3.4.15 · D5Coordination Chemistry
Question bank — Applications — biological (haemoglobin, chlorophyll, vit B₁₂), medicinal (cisplatin), industrial (catalysts)
This page assumes you have met the parent note Applications of Coordination Compounds. Words like macrocycle, axial site, cis/trans, reversible binding are all built there — flip back if any feels unearned.
True or false — justify
Haemoglobin carries oxygen because iron reacts with (oxidises by) O₂.
False — the whole point is that Fe(II) binds O₂ reversibly without changing its oxidation state; if it truly oxidised to Fe(III) you get useless methaemoglobin.
Chlorophyll and haemoglobin have chemically identical central metals doing the same kind of job.
False — both sit in a porphyrin ring, but Mg(II) is redox-inert and only holds the light-catching ring rigid, while Fe(II) is redox-active and binds a gas molecule.
The corrin ring in vitamin B₁₂ is just another name for porphyrin.
False — corrin is a smaller, more contracted macrocycle (one fewer bridging carbon), still 4 N-donors, but distinct from porphyrin.
Cisplatin and transplatin have the same molecular formula, so they are the same compound.
False — same formula but different spatial arrangement = geometric isomers; only the cis one bites two adjacent DNA bases, so only cis is the drug. (See Isomerism in coordination compounds.)
A macrocyclic ligand binds the metal through only one donor atom at a time.
False — the defining feature is binding through several donor atoms at once (porphyrin uses 4 N in a plane), which is exactly why it grips so tightly (the chelate/macrocyclic effect, Stability and chelate effect).
In haemoglobin all six coordination sites of iron are occupied by the porphyrin ring.
False — porphyrin gives only 4 equatorial N-donors; the 5th axial site holds a histidine of the protein and the 6th is the free site O₂ binds to.
Wilkinson's and Ziegler–Natta catalysts are heterogeneous (solid) catalysts.
False — Wilkinson's is a classic homogeneous (dissolved) catalyst; the metal complex works in the same phase as the reactants.
Mg(II) in chlorophyll can cycle through several oxidation states during photosynthesis.
False — Mg(II) is deliberately redox-inert; the ring passes the excited electron along, not the magnesium.
Spot the error
"CO is toxic because it chemically destroys the haemoglobin molecule."
Error — CO out-competes O₂ for the same 6th axial site (~200× stronger, effectively irreversible). It's coordination competition, not destruction; treatment = high O₂ to shift the equilibrium.
"Cisplatin works because Pt is octahedral, giving six sites to grab DNA."
Error — Pt(II) is d⁸ → square planar, four sites in a plane. Its two cis chlorides (90° apart) crosslink two adjacent bases; geometry, not six sites, is the mechanism. (See Square planar complexes.)
"Methaemoglobin can't carry O₂ because its iron fell out of the ring."
Error — the Fe is still in the porphyrin; it is simply oxidised to Fe(III), which binds water/OH⁻ tightly and blocks the reversible O₂ site.
"Transplatin is a better anticancer drug because trans complexes are more stable."
Error — stability is irrelevant here; transplatin's two Cl are 180° apart so they cannot reach two neighbouring DNA bases → biologically inactive.
"Chlorophyll is green because it absorbs green light."
Error — it absorbs red and blue and reflects/transmits green; the colour you see is the light it does not absorb (complementary colour).
"The metal-carbon bond in vitamin B₁₂ is common throughout human biochemistry."
Error — the Co–C bond in B₁₂ is famously the only metal–carbon bond in human biochemistry.
"Ziegler–Natta catalysis needs no metal complex — TiCl₄ alone polymerises ethene."
Error — it needs the combination ; the Al alkyl activates Ti and creates the active coordination site.
Why questions
Why must iron in haemoglobin specifically be Fe(II), not Fe(III)?
The reversible O₂–Fe(II) interaction is finely balanced so O₂ is grabbed in the lungs and dropped in tissues; Fe(III) binds water so tightly the site stays blocked.
Why does nature choose Mg for chlorophyll but Fe for haemoglobin?
Chlorophyll's job is light harvesting, which wants a redox-inert metal that just holds the conjugated ring rigid; haemoglobin's job is gas binding, which wants a redox-flexible, gas-binding Fe(II).
Why do only the two cis chlorides of cisplatin leave and get replaced by DNA nitrogens?
They hydrolyse in the cell's low-Cl⁻ interior, and being 90° apart they leave two adjacent open sites that fit two neighbouring guanine N atoms — a 90° bite that crosslinks DNA.
Why are variable oxidation states useful for a catalyst?
The metal can accept and donate electrons as it binds, activates, and releases substrate (e.g. splitting H₂ onto Rh), letting one metal centre run a redox cycle over and over.
Why does a macrocycle leave the axial positions "free" instead of wrapping the whole metal?
A flat 4-N ring saturates only the equatorial plane, deliberately leaving the two axial sites open for the reactive chemistry (O₂ binding, histidine anchoring, –CN in B₁₂).
Why is homogeneous (dissolved) catalysis often more selective than a metal surface?
A dissolved complex has a defined geometry around one metal centre, so it holds the substrate in one precise orientation, whereas a surface offers many uncontrolled sites.
Edge cases
Edge case: what happens to O₂ transport if every haemoglobin iron is oxidised to Fe(III)?
You get pure methaemoglobin — no reversible O₂ uptake at all, because every 6th site is blocked by water/OH⁻; this is lethal methaemoglobinaemia.
Edge case: if a patient inhales pure CO, why doesn't simply "breathing normal air" fix it?
CO binds ~200× more strongly and effectively irreversibly, so normal can't displace it; you must supply high-concentration O₂ to force the equilibrium back.
Edge case: could a trans Pt complex ever crosslink DNA at two adjacent bases?
No — its leaving groups are fixed at 180°, geometrically unable to reach two neighbouring sites; this is a hard geometric limit, not a matter of concentration.
Edge case: what is the biological effect of severe vitamin B₁₂ deficiency?
Impaired DNA synthesis and red-blood-cell formation → pernicious anaemia, since B₁₂'s Co(III)-corrin coenzyme drives those reactions.
Edge case: if you swapped the Fe in haem for the redox-inert Mg, could it still carry O₂?
No — Mg(II) cannot do the reversible electron/gas binding; you'd lose O₂ transport entirely, illustrating why the metal choice matters as much as the ring.
Edge case: does replacing the –CN axial group of B₁₂ with –CH₃ or –adenosyl destroy the vitamin?
No — those are the active coenzyme forms; the –CN form is just the stable storage/vitamin form, all sharing the same Co(III)-corrin core.
Connections
- Isomerism in coordination compounds — cis vs trans decides cisplatin's activity
- Crystal Field Theory — colour of haem/chlorophyll via d-orbital splitting
- Square planar complexes — why d⁸ Pt(II) is square planar
- Stability and chelate effect — why macrocycles grip so tightly
- Oxidation states of transition metals — Fe(II) vs Fe(III), Co(III) matter here