Metal carbonyls and applications

The essentials

Metal carbonyls contain carbon monoxide as the ligand. Examples are tetracarbonylnickel(0) [Ni(CO)₄] (tetrahedral), pentacarbonyliron(0) [Fe(CO)₅] (trigonal bipyramidal) and hexacarbonylchromium(0) [Cr(CO)₆] (octahedral). The M–C bond has both σ and π character.

Key points

• Synergic bonding: CO donates its carbon lone pair into a vacant metal orbital (σ bond), and the metal donates electrons from filled d orbitals back into the empty antibonding π* orbitals of CO (π back-bonding). Each strengthens the other, making the M–CO bond strong.
• Analytical uses: EDTA titration estimates hardness of water (Ca²⁺ and Mg²⁺); dimethylglyoxime tests for Ni²⁺.
• Metallurgy: silver and gold are extracted as cyanide complexes [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ and recovered with zinc; nickel is purified by forming volatile Ni(CO)₄ (Mond process) which decomposes to pure Ni.
• Biology: chlorophyll contains Mg; haemoglobin contains Fe; vitamin B₁₂ (cyanocobalamin) contains Co.
• Catalysis: Wilkinson's catalyst [(Ph₃P)₃RhCl] is used for hydrogenation of alkenes.
• Medicine: cis-platin, cis-[Pt(NH₃)₂Cl₂], is used against some cancers; EDTA is used in treating lead poisoning; D-penicillamine removes excess copper and desferrioxamine B removes excess iron by chelation.
• Electroplating: [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ give smooth, even coatings.
• Photography: undecomposed AgBr is removed from film with hypo, forming [Ag(S₂O₃)₂]³⁻.

Exam tips

• Remember the geometry–formula pairs for the three common carbonyls.

Common mistakes

• Thinking the metal–carbonyl bond is purely σ; back-bonding is essential.