Exercises — Applications — biological (haemoglobin, chlorophyll, vit B₁₂), medicinal (cisplatin), industrial (catalysts)

Why these facts matter: Fe(II) is the only state that binds O₂ reversibly; Mg(II) is redox-inert (holds the ring rigid); Co(III) sits in the smaller corrin ring and forms a rare metal–carbon bond. See Oxidation states of transition metals.

Formula: $\text{cis-}[\text{Pt}({\text{NH}}_3{)}_2{\text{Cl}}_2]$. Geometry: square planar, Pt is Pt(II), a $d^8$ ion. Square planar geometry is why the two chlorides can sit either $90^{\circ }$ apart (cis) or $180^{\circ }$ apart (trans). See Square planar complexes.

(a) Wilkinson's catalyst, $[\text{RhCl}({\text{PPh}}_3{)}_3]$ — adds ${\text{H}}_2$ across a $\text{C=C}$ double bond. (b) Ziegler–Natta catalyst, ${\text{TiCl}}_4+\text{Al}({\text{C}}_2{\text{H}}_5{)}_3$ — links ethene molecules into polythene.

Step 1 — WHAT is happening: CO and O₂ fight for the same 6th axial site on Fe(II). CO wins because it binds $\sim 200\times$ tighter, so the forward reaction (making HbCO) is favoured. Step 2 — WHY pure O₂ helps: Le Chatelier — flood the system with O₂ on the right side. A large $[{\text{O}}_2]$ pushes the equilibrium backwards, reforming ${\text{HbO}}_2$ and releasing CO. Step 3 — WHAT it means: CO never destroyed anything. It was pure coordination competition. Raise $p_{O_2}$ and you out-vote the CO.

"Relative pull" of each gas $=$ (binding strength) $\times$ (pressure).

• O₂ pull $=1\times p_{O_2}$.
• CO pull $=200\times p_{CO}=200\times \frac{1}{200}{p}_{O_2}=1\times p_{O_2}$.

They are equal! So CO takes $50\%$ of sites even though its pressure is $200\times$ smaller. Fraction on CO $=0.5$. That is why tiny traces of CO are lethal — its $200\times$ strength exactly cancels a $200\times$ dilution.

Step 1 — geometry: Pt(II) is $d^8$ → square planar (four ligands at the corners of a square, $90^{\circ }$ between neighbours, $180^{\circ }$ across the diagonal). See Crystal Field Theory for why $d^8$ gives square planar. Step 2 — the leaving groups: Inside the cell the two ${\text{Cl}}^{-}$ hydrolyse off, leaving two open bonds where DNA nitrogen atoms will attach. Step 3 — the bite distance:

• cis: the two Cl are $90^{\circ }$ apart → the two new bonds point at neighbouring corners → Pt can grab two adjacent guanine bases, kinking the DNA. Replication stops → cancer cell dies.
• trans: the two Cl are $180^{\circ }$ apart → the bonds point in opposite directions → Pt cannot reach two neighbouring bases → no crosslink → inactive.

This is geometric isomerism (Isomerism in coordination compounds) deciding biological life and death. Same formula, different arrangement, different job.

Haemoglobin's job = grab and release O₂. Reversible binding of a small molecule needs a metal whose $d$-orbitals can accept and back-donate electrons — Fe(II) does exactly this. It must not be permanently oxidised (Fe(III) = methaemoglobin can't carry O₂), but it must be able to interact electronically. Chlorophyll's job = hold a light-catching antenna rigid. Here you don't want the metal changing oxidation state — a redox event would drain the captured light energy into the wrong place. Mg(II) has no easy oxidation-state change, so it simply clamps the conjugated ring flat, keeping the absorption band sharp and channelling the excited electron cleanly. Conclusion: the required function selects the metal. Reactivity → Fe. Rigidity → Mg.

Idea 1 — the chelate effect: A ligand that binds through several donor atoms at once (a chelate) makes a far more stable complex than the same number of separate single-donor ligands. Reason: releasing one arm doesn't release the whole ligand — it's still tethered — so the metal stays locked. Idea 2 — a macrocycle is the extreme chelate: all four N-donors are pre-organised in one rigid ring (the "macrocyclic effect"), even more stable than an open chain chelate. The metal essentially cannot fall out. Idea 3 — geometry for free: the flat ring forces four donors into the equatorial plane and leaves the two axial sites open for the actual chemistry (O₂, histidine, –CN). Synthesis: Life needs a metal that (a) stays put for the cell's whole lifetime and (b) has a clean, reserved reactive site. A macrocycle delivers both — maximum stability and a pre-shaped axial pocket. Four loose ligands would drift off and would not organise the axial sites.

Building block 1 — open site: a catalytic complex has a vacant coordination position. The reactant (${\text{H}}_2$, alkene) docks there, in one precise orientation — not randomly like on a metal surface. Building block 2 — variable oxidation state: the metal can temporarily change its oxidation number (e.g. Rh(I) → Rh(III) as ${\text{H}}_2$ adds on, then back). This lets it hold the reactant, rearrange it, and release the product, returning to the start — a genuine catalytic cycle. See Oxidation states of transition metals. Synthesis: because every catalyst molecule is identical and dissolved, each one binds and orients the substrate the same way → high selectivity and mild conditions. It is a reusable molecular workbench: dock → transform → eject → repeat.

Claim under test: charge alone ($+2$) determines O₂ carrying. Verdict: false — here is the chain.

1. Carrying O₂ = reversibly forming and breaking an Fe–O₂ bond. That requires the metal's $d$-orbitals to accept O₂'s electrons and back-donate — a redox-capable metal.
2. Fe(II) is a transition metal with partly filled $d$-orbitals → it can do this delicate electron give-and-take (and can be pushed to Fe(III), which is why oxidation to met-Hb must be avoided).
3. Mg(II) has a noble-gas electron configuration — no accessible $d$-electrons, redox-inert. It cannot form the reversible dative O₂ bond at all; it would just sit there or bind water.
4. Matching charge ($+2$) does not match electronic capability. Chlorophyll deliberately uses Mg(II) precisely because it won't do redox (Q3.2). Conclusion: the swap fails — same charge, wrong electronics. Function is set by the $d$-electron chemistry, not by the ionic charge.

Unifying principle: A metal ion's usefulness is tuned by wrapping it in the right ligand cage with the right geometry, leaving a controlled reactive site. Fe(II) + porphyrin + one open axial site → reversible O₂ transport. Mg(II) + porphyrin (redox-inert core) → rigid light antenna. Co(III) + corrin + a metal–carbon axial bond → DNA-building enzyme. Pt(II) + square-planar geometry + two cis leaving groups → a DNA crosslinker. Change the metal, the ring, the oxidation state, or the geometry — and you change the job entirely. The cage makes the function.