Applications of Coordination Compounds

Introduction

Coordination chemistry isn’t just abstract theory - it’s the science behind life itself! From the oxygen you breathe to the photos you take, from life-saving cancer drugs to the green plants around you, coordination compounds are working behind the scenes everywhere.

1. Biological Applications

A. Hemoglobin - Oxygen Transport

Structure:

• Central atom: Fe²⁺
• Ligand system: Porphyrin (macrocyclic tetradentate)
• Additional ligands: Histidine (from protein) + O₂ (reversibly bound)
• Coordination number: 6 (octahedral)

graph TD
    A[Lungs: High O₂] --> B[O₂ binds to Fe²⁺]
    B --> C[Oxy-hemoglobin
Bright red]
    C --> D[Tissues: Low O₂]
    D --> E[O₂ released]
    E --> F[Deoxy-hemoglobin
Dark purple-red]
    F --> A

    style C fill:#ff0000
    style F fill:#8b0000

Key Features:

• Reversible binding: O₂ must bind AND release easily
• Spin state change: Low O₂ → high spin (paramagnetic), High O₂ → low spin
• Color change: Explains why arterial blood (O₂-rich) is bright red, venous blood is darker

Carbon Monoxide Poisoning:

• CO binds 200 times more strongly than O₂!
• CO is strong field ligand → doesn’t release easily
• Blocks oxygen transport → fatal
• Treatment: High concentration O₂ therapy

B. Chlorophyll - Photosynthesis

Structure:

• Central atom: Mg²⁺
• Ligand: Porphyrin ring (similar to heme, but with Mg instead of Fe)
• Function: Absorbs light for photosynthesis

Why green?

• Chlorophyll absorbs red (~680 nm) and blue (~430 nm) light
• Reflects green light (~550 nm)
• The Mg-porphyrin coordination complex is responsible for this selective absorption

Comparison: Hemoglobin vs Chlorophyll

Both use the same basic porphyrin structure - nature’s favorite coordination ligand!

C. Vitamin B₁₂ (Cyanocobalamin)

Structure:

• Central atom: Co³⁺
• Ligand: Corrin ring (similar to porphyrin)
• Axial ligands: CN⁻ and dimethylbenzimidazole
• Most complex non-polymer molecule synthesized by living organisms!

Function:

• Essential for DNA synthesis
• Red blood cell formation
• Nervous system function

Deficiency:

• Pernicious anemia
• Nerve damage

Fun fact: First coordination compound whose structure was determined using X-ray crystallography by Dorothy Hodgkin (Nobel Prize 1964)!

D. Other Biological Examples

2. Medicinal Applications

A. Cisplatin - Anticancer Drug

Structure: cis-[Pt(NH₃)₂Cl₂]

Discovery:

• 1965: Barnett Rosenberg accidentally discovered it inhibits bacterial cell division
• 1978: FDA approved for testicular and ovarian cancer
• Now: One of the most widely used chemotherapy drugs

Mechanism:

1. Enters cancer cell
2. Cl⁻ ligands replaced by water → activated complex
3. Binds to DNA (coordinates with N atoms of adjacent guanine bases)
4. Forms cross-links between DNA strands
5. Prevents DNA replication → cell death

graph LR
    A[cis-Pt(NH₃)₂Cl₂] --> B[Inside cell]
    B --> C[Cl⁻ replaced by H₂O]
    C --> D[Binds to DNA]
    D --> E[Cross-links DNA]
    E --> F[Cell cannot divide]
    F --> G[Cancer cell dies]

    style A fill:#3498db
    style G fill:#e74c3c

Why only CIS, not TRANS?

• cis isomer: Two Cl⁻ on same side → can bind to adjacent guanines on DNA
• trans isomer: Cl⁻ on opposite sides → cannot form cross-link effectively
• This is a perfect example of how stereochemistry matters in medicine!

