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Concept 06 / 06Coordination Compounds

Metal Carbonyls, Stability of Complexes and Applications of Coordination Compounds

Structures of homoleptic metal carbonyls and the synergic sigma donation and pi back-bonding in the metal-carbon bond, stepwise and overall stability constants, factors affecting stability including the chelate effect, and applications of coordination compounds in analysis, metallurgy, biology, medicine, catalysis and photography.

Figure 1
Metal carbonyls. Homoleptic carbonyls contain only CO ligands: [Ni(CO)4]\mathrm{[Ni(CO)_{4}]} (tetrahedral), [Fe(CO)5]\mathrm{[Fe(CO)_{5}]} (trigonal bipyramidal), [Cr(CO)6]\mathrm{[Cr(CO)_{6}]} (octahedral), [Mn2(CO)10]\mathrm{[Mn_{2}(CO)_{10}]} (two square pyramidal Mn(CO)5\mathrm{Mn(CO)_{5}} units joined by a Mn-Mn bond), [Co2(CO)8]\mathrm{[Co_{2}(CO)_{8}]} (a Co-Co bond bridged by two CO groups). The metal is in a low oxidation state (often zero). Bonding (see figure): the M-C bond has both σ\sigma and π\pi character. CO donates its carbon lone pair into an empty metal orbital (σ\sigma bond); the metal donates electrons from a filled d orbital into the empty antibonding π∗\pi^{*} orbital of CO (π\pi back-bond). The two effects reinforce each other (synergic bonding): the M-C bond becomes stronger, and the C-O bond weaker (lower C-O stretching frequency than in free CO). This explains why CO, though neutral and a weak base, is the strongest ligand in the spectrochemical series. Stability of complexes. In solution, complex formation proceeds stepwise. For M+4L⇌ML4\mathrm{M + 4L \rightleftharpoons ML_{4}}: stepwise constants: M+L⇌ML\mathrm{M + L \rightleftharpoons ML}, K1=[ML][M][L]K_{1} = \frac{[\mathrm{ML}]}{[\mathrm{M}][\mathrm{L}]}, and so on to K4K_{4}; overall stability constant: β4=K1K2K3K4=[ML4][M][L]4\beta_{4} = K_{1}K_{2}K_{3}K_{4} = \frac{[\mathrm{ML_{4}}]}{[\mathrm{M}][\mathrm{L}]^{4}}. A larger β\beta means a more stable complex; the instability constant is 1β\frac{1}{\beta}. For [Cu(NH3)4]2+\mathrm{[Cu(NH_{3})_{4}]^{2+}}, β4=4.5×1011\beta_{4} = 4.5 \times 10^{11}. Usually K1>K2>K3>…K_{1} > K_{2} > K_{3} > \ldots. Factors: higher charge and smaller size of the metal ion; more basic ligands; chelation (the chelate effect: a ring-forming ligand gives a much more stable complex than equivalent unidentate ligands, mainly for entropy reasons); crystal field stabilisation. Applications. Analytical chemistry: detection and estimation of metal ions: Ni2+\mathrm{Ni^{2+}} with dimethylglyoxime (red precipitate); Cu2+\mathrm{Cu^{2+}} with ammonia (deep blue); Fe3+\mathrm{Fe^{3+}} with thiocyanate (blood red). Water hardness is estimated by EDTA titration of Ca2+\mathrm{Ca^{2+}} and Mg2+\mathrm{Mg^{2+}}. Metallurgy: extraction of silver and gold as cyanide complexes [Ag(CN)2]−\mathrm{[Ag(CN)_{2}]^{-}}, [Au(CN)2]−\mathrm{[Au(CN)_{2}]^{-}}; purification of nickel via [Ni(CO)4]\mathrm{[Ni(CO)_{4}]} (Mond process). Biology: chlorophyll is a magnesium complex; haemoglobin (oxygen carrier) is an iron complex; vitamin B12\mathrm{B_{12}} (cyanocobalamin) is a cobalt complex; many enzymes contain metal ions. Medicine: EDTA and related chelating agents remove excess toxic metals (lead poisoning); D-penicillamine and desferrioxamine B for copper and iron excess; cisplatin, cis-[Pt(NH3)2Cl2]\mathrm{[Pt(NH_{3})_{2}Cl_{2}]}, inhibits the growth of tumours. Catalysis: Wilkinson's catalyst [(Ph3P)3RhCl]\mathrm{[(Ph_{3}P)_{3}RhCl]} for hydrogenation of alkenes; Ziegler-Natta catalysts. Photography: undeveloped AgBr is dissolved by hypo as [Ag(S2O3)2]3−\mathrm{[Ag(S_{2}O_{3})_{2}]^{3-}}. Electroplating: smooth, even coatings from [Ag(CN)2]−\mathrm{[Ag(CN)_{2}]^{-}} and [Au(CN)2]−\mathrm{[Au(CN)_{2}]^{-}} solutions, which release metal ions slowly. Key Takeaways: - Carbonyl shapes: Ni(CO)4\mathrm{Ni(CO)_{4}} tetrahedral, Fe(CO)5\mathrm{Fe(CO)_{5}} trigonal bipyramidal, Cr(CO)6\mathrm{Cr(CO)_{6}} octahedral. - Synergic bonding: σ\sigma donation and π\pi back-donation; M-C strengthened, C-O weakened. - βn=K1K2…Kn\beta_{n} = K_{1}K_{2}\ldots K_{n}; chelation increases stability. - Applications: EDTA (hardness, lead poisoning), cyanide extraction, Mond process, haemoglobin, chlorophyll, B12\mathrm{B_{12}}, cisplatin, Wilkinson's catalyst, hypo in photography. - Pitfall: the C-O bond in a metal carbonyl is weaker (longer) than in free CO because of back-donation into π∗\pi^{*}. Brain-Teaser 1: Explain why the C-O stretching frequency is lower in [Fe(CO)5]\mathrm{[Fe(CO)_{5}]} than in free CO, and why it is even lower in [Fe(CO)4]2−\mathrm{[Fe(CO)_{4}]^{2-}}. Solution: Back-donation from filled metal d orbitals into CO π∗\pi^{*} orbitals lowers the C-O bond order. In the anion [Fe(CO)4]2−\mathrm{[Fe(CO)_{4}]^{2-}}, the metal is more electron-rich (negative charge), so back-donation is stronger, the C-O bond weaker, and the stretching frequency lower still. Brain-Teaser 2: For [Cu(NH3)4]2+\mathrm{[Cu(NH_{3})_{4}]^{2+}}, β4=4.5×1011\beta_{4} = 4.5 \times 10^{11}. Find the instability constant and log⁡β4\log\beta_{4}. Solution: Instability constant =1β4=2.2×10−12= \frac{1}{\beta_{4}} = 2.2 \times 10^{-12}. log⁡β4=log⁡(4.5×1011)=11.65\log\beta_{4} = \log(4.5 \times 10^{11}) = 11.65. Brain-Teaser 3: Why is cisplatin, and not its trans isomer, an effective anticancer drug? Solution: In cis-[Pt(NH3)2Cl2]\mathrm{[Pt(NH_{3})_{2}Cl_{2}]}, the two chloride ligands are adjacent. After they are replaced by water inside the cell, the platinum binds to two neighbouring guanine bases on the same DNA strand, forming a crosslink that distorts DNA and blocks replication. The trans geometry cannot form this adjacent crosslink.

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