Metal carbonyls and uses of coordination compounds
In metal carbonyls, CO gives a σ pair to the metal and takes back d electrons into its empty π* orbital: synergic bonding. Complexes matter in medicine, metallurgy and analysis.
Structures of some metal carbonyls
| Carbonyl | IUPAC name | Structure |
|---|---|---|
| [Ni(CO)₄] | tetracarbonylnickel(0) | Tetrahedral |
| [Fe(CO)₅] | pentacarbonyliron(0) | Trigonal bipyramidal |
| [Cr(CO)₆] | hexacarbonylchromium(0) | Octahedral |
| [Mn₂(CO)₁₀] | decacarbonyldimanganese(0) | Two square pyramidal Mn(CO)₅ units joined by a Mn-Mn bond |
| [Co₂(CO)₈] | octacarbonyldicobalt(0) | A Co-Co bond bridged by two CO groups |
How to draw and explain synergic bonding
- 1σ donation: the lone pair on the carbon atom of CO enters a vacant orbital of the metal. This forms a metal-carbon σ bond.
- 2π back donation: a filled d orbital of the metal gives a pair of electrons into the vacant antibonding π* orbital of CO. This forms a metal-carbon π bond.
- 3σ donation makes the metal electron-rich, which helps back donation; back donation removes that extra charge, which helps σ donation. This mutual help is the synergic effect.
- 4★Result: the metal-carbon bond is strengthened, and the C-O bond is slightly weakened, because electrons enter its antibonding orbital.
- 5Drawing: write M on the left and C≡O on the right. Draw an arrow from the lone pair on C to an empty metal orbital (σ). Draw a curved arrow from a filled metal d lobe to an empty π* lobe on CO (π).
★ key step
Uses in analysis and metallurgy
- Detecting ions: Ni²⁺ gives a red precipitate with dimethylglyoxime (DMG); Cu²⁺ gives a deep blue [Cu(NH₃)₄]²⁺ with ammonia.
- Hardness of water: Ca²⁺ and Mg²⁺ are estimated by titration with Na₂EDTA, because they form stable complexes with it.
- Extraction of gold and silver: the metal dissolves as a cyanide complex and is recovered with zinc: 4Au + 8CN⁻ + 2H₂O + O₂ → 4[Au(CN)₂]⁻ + 4OH⁻, then 2[Au(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Au.
- Purifying nickel (Mond process): impure nickel is turned into volatile [Ni(CO)₄], Ni + 4CO → [Ni(CO)₄], which is then decomposed by stronger heating, [Ni(CO)₄] → Ni + 4CO, giving pure nickel. (Extra, not in your textbook: about 330 to 350 K to form the carbonyl and 450 to 470 K to decompose it.)
- Electroplating: silver and gold plate more smoothly and evenly from solutions of [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ than from simple metal ions.
- Black and white photography: unexposed AgBr on the film is dissolved by hypo: AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr.
Uses in biology, medicine and industry
- Chlorophyll, the green pigment for photosynthesis, is a complex of magnesium. Haemoglobin, the oxygen carrier in blood, is a complex of iron. Vitamin B₁₂ (cyanocobalamin) is a complex of cobalt.
- Chelate therapy: excess copper and iron are removed with the chelating ligands D-penicillamine and desferrioxamine B (spelt desferrioxime B in your textbook); EDTA is used to treat lead poisoning.
- Cisplatin, cis-[PtCl₂(NH₃)₂], and related platinum complexes stop the growth of tumours.
- Catalysis: Wilkinson's catalyst, [RhCl(PPh₃)₃], is used for the hydrogenation of alkenes.
Carbon monoxide from a smoky chulha or a car running in a closed garage binds to the Fe²⁺ of haemoglobin far more strongly than oxygen does. That is why CO poisoning is so dangerous and why kitchens need good ventilation.
2 marks: “How does CO bond to a metal in a metal carbonyl? Why is the metal-carbon bond strong?” σ bond: lone pair on C of CO donated into an empty metal orbital. π bond: filled metal d orbital donates into the empty π* orbital of CO. The synergic effect strengthens the metal-carbon bond.
2 marks: “Give two uses of coordination compounds in medicine.” Cisplatin against cancer; EDTA for lead poisoning (also D-penicillamine for excess copper and desferrioxamine B for excess iron).
2 marks: “How is impure nickel purified using a coordination compound? Write the equations.” Impure nickel is heated with CO to form volatile [Ni(CO)₄]: Ni + 4CO → [Ni(CO)₄]. Stronger heating decomposes it: [Ni(CO)₄] → Ni + 4CO, leaving pure nickel (the Mond process).
CO gives and takes: σ in, π back. Metals of life: Mg in the green leaf, Fe in red blood, Co in B₁₂.