

The following standard definitions are strictly based on NCERT terminology. These are frequently asked as 1–2 mark questions in Board examinations and are essential for understanding nomenclature and isomerism.

3.1 Coordination Entity & Coordination Sphere

• Coordination Entity: A central metal atom or ion bonded to a fixed number of ions or molecules called ligands.
  Example: $[CoCl_3(NH_3)_3]$.
• Coordination Sphere: The central metal ion together with the ligands attached to it and enclosed within square brackets $[ \ ]$ is called the coordination sphere.
  It represents the non-ionisable part.
• Counter Ions: The ions present outside the coordination sphere which ionise in solution.

3.2 Ligands

Ligands are ions or molecules bound to the central metal atom/ion. They act as Lewis bases (electron pair donors).

(A) Classification Based on Charge

• Neutral: $NH_3$ (ammine), $H_2O$ (aqua), $CO$ (carbonyl), $en$ (ethylenediamine).
• Anionic: $Cl^-$ (chlorido), $CN^-$ (cyanido), $OH^-$ (hydroxo), $C_2O_4^{2-}$ (oxalato).
• Cationic: $NO^+$ (nitrosonium).

(B) Classification Based on Denticity

Denticity: Number of donor atoms in a single ligand molecule directly attached to the metal.

Type	Denticity	Examples
Monodentate	1	$H_2O$, $NH_3$, $Cl^-$
Bidentate	2	Ethane-1,2-diamine ($en$), Oxalate ($C_2O_4^{2-}$)
Hexadentate	6	EDTA$^{4-}$

(C) Ambidentate Ligands

Ligands which can ligate through two different donor atoms are called ambidentate ligands.

3.3 Coordination Number (C.N.)

Total number of ligand donor atoms directly bonded to the central metal atom.

3.4 Coordination Polyhedron (Geometry)

The spatial arrangement of the ligand donor atoms around the central atom/ion defines the Coordination Polyhedron.
Common Shapes: Octahedral, Tetrahedral, Square Planar.

3.5 Oxidation Number Calculation

Charge central metal would carry if all ligands are removed.

3.6 Chelation

When a di- or polydentate ligand binds using two or more donor atoms to form a ring structure, it is called chelation.

4. IUPAC Nomenclature of Coordination Compounds

Systematic naming ensures uniformity. Following are the rules strictly based on IUPAC recommendations adopted by NCERT.

4.1 Rules for Naming

• 1. Order of Naming: The cation is always named first, followed by the anion, irrespective of whether the complex ion appears in the cationic or anionic part.
• 2. Naming the Coordination Sphere: Within a coordination entity, the ligands are named first in alphabetical order, ignoring numerical prefixes such as di-, tri-, tetra-, followed by the name of the central metal atom.
• 3. Prefixes for Ligands:
  • For simple ligands, prefixes such as di-, tri-, tetra- are used.
  • For ligands containing numerical prefixes in their names (e.g. ethane-1,2-diamine), prefixes bis-, tris-, tetrakis- are used.
• 4. Oxidation State: The oxidation number of the metal is indicated by Roman numerals in parentheses immediately after the metal name (without space).
• 5. Ending of Metal Name:
  • For neutral or cationic complexes, the metal name remains unchanged.
  • For anionic complexes, the metal name ends with the suffix –ate.

4.2 Naming of Ligands (Common Ligands)

Ligand	IUPAC Name	Nature
$H_2O$	aqua	Neutral
$NH_3$	ammine	Neutral
$CO$	carbonyl	Neutral
$Cl^-$	chlorido	Anionic
$NO_2^-$	nitrito-N	Ambidentate (via N)
$ONO^-$	nitrito-O	Ambidentate (via O)
$en$	ethane-1,2-diamine	Polydentate (use bis-, tris-)

4.3 Naming of Metals in Anionic Complexes

If the complex ion carries a negative charge, the Latin or conventional name of the metal is used with the suffix –ate.