Limitations:

• Severe side effects (kidney damage, nausea)
• Some cancers develop resistance

Next generation:

• Carboplatin: Less toxic, same mechanism
• Oxaliplatin: Effective against different cancers

B. Gold Complexes - Rheumatoid Arthritis

Examples:

• Auranofin: [Au(PEt₃)(thioglucose)]
• Gold sodium thiomalate: [Au(S-malate)₂]³⁻

Function:

• Reduces inflammation
• Slows disease progression
• Mechanism: Inhibits enzymes involved in inflammatory response

C. Chelation Therapy

For Heavy Metal Poisoning:

How it works:

1. Chelating agent forms very stable complex with toxic metal
2. Complex is water-soluble
3. Kidneys filter and excrete the complex
4. Toxic metal is removed from body

Example: EDTA for Lead Poisoning

EDTA (hexadentate) “wraps around” Pb²⁺:

Stability constant: β ≈ 10¹⁸ (extraordinarily stable!)

D. Imaging and Diagnosis

Gadolinium Complexes in MRI:

• [Gd(DTPA)]²⁻ used as contrast agent
• Gd³⁺ is paramagnetic → enhances MRI signal
• DTPA chelation makes it safe (free Gd³⁺ is toxic)

Radioactive Complexes:

• ⁹⁹ᵐTc complexes for diagnostic imaging
• ¹⁷⁷Lu complexes for targeted radiotherapy

3. Analytical Chemistry

A. Qualitative Analysis

Detection of Metal Ions:

Example: Nickel Detection

Nickel + dimethylglyoxime (DMG) forms a rose-red precipitate:

This is a specific test for Ni²⁺!

B. Complexometric Titrations

EDTA Titrations for Water Hardness:

Hard water contains Ca²⁺ and Mg²⁺. EDTA forms 1:1 complexes:

Procedure:

1. Add indicator (Eriochrome Black T)
2. Titrate with EDTA solution
3. EDTA “steals” metal from indicator
4. Color change indicates endpoint

Advantages:

• Very precise
• Works for many metal ions
• Simple procedure

C. Extraction and Separation

Solvent Extraction:

Neutral complexes are organic-soluble and can be extracted:

Example: Uranium extraction using TBP (tributyl phosphate)

The neutral complex dissolves in organic solvent (kerosene), separating U from other metals.

4. Industrial Applications

A. Metallurgy - Metal Extraction

Gold and Silver Extraction

Cyanide Process:

• Gold forms very stable cyano complex (β ≈ 10³⁸)
• Complex is water-soluble
• Gold can be separated from ore
• Later reduced to metallic gold with Zn

Similarly for silver:

Nickel Purification - Mond Process

• Ni reacts with CO to form volatile complex
• [Ni(CO)₄] is heated to 200°C
• Decomposes to pure Ni metal

Advantages:

• Gives very pure nickel (99.99%)
• Separates Ni from impurities

B. Catalysis

Many industrial catalysts are coordination compounds:

Wilkinson’s Catalyst

[RhCl(PPh₃)₃] - Hydrogenation of alkenes

Ziegler-Natta Catalyst

TiCl₄ + Al(C₂H₅)₃ - Polymerization of ethylene and propylene

• Used to make high-density polyethylene (HDPE)
• Used to make polypropylene

Monsanto Process

[Rh(CO)₂I₂]⁻ - Methanol → Acetic acid

C. Photography

Fixing Agent - Sodium Thiosulfate (“Hypo”)

After exposure to light, film contains:

• Exposed areas: Metallic Ag (black)
• Unexposed areas: AgBr (sensitive to light)

To “fix” the image, unexposed AgBr must be removed:

The thiosulfato complex is water-soluble and washes away, leaving permanent image.

Why it works:

• [Ag(S₂O₃)₂]³⁻ is very stable (large β)
• Removes AgBr without affecting metallic Ag

D. Electroplating

Silver Plating:

Uses [Ag(CN)₂]⁻ instead of Ag⁺ because:

1. Slow deposition → smoother, uniform coating
2. Better adhesion
3. Less wastage

(at cathode)

Gold Plating:

Similarly uses [Au(CN)₂]⁻ complex.

Interactive Demo: Visualize Electrochemical Cells

See how coordination compounds work in electrochemical cells and electroplating processes.

5. Environmental Applications

A. Water Treatment

EDTA for Heavy Metal Removal:

Industrial wastewater often contains toxic metals:

The complex can be:

• Removed by ion exchange
• Precipitated
• Filtered out

Preventing Scale Formation:

Hard water deposits CaCO₃ scales in pipes. Adding phosphates forms soluble complexes:

B. Pollution Control

Mercury Removal:

Chelating resins contain ligands that bind Hg²⁺:

• Water flows through resin bed
• Hg²⁺ forms stable complex with resin
• Clean water emerges

Fertilizer Enhancement:

Metal micronutrients (Fe, Zn, Cu, Mn) are often insoluble in soil. Using chelated micronutrients:

• Remains soluble in soil
• Plants can absorb it
• Better crop yields

6. Material Science

A. Dyes and Pigments

Many commercial dyes are coordination compounds:

B. Magnetic Materials

Magnetic Recording:

• Uses coordination compounds of Fe and Co
• Different spin states = different magnetic properties
• Data storage in hard drives

7. Advanced Applications (JEE Advanced)

A. Supramolecular Chemistry

Host-Guest Chemistry:

• Crown ethers (macrocyclic ligands) selectively bind metal ions
• Used in ion-selective sensors
• Basis for artificial enzymes

B. Molecular Electronics

Metal-Organic Frameworks (MOFs):

• 3D networks of metal ions connected by organic ligands
• Applications: Gas storage, catalysis, drug delivery
• Can store H₂ for fuel cells

C. Luminescent Materials

LEDs and OLEDs:

• Iridium and platinum complexes
• Emit light when electrons transition between energy levels
• Used in displays and lighting

Example: [Ir(ppy)₃] (green OLED emitter)

Summary of Applications

graph TD
    A[Coordination Compounds] --> B[Biological]
    A --> C[Medicinal]
    A --> D[Analytical]
    A --> E[Industrial]
    A --> F[Environmental]

    B --> B1[Hemoglobin
Chlorophyll
Vitamin B₁₂]
    C --> C1[Cisplatin
Chelation therapy
MRI contrast]
    D --> D1[Qualitative tests
EDTA titrations
Extractions]
    E --> E1[Metal extraction
Catalysis
Photography]
    F --> F1[Water treatment
Pollution control
Fertilizers]

    style A fill:#3498db
    style B fill:#2ecc71
    style C fill:#e74c3c
    style D fill:#f39c12
    style E fill:#9b59b6
    style F fill:#1abc9c

Memory Tricks

“BMAIE” for Application Categories

Biological (Hemoglobin, Chlorophyll) Medicinal (Cisplatin, Chelation) Analytical (Qualitative tests, Titrations) Industrial (Metallurgy, Catalysis) Environmental (Water treatment, Pollution)

Metal-Color Association

“Copper turns BLUE with ammonia” → [Cu(NH₃)₄]²⁺ (deep blue) “Iron turns RED with thiocyanate” → [Fe(SCN)]²⁺ (blood red) “Nickel gives ROSE-RED with DMG” → [Ni(DMG)₂] (rose-red)

Common Mistakes

Practice Problems

Level 1: Basic Understanding

Q1. Explain why carbon monoxide is poisonous in terms of coordination chemistry.

Q2. What is the role of coordination compounds in: a) Photography b) Electroplating c) Qualitative analysis of Fe³⁺

Q3. Why is chlorophyll green while hemoglobin is red, despite both having similar porphyrin structures?

Level 2: Application

Q4. A patient is suffering from lead poisoning. Explain: a) Why EDTA is used as treatment b) Why the stability constant of [Pb(EDTA)]²⁻ must be very large c) How the complex is removed from the body

Q5. In gold extraction:

Explain: a) Why gold, a noble metal, dissolves in cyanide solution b) Why this process requires oxygen c) How gold is recovered from [Au(CN)₂]⁻

Q6. Why is cis-platin effective against cancer while trans-platin is not?

Q7. A solution containing Cu²⁺ is treated with excess NH₃. Describe: a) Color change observed b) Complex formed c) How this is used in qualitative analysis

Level 3: JEE Advanced

Q8. Compare and contrast hemoglobin and hemocyanin in terms of: a) Metal center b) Color of blood c) Coordination number d) Oxygen binding

Q9. The Mond process for nickel purification uses [Ni(CO)₄]. Explain: a) Why this complex is volatile while most coordination compounds aren’t b) Why heating to 200°C decomposes it back to Ni c) Why this gives very pure nickel

Q10. In complexometric titration with EDTA: a) Why is EDTA preferred over other ligands? b) How does the chelate effect make titrations sharp? c) Why can one EDTA solution be used for many different metal ions?

Q11. Cisplatin’s mechanism involves:

cis-[Pt(NH₃)₂Cl₂] → cis-[Pt(NH₃)₂(H₂O)₂]²⁺ → DNA cross-linking

Explain: a) Why Cl⁻ is replaced by H₂O inside cells b) Why the aqua complex then binds to DNA c) Why the cross-link prevents cell division

Q12. A student wants to electroplate an object with silver. Compare: a) Using AgNO₃ solution b) Using K[Ag(CN)₂] solution

Which gives better plating and why?

Solutions to Selected Problems

Q1. CO binds to Fe²⁺ in hemoglobin 200× more strongly than O₂. It forms [Hb-CO] which doesn’t release, blocking oxygen transport.

Q3. Different metal centers (Fe²⁺ vs Mg²⁺) cause different d-d transitions, absorbing different wavelengths. The reflected/transmitted light gives the observed colors.

Q5. a) Formation of very stable [Au(CN)₂]⁻ (β ≈ 10³⁸) provides driving force b) O₂ oxidizes Au from 0 to +1 oxidation state c) Add Zn, which reduces Au⁺ back to Au(0) while forming [Zn(CN)₄]²⁻

Q6. cis-platin’s two Cl⁻ are adjacent, allowing binding to two adjacent guanines on DNA, forming cross-link. trans-platin’s Cl⁻ are opposite, cannot form effective cross-link.

Q9. a) [Ni(CO)₄] is neutral and has no permanent dipole → low intermolecular forces → volatile b) Thermal decomposition (ΔH > 0) is favored at high temperature c) All impurities remain as solids while only [Ni(CO)₄] vaporizes and is collected separately

Q12. K[Ag(CN)₂] is better because:

• Slow reduction → uniform deposition
• Better adhesion to surface
• Smooth, shiny finish

Real-World Impact

Nobel Prizes Related to Coordination Chemistry

1. Alfred Werner (1913): Structure of coordination compounds
2. Dorothy Hodgkin (1964): X-ray structure of Vitamin B₁₂
3. Wilkinson & Fischer (1973): Organometallic chemistry (catalysts)
4. Jean-Marie Lehn (1987): Supramolecular chemistry
5. Yves Chauvin (2005): Metathesis catalysis

Economic Impact

• Cisplatin alone: Saves thousands of lives annually, billions in healthcare impact
• Ziegler-Natta catalysts: Entire plastics industry (polyethylene, polypropylene)
• Gold/silver extraction: Enables modern electronics (need pure metals)
• EDTA: Multibillion-dollar water treatment industry

Looking Forward

Emerging Applications

1. Artificial photosynthesis: Ru and Ir complexes to split water for H₂ fuel
2. CO₂ capture: MOFs for greenhouse gas reduction
3. Targeted drug delivery: Stimulus-responsive coordination polymers
4. Quantum computing: Single-molecule magnets
5. Solar cells: Ru-based dye-sensitized solar cells

Related Topics

Within Coordination Compounds

• Werner’s Theory — Foundation of all these applications
• Stability Constants — Why EDTA works for chelation
• Crystal Field Theory — Why complexes are colored
• Isomerism — Why cis-platin ≠ trans-platin

Cross-Chapter Connections

• d-Block Elements — Source of metals in complexes
• Chemical Kinetics — Catalyst mechanisms
• Electrochemistry — Electroplating
• Biomolecules — Proteins and enzymes

← Stability Constants | Coordination Compounds Index