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Overview of Coordination Compounds

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Estimated time: 41 minutes
Maharashtra State Board: Class 12

Definition: Coordination compound

A compound in which a central metal atom or ion is surrounded by ligands bonded through coordinate bonds is called a coordination compound.

Maharashtra State Board: Class 12

Definition: Ligand

An ion or molecule that donates a pair of electrons to the central metal atom or ion to form a coordinate bond is called a ligand.

Maharashtra State Board: Class 12

Definition: Monodentate ligand

A ligand that contains only one donor atom and forms only one coordinate bond with the central metal ion is called a monodentate ligand.

Maharashtra State Board: Class 12

Definition: Polydentate ligand

A ligand that contains two or more donor atoms and forms multiple coordinate bonds with the central metal ion is called a polydentate ligand.

Maharashtra State Board: Class 12

Definition: Ambidentate ligand

A ligand that can coordinate to the metal through two different donor atoms but only one at a time is called an ambidentate ligand.

Maharashtra State Board: Class 12

Definition: Coordination number

The number of ligand donor atoms directly bonded to the central metal ion in a complex is called the coordination number.

Maharashtra State Board: Class 12

Definition: Coordination complex

A compound that retains its identity in solution and does not completely dissociate into simple ions is called a coordination complex.

Maharashtra State Board: Class 12

Definition: Homoleptic complex

A complex containing only one type of ligand attached to the central metal ion is called a homoleptic complex.

Maharashtra State Board: Class 12

Definition: Heteroleptic complex

A complex containing more than one type of ligand attached to the central metal ion is called a heteroleptic complex.

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Definition: Cationic complex

A complex having a positively charged coordination sphere is called a cationic complex.

Maharashtra State Board: Class 12

Definition: Anionic complex

A complex having a negatively charged coordination sphere is called an anionic complex.

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Definition: Neutral complex

A complex having no charge on the coordination sphere is called a neutral complex.

Maharashtra State Board: Class 12

Definition: Linkage isomerism

Isomerism arising due to different donor atoms of the same ligand is called linkage isomerism.

Maharashtra State Board: Class 12

Definition: Ionization isomerism

Isomerism arising due to exchange of ligands between coordination sphere and ionization sphere is called ionization isomerism.

Maharashtra State Board: Class 12

Definition: Coordination isomerism

Isomerism arising due to interchange of ligands between cationic and anionic complexes is called coordination isomerism.

Maharashtra State Board: Class 12

Definition: Solvate (hydrate) isomerism

Isomerism arising due to the presence of solvent molecules inside or outside the coordination sphere is called solvate (hydrate) isomerism.

Maharashtra State Board: Class 12

Definition: Geometrical isomerism

Isomerism in which ligands differ in their spatial arrangement around the central metal ion is called geometrical isomerism.

Definition: Optical isomerism

Isomerism in which two compounds are non-superimposable mirror images of each other is called optical isomerism.

Maharashtra State Board: Class 12

Definition: Effective atomic number (EAN)

The total number of electrons surrounding the central metal ion after complex formation is called the effective atomic number (EAN).

EAN = number of electrons of metal ion + total number of electrons donated by ligands
= atomic number of metal (Z) - number of electrons lost by metal to form the ion (X) + number of electrons donated by ligands (Y).
= Z - X + Y 

Maharashtra State Board: Class 12

Definition: Strong field ligands

Ligands that cause large splitting of d-orbitals and pairing of electrons are called strong field ligands.

Maharashtra State Board: Class 12

Definition: Weak field ligands

Ligands that cause small splitting of d-orbitals and do not cause pairing of electrons are called weak field ligands.

Maharashtra State Board: Class 12

Definition: Electroplating

The process of depositing a thin layer of metal over another material using coordination compounds is called electroplating.

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Definition: Chelation

The removal of metal ions from solution by forming stable coordination complexes is called chelation.

Maharashtra State Board: Class 12

Key Points: Werner’s Theory of Coordination Complexes

Werner’s Theory of Coordination Complexes explains the bonding and structure of coordination compounds through the concept of valencies of the metal ion.

  1. The central metal ion exhibits two types of valencies, namely primary valency and secondary valency.
  2. Primary valency is ionizable and corresponds to the oxidation state of the metal ion, and it is generally satisfied by anions present outside the coordination sphere.
  3. Secondary valency is non-ionizable and corresponds to the coordination number of the metal ion, and it is satisfied by ligands directly bonded to the metal ion.
  4. Secondary valencies are fixed in number and have definite spatial arrangement, which determines the geometry of the complex, such as octahedral, tetrahedral, or square planar.
Maharashtra State Board: Class 12

Key Points: Valence bond theory (VBT)

Valence Bond Theory (VBT) explains bonding in coordination compounds using hybridisation of metal orbitals.

  • The central metal ion provides vacant orbitals (s, p and d) which hybridize to form equivalent hybrid orbitals.
  • The number of hybrid orbitals formed equals the coordination number of the metal ion.
  • Coordinate bonds are formed by overlap of vacant hybrid orbitals of the metal ion with filled orbitals of ligands.
  • Hybridisation determines geometry:
  1. sp³ → Tetrahedral
  2. dsp² → Square planar
  3. d²sp³ → Octahedral (inner orbital)
  4. sp³d² → Octahedral (outer orbital)
  • Inner orbital complexes (d²sp³) involve pairing of (n−1)d electrons before hybridisation and are generally low spin.
  • Outer orbital complexes (sp³d²) do not involve pairing of (n−1)d electrons before hybridisation and are generally high spin; magnetic nature depends on the number of unpaired electrons.
Maharashtra State Board: Class 12

Key Points: Crystal Field Theory (CFT)

  • CFT assumes that ligands are point charges or dipoles and metal–ligand interaction is purely electrostatic.
  • In an isolated metal ion, all five d-orbitals are degenerate (equal in energy).
  • In an octahedral field, d-orbitals split into two sets:
    Lower energy: t2g(dxy, dyz, dxz)
    Higher energy: eg (dx²–y², dz²)
  • The energy difference between t₂g and e_g orbitals is called crystal field splitting energy (Δ₀), and Δ₀ = 10 Dq.
  • Energy change in octahedral field: 
    t2g orbitals are stabilized by −2/5 Δ₀
    eg orbitals are destabilized by +3/5 Δ₀
  • Magnitude of Δ₀ depends on ligand strength and oxidation state of the metal ion.
    Strong field ligands → large Δ₀ → low spin
    Weak field ligands → small Δ₀ → high spin
  • Colour of coordination compounds is due to d–d transitions, where electrons absorb energy equal to Δ₀ and move from t2g to eg orbitals.
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