Chemistry of transition elements
Transition element properties
A transition element forms one or more stable ions with an incomplete d subshell (so Sc and Zn are d-block but not transition elements). Configurations: 4s fills before 3d but is emptied first on ionisation; Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹. Properties: variable oxidation states (similar 4s and 3d energies), coloured ions, complex formation and catalysis (Fe in the Haber process, V₂O₅ in the Contact process, Fe²⁺/Fe³⁺ in I⁻/S₂O₈²⁻). Redox titrations: (self-indicating: purple to colourless); reacts 1 : 6 with Fe²⁺.
≈ 50 minComplex ions, ligands and colour
A complex ion is a central metal ion with ligands bonded by dative (coordinate) bonds. Coordination number = number of dative bonds: 6 → octahedral ([Cu(H₂O)₆]²⁺); 4 → tetrahedral ([CuCl₄]²⁻, with large Cl⁻) or square planar (cisplatin, Pt); 2 → linear ([Ag(NH₃)₂]⁺). Ligands may be monodentate (H₂O, NH₃, Cl⁻), bidentate (en, C₂O₄²⁻) or polydentate (EDTA⁴⁻, 6 bonds). Colour: ligands split the d orbitals; an electron absorbs visible light of energy to jump up; the complementary colour is seen. Changing ligand, coordination number or oxidation state changes ΔE and so the colour. Ligand exchange is favoured by larger and by the chelate effect (entropy increase). Complexes show cis/trans and optical isomerism.
≈ 55 minReactions of aqua ions
Metal ions in water exist as hexaaqua ions [M(H₂O)₆]ⁿ⁺. 3+ ions are more acidic than 2+: higher charge density polarises the O–H bonds in water ligands, releasing H⁺. With OH⁻ or a little NH₃, ions are deprotonated to neutral hydroxide precipitates: Cu²⁺ blue; Fe²⁺ green (turns brown in air); Fe³⁺ orange-brown; Cr³⁺ grey-green; Al³⁺ white; Mn²⁺ off-white (browns). Excess NaOH dissolves amphoteric Al(OH)₃ and Cr(OH)₃. Excess NH₃ causes ligand exchange: Cu²⁺ gives deep blue ; Cr³⁺ gives purple . With carbonate, 2+ ions give MCO₃ precipitates; 3+ ions are acidic enough to give the hydroxide + CO₂.
≈ 45 min