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Concept 03 / 06The d and f Block Elements

Characteristic Properties: Magnetism, Colour, Complex Formation, Catalysis, Interstitial Compounds and Alloys

Paramagnetism and the spin-only formula, colour of transition metal ions from d-d transitions and colours of common aqueous ions, formation of complex compounds, catalytic properties, interstitial compounds and their properties, and alloy formation.

Figure 1
Magnetic properties. Substances with unpaired electrons are paramagnetic (attracted by a magnetic field); those with all electrons paired are diamagnetic. Most transition metal ions have unpaired d electrons. For 3d ions, the orbital contribution is largely quenched, and the spin-only formula gives the moment (see figure): μ=n(n+2)\mu = \sqrt{n(n + 2)} BM (Bohr magnetons), where nn is the number of unpaired electrons. n=1n = 1: 1.731.73; 22: 2.832.83; 33: 3.873.87; 44: 4.904.90; 55: 5.925.92 BM. Measuring μ\mu reveals nn and hence the oxidation state and electron arrangement. Colour. Most transition metal compounds are coloured. In the presence of ligands, the five d orbitals split into groups of different energy. An electron can absorb a photon of visible light and jump between these levels (d-d transition); the colour observed is complementary to the colour absorbed. Ions with d0d^{0} (Sc3+\mathrm{Sc^{3+}}, Ti4+\mathrm{Ti^{4+}}) or d10d^{10} (Zn2+\mathrm{Zn^{2+}}, Cu+\mathrm{Cu^{+}}) configurations are colourless, since no d-d transition is possible. Colours in water: Ti3+\mathrm{Ti^{3+}} purple, V3+\mathrm{V^{3+}} green, V2+\mathrm{V^{2+}} violet, VO2+\mathrm{VO^{2+}} (V4+\mathrm{V^{4+}}) blue, Cr3+\mathrm{Cr^{3+}} violet (green in chloride complexes), Mn2+\mathrm{Mn^{2+}} pale pink, Fe2+\mathrm{Fe^{2+}} pale green, Fe3+\mathrm{Fe^{3+}} yellow, Co2+\mathrm{Co^{2+}} pink, Ni2+\mathrm{Ni^{2+}} green, Cu2+\mathrm{Cu^{2+}} blue. Some d0d^{0} species are intensely coloured for a different reason: MnO4−\mathrm{MnO_{4}^{-}} (purple) and Cr2O72−\mathrm{Cr_{2}O_{7}^{2-}} (orange) owe their colour to charge transfer from oxygen to the metal. Complex formation. Transition metal ions form many complexes, such as [Fe(CN)6]3−\mathrm{[Fe(CN)_{6}]^{3-}}, [Cu(NH3)4]2+\mathrm{[Cu(NH_{3})_{4}]^{2+}}, [Ni(CN)4]2−\mathrm{[Ni(CN)_{4}]^{2-}}, because of their small size, high charge, and availability of vacant d orbitals of suitable energy to accept lone pairs from ligands. Catalytic properties. Many transition metals and their compounds are catalysts, owing to their ability to adopt multiple oxidation states and to form complexes and to adsorb reactants on their surfaces (bonds with reactant molecules using 3d and 4s electrons), which increases reactant concentration at the surface and weakens reactant bonds. Examples: V2O5\mathrm{V_{2}O_{5}} (contact process), finely divided Fe (Haber process), Ni (hydrogenation), TiCl4+Al(CH3)3\mathrm{TiCl_{4} + Al(CH_{3})_{3}} (Ziegler-Natta polymerisation), Pt/Rh (Ostwald process). In