General characteristic chemical properties of the transition elements, titanium to copper

Chemistry A2 · Chemistry of transition elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A ligand is a species with a lone pair of electrons that forms a dative covalent bond to a central metal atom or ion. The metal ion with its ligands is a complex. The coordination number is the number of dative bonds to the metal, not the number of ligands.

One ligand can replace another. This is ligand exchange, and it often changes the colour, and sometimes the shape and the charge, of the complex.

Transition element ions also take part in redox reactions, because they can change oxidation state. E^⦵ values show which reactions are feasible, and titrations with manganate(VII) or iodine and thiosulfate are used to find amounts.

Rules to remember

Monodentate (one bond): H₂O, NH₃, Cl⁻, CN⁻. Bidentate (two): 1,2-diaminoethane (en), C₂O₄²⁻. Polydentate: EDTA⁴⁻ forms six bonds.
Shapes: octahedral (6 bonds, 90°); tetrahedral (4, 109.5°); square planar (4, 90°); linear (2, 180°).
Charge on a complex = charge on the metal ion + total charge of the ligands.
Copper(II): [Cu(H₂O)₆]²⁺ blue; with OH⁻ or a little NH₃, pale blue precipitate of Cu(OH)₂; excess NH₃ gives deep blue [Cu(NH₃)₄(H₂O)₂]²⁺; concentrated HCl gives yellow [CuCl₄]²⁻.
Cobalt(II): [Co(H₂O)₆]²⁺ pink; with OH⁻, blue precipitate of Co(OH)₂; excess NH₃ gives [Co(NH₃)₆]²⁺; concentrated HCl gives blue [CoCl₄]²⁻.
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O (1 : 5); 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O (2 : 5)
2Cu²⁺ + 4I⁻ → 2CuI + I₂, then I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻; so moles of Cu²⁺ = moles of S₂O₃²⁻.

Common mistake

Students count ligands instead of bonds. In [Cr(C₂O₄)₃]³⁻ there are three ligands but each is bidentate, so the coordination number is 6 and the shape is octahedral.

Worked example

25.0 cm³ of a solution of Fe²⁺ ions is acidified and titrated with 0.0200 mol dm⁻³ KMnO₄; 22.50 cm³ is needed. What is the concentration of Fe²⁺?

Moles of MnO₄⁻ = 0.0200 × (22.50 ÷ 1000) = 4.50 × 10⁻⁴ mol.
MnO₄⁻ gains 5 electrons and Fe²⁺ loses 1, so the ratio is 1 MnO₄⁻ : 5 Fe²⁺.
Moles of Fe²⁺ = 5 × 4.50 × 10⁻⁴ = 2.25 × 10⁻³ mol.
Concentration = 2.25 × 10⁻³ ÷ (25.0 ÷ 1000) = 0.0900 mol dm⁻³.

Answer: [Fe²⁺] = 0.0900 mol dm⁻³

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