Metal	Name in Anionic Complex
Iron (Fe)	Ferrate
Copper (Cu)	Cuprate
Silver (Ag)	Argentate
Gold (Au)	Aurate
Lead (Pb)	Plumbate

4.4 Solved Board Examples (Step-by-Step)

5. Isomerism in Coordination Compounds

Isomers are coordination compounds having the same molecular formula but a different arrangement of ligands or donor atoms around the central metal ion. This phenomenon is known as isomerism.

5.1 Structural Isomerism

(1) Linkage Isomerism

Linkage isomerism arises in coordination compounds containing ambidentate ligands which can coordinate through two different donor atoms.

(2) Solvate (Hydrate) Isomerism

Solvate isomerism arises when solvent molecules occupy different positions inside or outside the coordination sphere. When the solvent is water, it is called hydrate isomerism.

(3) Ionisation Isomerism

Ionisation isomerism occurs when a ligand inside the coordination sphere and a counter ion outside exchange their positions, producing different ions in solution.

(4) Coordination Isomerism

Coordination isomerism occurs when both cation and anion are complex ions and ligands are interchanged between them.

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5.2 Stereoisomerism

(A) Geometrical Isomerism

(B) Optical Isomerism

Optical isomerism is shown by coordination compounds which are chiral, i.e., they do not possess a plane of symmetry. Such isomers exist as non-superimposable mirror images called enantiomers.

6. Bonding in Coordination Compounds (Valence Bond Theory)

Valence Bond Theory (VBT) explains the formation of coordination compounds on the basis of hybridisation of atomic orbitals of the central metal atom or ion.

6.1 Postulates of Valence Bond Theory

• The central metal atom/ion makes available a number of empty orbitals equal to its coordination number.
• These orbitals undergo hybridisation to form an equal number of equivalent hybrid orbitals.
• Ligands donate lone pair(s) of electrons into these hybrid orbitals to form coordinate bonds.
• Inner orbital complexes are formed when pairing of electrons occurs in the inner $(n-1)d$ orbitals.
• Outer orbital complexes are formed when pairing does not occur and outer $nd$ orbitals are used.

6.2 Hybridisation and Geometry

Coordination Number	Hybridisation	Geometry
4	$sp^3$	Tetrahedral
4	$dsp^2$	Square Planar
6	$d^2sp^3$	Octahedral (Inner orbital)
6	$sp^3d^2$	Octahedral (Outer orbital)

6.3 Case Studies (NCERT & Board Focused)

(A) Octahedral Complexes (C.N. = 6)

(B) Complexes with Coordination Number 4

6.4 Limitations of Valence Bond Theory

• It involves a number of assumptions regarding orbital hybridisation.
• It does not explain the colour exhibited by coordination compounds.
• It does not distinguish quantitatively between weak and strong field ligands.
• It does not explain the relative stability of coordination compounds.

8. Bonding in Metal Carbonyls

Metal carbonyls are homoleptic coordination compounds in which carbon monoxide (CO) acts as a ligand and the metal is in zero oxidation state. Examples include $Ni(CO)_4$ (tetrahedral), $Fe(CO)_5$ (trigonal bipyramidal) and $Cr(CO)_6$ (octahedral).

8.1 Nature of Bonding (Synergic Effect)

The metal–carbon bond in metal carbonyls possesses both $\sigma$-bond and $\pi$-bond character. This dual interaction is known as synergic bonding.

• (i) $\sigma$-bonding: The carbon atom of CO donates a lone pair of electrons into a vacant orbital of the metal atom ($C \rightarrow M$).
• (ii) $\pi$-back bonding: The filled metal d-orbitals donate electrons into the vacant antibonding $\pi^*$ orbitals of CO ($M \rightarrow C$).

These two interactions reinforce each other, hence the term synergic effect.

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9. Importance and Applications of Coordination Compounds

Coordination compounds are of immense importance in biological systems, analytical chemistry, metallurgy and medicine.