solution, Fe3+\mathrm{Fe^{3+}} catalyses the reaction of iodide with peroxodisulfate by switching between oxidation states: 2Fe3++2I−→2Fe2++I2\mathrm{2Fe^{3+} + 2I^{-} \rightarrow 2Fe^{2+} + I_{2}}, then 2Fe2++S2O82−→2Fe3++2SO42−\mathrm{2Fe^{2+} + S_{2}O_{8}^{2-} \rightarrow 2Fe^{3+} + 2SO_{4}^{2-}}. Interstitial compounds. Small atoms (H, C, N, B) trapped in the interstitial holes of metal lattices: TiC, Mn4N\mathrm{Mn_{4}N}, Fe3H\mathrm{Fe_{3}H}, VH0.56\mathrm{VH_{0.56}}, TiH1.7\mathrm{TiH_{1.7}}. They are usually non-stoichiometric, neither typically ionic nor covalent. Properties: very high melting points (higher than the pure metals), very hard (some borides approach diamond), retain metallic conductivity, and are chemically inert. Alloy formation. Because transition metals have similar atomic radii (within about 15%15\%), atoms of one can substitute for another in the lattice, giving alloys: steel and stainless steel (Fe with Cr, Ni, Mn, V, Mo, W), brass (Cu + Zn), bronze (Cu + Sn). Alloys are usually harder and have higher melting points than the component metals. Key Takeaways: - μ=n(n+2)\mu = \sqrt{n(n + 2)} BM; maximum 5.925.92 for d5d^{5}. - Colour from d-d transitions; d0d^{0} and d10d^{10} ions are colourless (except charge-transfer species like MnO4−\mathrm{MnO_{4}^{-}}). - Complexes form due to small, highly charged ions with vacant d orbitals. - Catalysis via variable oxidation states and surface adsorption. - Interstitial compounds: hard, high-melting, conducting, non-stoichiometric. - Pitfall: KMnO4\mathrm{KMnO_{4}} is intensely coloured although Mn7+\mathrm{Mn^{7+}} is d0d^{0}; the colour is charge transfer, not d-d. Brain-Teaser 1: Calculate the spin-only magnetic moments of Fe2+\mathrm{Fe^{2+}}, Fe3+\mathrm{Fe^{3+}} and Cu2+\mathrm{Cu^{2+}} ions in water. Solution: Fe2+\mathrm{Fe^{2+}} 3d63d^{6}, 44 unpaired: 24=4.90\sqrt{24} = 4.90 BM. Fe3+\mathrm{Fe^{3+}} 3d53d^{5}, 55 unpaired: 35=5.92\sqrt{35} = 5.92 BM. Cu2+\mathrm{Cu^{2+}} 3d93d^{9}, 11 unpaired: 3=1.73\sqrt{3} = 1.73 BM. Brain-Teaser 2: Which of these ions are coloured in aqueous solution: Sc3+\mathrm{Sc^{3+}}, Ti3+\mathrm{Ti^{3+}}, Cu+\mathrm{Cu^{+}}, Zn2+\mathrm{Zn^{2+}}, Co2+\mathrm{Co^{2+}}, V3+\mathrm{V^{3+}}? Solution: Ti3+\mathrm{Ti^{3+}} (d1d^{1}, purple), Co2+\mathrm{Co^{2+}} (d7d^{7}, pink) and V3+\mathrm{V^{3+}} (d2d^{2}, green) are coloured. Sc3+\mathrm{Sc^{3+}} (d0d^{0}), Cu+\mathrm{Cu^{+}} (d10d^{10}) and Zn2+\mathrm{Zn^{2+}} (d10d^{10}) are colourless. Brain-Teaser 3: A 3d ion in the +2+2 state has a spin-only moment of 3.873.87 BM. Identify possible ions. Solution: n(n+2)=3.87\sqrt{n(n + 2)} = 3.87 gives n=3n = 3 unpaired electrons: d3d^{3} or d7d^{7} (high spin). M2+\mathrm{M^{2+}} with d3d^{3} is V2+\mathrm{V^{2+}}; with d7d^{7} is Co2+\mathrm{Co^{2+}}.

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Characteristic Properties: Magnetism, Colour, Complex Formation, Catalysis, Interstitial Compounds and Alloys — DPP

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