Coordination Compounds

Coordination compounds contain a central metal atom/ion surrounded by ligands. They’re important in biological systems and catalysis.

Overview

graph TD
    A[Coordination Compounds] --> B[Werner's Theory]
    A --> C[Nomenclature]
    A --> D[Isomerism]
    A --> E[Bonding]
    E --> E1[VBT]
    E --> E2[CFT]

Basic Terms

Werner’s Theory

1. Metals show two types of valency: primary and secondary
2. Primary valency (ionizable): Satisfied by anions
3. Secondary valency (non-ionizable): Satisfied by ligands
4. Secondary valency = Coordination number
5. Ligands are arranged in definite geometry

Ligands

Classification by Denticity

Ambidentate Ligands

Can coordinate through different atoms:

• NO₂⁻: Through N (nitro) or O (nitrito)
• SCN⁻: Through S (thiocyanato) or N (isothiocyanato)

Chelates

Complexes with polydentate ligands forming ring structures.

IUPAC Nomenclature

Order of Naming

1. Cation before anion
2. Within coordination sphere:
   • Ligands alphabetically (before metal)
   • Metal name with oxidation state

Ligand Names

Prefixes for Number

di-, tri-, tetra-, penta-, hexa- (bis-, tris-, tetrakis- for complex ligands)

Examples

• [Co(NH₃)₆]Cl₃: Hexaamminecobalt(III) chloride
• K₄[Fe(CN)₆]: Potassium hexacyanidoferrate(II)

Isomerism

Structural Isomerism

graph TD
    A[Structural Isomerism] --> B[Ionization]
    A --> C[Hydrate/Solvate]
    A --> D[Linkage]
    A --> E[Coordination]

1. Ionization: Different ions in/outside sphere

   • [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br

2. Hydrate: Water in/outside sphere

   • [Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂·H₂O

3. Linkage: Ambidentate ligand coordination

   • [Co(NH₃)₅NO₂]²⁺ vs [Co(NH₃)₅ONO]²⁺

4. Coordination: Ligand exchange between cation and anion

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

Stereoisomerism

1. Geometrical (cis-trans): Same formula, different spatial arrangement

For square planar [MA₂B₂]:

• cis: Same ligands on same side
• trans: Same ligands on opposite sides

For octahedral [MA₂B₄]:

• cis: A’s adjacent
• trans: A’s opposite

2. Optical: Non-superimposable mirror images (enantiomers)

Required: Absence of plane of symmetry

Bonding in Coordination Compounds

Valence Bond Theory (VBT)

• Ligands donate electron pairs to empty metal orbitals
• Hybridization determines geometry:

Crystal Field Theory (CFT)

Ligands are point charges/dipoles creating an electrostatic field.

In octahedral field:

• d orbitals split into t₂g (lower) and eg (higher)
• Crystal field splitting energy = Δ₀

Spectrochemical series:

Weak field ←→ Strong field

High Spin vs Low Spin

Magnetic Properties

where n = number of unpaired electrons

Applications

1. Qualitative analysis: Characteristic colors
2. Extraction of metals: Cyanide complex in gold extraction
3. Biological systems: Hemoglobin, Chlorophyll, Vitamin B₁₂
4. Medicine: Cisplatin (anticancer)
5. Photography: Silver bromide complexes

Practice Problems

1. Write IUPAC names for:

   • [Cr(NH₃)₄Cl₂]⁺
   • [Pt(en)₂Cl₂]²⁺

2. Predict the geometry and magnetic nature of [Ni(CN)₄]²⁻ and [NiCl₄]²⁻.

3. How many geometrical isomers are possible for [Co(NH₃)₃Cl₃]?

4. Calculate the magnetic moment of [Fe(H₂O)₆]²⁺.

Further Reading

• d- and f-Block Elements
• Chemical Bonding