9.1 Biological Importance

Complex	Central Metal	Function
Haemoglobin	$Fe^{2+}$	Transport of oxygen in blood
Chlorophyll	$Mg^{2+}$	Photosynthesis in plants
Vitamin B12	$Co^{3+}$	Treatment of pernicious anaemia

9.2 Analytical Chemistry

• Detection of Nickel: $Ni^{2+}$ ions give a red precipitate of $[Ni(DMG)_2]$ with dimethylglyoxime (DMG).
• Estimation of Hardness of Water: Calcium and magnesium ions are estimated using EDTA titration.

9.3 Medicinal Uses

• Cis-platin $[Pt(NH_3)_2Cl_2]$ is used in cancer chemotherapy.
• EDTA is used in the treatment of heavy metal poisoning (e.g., lead).

9.4 Metallurgical Applications

• Cyanide Process: Gold and silver are extracted as soluble complexes $[Au(CN)_2]^-$ and $[Ag(CN)_2]^-$.
• Mond Process: Purification of nickel using volatile $Ni(CO)_4$.

9.5 Catalysis

• Wilkinson’s Catalyst $[(PPh_3)_3RhCl]$ is used for hydrogenation of alkenes.

10. Practice Questions with Answers

Class 12 Chemistry – Chapter 5: Coordination Compounds

These questions are curated based on CBSE & RBSE Previous Year Papers.

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A. Very Short Answer Questions (1 Mark)

1. What is a coordination compound?
   Ans: A compound in which a central metal atom or ion is bonded to a fixed number of ligands through coordinate bonds.
2. Define coordination number.
   Ans: The total number of ligand donor atoms directly bonded to the central metal atom.
3. What is the coordination number of Co in $[Co(NH_3)_6]Cl_3$?
   Ans: 6 (Six monodentate ligands).
4. What is an ambidentate ligand?
   Ans: A ligand that can coordinate through two different donor atoms (e.g., $NO_2^-$, $SCN^-$).
5. Give one example of a chelating ligand.
   Ans: Ethane-1,2-diamine ($en$) or Oxalate ion ($C_2O_4^{2-}$).
6. What is a homoleptic complex?
   Ans: A complex containing only one type of ligands. Example: $[Co(NH_3)_6]^{3+}$.
7. What is a heteroleptic complex?
   Ans: A complex containing more than one type of ligands. Example: $[Co(NH_3)_4Cl_2]^+$.
8. What does primary valency represent in Werner’s theory?
   Ans: It represents the oxidation state of the metal (ionisable valency).
9. Which hybridisation is associated with square planar geometry?
   Ans: $dsp^2$
10. Is $[Ni(CN)_4]^{2-}$ paramagnetic or diamagnetic?
    Ans: Diamagnetic (All electrons are paired due to strong field $CN^-$).
11. What is the spectrochemical series?
    Ans: The arrangement of ligands in increasing order of their crystal field splitting power.
12. Why is $CN^-$ considered a strong field ligand?
    Ans: Because it has high crystal field splitting energy ($\Delta_o > P$) due to extensive $\pi$-back bonding.
13. What is $\Delta_o$?
    Ans: The energy difference between $t_{2g}$ and $e_g$ orbitals in an octahedral crystal field.
14. Why are tetrahedral complexes mostly high spin?
    Ans: Because the crystal field splitting energy ($\Delta_t$) is small and rarely exceeds the pairing energy ($P$).
15. What is the medicinal use of cis-platin?
    Ans: It is used in the treatment of cancer.

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B. Short Answer Questions (2 Marks)

