Transition metals

Complexes, colour, catalysis, reactions of aqueous ions

Four characteristic properties

• Variable oxidation states · coloured ions · catalytic activity · complex formation.

Complexes

• A ligand donates a lone pair to the metal ion (co-ordinate bond); the co-ordination number is the number of bonds. [Cu(H₂O)₆]²⁺ is octahedral, 6.
• Bidentate: ethanedioate, ethane-1,2-diamine. