Applications — biological (haemoglobin, chlorophyll, vit B₁₂), medicinal (cisplatin), industrial (catalysts)
WHY do biology and industry love coordination compounds?
A free metal ion (, , ) has:
- variable oxidation states → can do redox,
- empty d-orbitals → can bind small molecules (O₂, CO, N₂) reversibly,
- a definite geometry forced by ligands → can position a reactive site precisely.
The trick of life is the porphyrin / corrin macrocycle — a flat ring with 4 nitrogen donors. It grabs the metal in a square plane and leaves the 2 axial positions free for chemistry.
1. Biological systems
(a) Haemoglobin — O₂ transport
- Metal: Fe(II) (the +2 state is essential; Fe(III) form = methaemoglobin can't carry O₂).
- Ligand: porphyrin (the "haem" group) gives 4 N-donors in a plane; a 5th position is held by a histidine of the globin protein; the 6th axial site binds O₂ reversibly.
- WHAT happens: . Binding is reversible so O₂ is picked up in lungs (high ) and released in tissues (low ).
(b) Chlorophyll — photosynthesis
- Metal: Mg(II) in a porphyrin-like ring.
- WHY Mg and not Fe? Mg²⁺ is redox-inert (no easy oxidation state change). Its job isn't to bind O₂ — it's to hold the conjugated ring rigid so it absorbs visible light (red+blue) and channels the excited electron. The green colour = light it doesn't absorb.
- Role: converts .
(c) Vitamin B₁₂ (cyanocobalamin)
- Metal: Co(III) in a corrin ring (smaller than porphyrin; 4 N-donors).
- One axial site has a –CN (in the vitamin form) or –CH₃/–adenosyl (active coenzyme forms).
- Role: the only metal-carbon bond in human biochemistry; enzyme for DNA synthesis & RBC formation. Deficiency → pernicious anaemia.
2. Medicinal — Cisplatin
WHY only the cis isomer works:
- Inside the cell (low Cl⁻ concentration), the two cis chlorides hydrolyse and are replaced by water/then by two adjacent N atoms of guanine bases on DNA.
- Because the two leaving groups are cis (90° apart), Pt can bind two neighbouring DNA sites → kinks DNA → stops replication → cancer cell dies.
- In trans-platin the two Cl are 180° apart → can't bridge adjacent bases → inactive. This is geometry deciding life and death.
3. Industrial catalysts
Coordination complexes act as homogeneous catalysts — they bind the substrate, lower the activation barrier, release product, and regenerate.
| Catalyst | Formula | Reaction |
|---|---|---|
| Wilkinson's | hydrogenation of alkenes ( alkane) | |
| Ziegler–Natta | polymerisation of ethene → polythene | |
| Wacker process | ethene → ethanal |
Worked reasoning examples
Recall Feynman: explain to a 12-year-old
Imagine a metal atom as a tiny magnet-hand. Plants put a magnesium hand inside a green frame that catches sunlight to make food. Your blood puts an iron hand inside a red frame that grabs oxygen in your lungs and drops it in your muscles. A vitamin uses a cobalt hand to help build your blood. And a cancer medicine uses a platinum hand with two "sticky fingers" that must be next to each other (cis) so it can grab and jam the cancer cell's instruction book (DNA). Same atom, different cage = totally different job.
Flashcards
Which metal & oxidation state is in haemoglobin?
Which metal is at the centre of chlorophyll, and why this metal?
Which metal and oxidation state is in vitamin B₁₂?
What macrocyclic ring binds the metal in haemoglobin & chlorophyll?
What ring is in vitamin B₁₂ (different from porphyrin)?
Why does CO poison haemoglobin?
Why can't methaemoglobin carry oxygen?
Formula and geometry of cisplatin?
Why is only cis-platin an anticancer drug?
Wilkinson's catalyst formula and use?
Ziegler–Natta catalyst and use?
Deficiency disease of vitamin B₁₂?
Why are metal complexes good homogeneous catalysts?
Connections
- Isomerism in coordination compounds — cis/trans decides cisplatin's activity
- Crystal Field Theory — explains colour of haem & chlorophyll (d-orbital splitting)
- Square planar complexes — geometry of Pt(II) d⁸
- Stability and chelate effect — why macrocyclic rings bind so tightly (macrocyclic effect)
- Oxidation states of transition metals — Fe(II)/Fe(III), Co(III)
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, coordination chemistry sirf exam ka chapter nahi hai — yeh literally zindagi chalati hai. Idea simple hai: ek metal ion ko ek bade organic "cage" (ligand) mein band kar do, aur uski properties — colour, reactivity, binding — tune ho jaati hain. Nature ne porphyrin naam ka ek flat ring banaya jo 4 nitrogen se metal ko pakadta hai aur do axial positions free chhodta hai chemistry ke liye.
Haemoglobin mein Fe(II) porphyrin ke beech baitha hai; uska 6th site O₂ ko reversibly pakadta hai — lungs mein utha lo, tissues mein chhod do. CO isi 6th site ko O₂ se 200 guna strongly pakad leta hai, isiliye poison karta hai (Hb ko todta nahi, sirf jagah cheen leta hai). Chlorophyll mein Mg(II) hai jo redox-inert hai — uska kaam sunlight absorb karna hai, isiliye plant green dikhta hai. Vitamin B₁₂ mein Co(III) corrin ring ke andar hai, jo DNA aur RBC banane mein help karta hai; iski kami se pernicious anaemia hota hai.
Medicine side pe cisplatin, cis-[Pt(NH₃)₂Cl₂], ek square planar Pt(II) complex hai. Sirf cis form kaam karta hai kyunki uske do Cl 90° pe hote hain, jisse Pt DNA ke do paas-paas wale bases ko crosslink kar deta hai aur cancer cell ki replication ruk jaati hai. Trans form mein Cl 180° pe hote hain — paas-paas bind nahi kar sakte — isliye inactive. Yahan geometry hi life-and-death decide karti hai.
Industry mein metal complexes shaandaar catalysts hote hain: Wilkinson's catalyst alkene ko hydrogenate karta hai, Ziegler–Natta ethene ko polythene banata hai. Reason — variable oxidation states aur khaali coordination sites unhe ek reusable molecular workbench bana dete hain. Bas yaad rakho: same metal, alag cage = bilkul alag kaam.