16. Define denticity with an example.
    Ans: The number of donor atoms in a single ligand that bind to the central metal. E.g., EDTA has a denticity of 6.
17. Explain linkage isomerism.
    Ans: Isomerism arising in coordination compounds containing ambidentate ligands. Example: $[Co(NH_3)_5(NO_2)]Cl_2$ can exist as Nitro (N-bonded) or Nitrito (O-bonded) isomers.
18. What is ionisation isomerism?
    Ans: Isomerism that arises when the counter ion in a coordination compound is itself a potential ligand and can displace a ligand.
19. What is coordination isomerism?
    Ans: Isomerism arising from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex.
20. What are optical isomers?
    Ans: Two isomers that are non-superimposable mirror images of each other (enantiomers). They are chiral.
21. Differentiate between inner and outer orbital complexes.
    Ans:
    • Inner: Involves $(n-1)d$ orbitals (e.g., $d^2sp^3$). Strong ligands.
    • Outer: Involves $nd$ orbitals (e.g., $sp^3d^2$). Weak ligands.
22. Give one application of the spectrochemical series.
    Ans: It helps in predicting whether a complex will be high spin (weak ligand) or low spin (strong ligand).
23. Why is CO (Carbon monoxide) the strongest ligand?
    Ans: Because it forms both $\sigma$-bond (C to M) and $\pi$-bond (M to C back donation), creating a strong synergic effect.

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C. Reasoning-Type Questions (3 Marks)

24. Why is $[Co(NH_3)_6]^{3+}$ diamagnetic while $[CoF_6]^{3-}$ is paramagnetic?
    Ans: $NH_3$ is a strong ligand and causes pairing of electrons ($d^2sp^3$, no unpaired $e^-$). $F^-$ is a weak ligand and does not cause pairing ($sp^3d^2$, 4 unpaired $e^-$).
25. Why is $[Ni(CN)_4]^{2-}$ square planar but $[NiCl_4]^{2-}$ tetrahedral?
    Ans: $CN^-$ is a strong field ligand, forcing pairing of electrons ($dsp^2$ hybridisation). $Cl^-$ is a weak field ligand, so no pairing occurs ($sp^3$ hybridisation).
26. Why are coordination compounds generally coloured?
    Ans: Due to d–d transitions. Electrons absorb energy from visible light to jump from lower ($t_{2g}$) to higher ($e_g$) d-orbitals. The transmitted light gives the complementary colour.
27. Why is $[Sc(H_2O)_6]^{3+}$ colourless?
    Ans: $Sc^{3+}$ has a $3d^0$ configuration. Since there are no d-electrons, no d–d transition is possible, hence it is colourless.
28. Why do chelating ligands form more stable complexes?
    Ans: Due to the Chelate Effect. Chelation increases the entropy of the system (more particles are released), making the formation constant value very high.

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D. Numerical & Naming Questions (Most Important)

29. Find the oxidation state of Fe in $[Fe(C_2O_4)_3]^{3-}$.
    Ans: Let Fe = $x$. Oxalate is $-2$.
    $x + 3(-2) = -3 \Rightarrow x - 6 = -3 \Rightarrow x = +3$.
30. Write the IUPAC name of $[Co(NH_3)_5Cl]Cl_2$.
    Ans: Pentaamminechloridocobalt(III) chloride.
31. Write the formula for: Potassium trioxalatoaluminate(III).
    Ans: $K_3[Al(C_2O_4)_3]$
    (Reason: $Al^{3+}$ and 3 $C_2O_4^{2-}$ give -3 charge on complex. Need 3 $K^+$ to balance).
32. A complex has spin-only magnetic moment of 2.83 BM. How many unpaired electrons does it have?
    Ans: $\sqrt{n(n+2)} \approx 2.83$. Since the first digit is 2, n = 2 unpaired electrons.

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E. Long Answer Questions (5 Marks)

33. Discuss the bonding in Metal Carbonyls with a diagram.
    Ans: (Draw Synergic Bonding diagram). Bonding involves:
    1. $\sigma$-donation from C lone pair to metal empty orbital.
    2. $\pi$-back donation from filled metal d-orbital to empty $\pi^*$ orbital of CO.
    This strengthens the M-C bond.
34. Using VBT, explain the geometry and magnetic behaviour of $[Co(NH_3)_6]^{3+}$.
    Ans: (Draw Orbital Box Diagram).
    • Config: $3d^6$.
    • $NH_3$ causes pairing.
    • Hybridisation: $d^2sp^3$ (Inner orbital).
    • Geometry: Octahedral.
    • Magnetic: Diamagnetic.

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✅ END OF CHAPTER 5

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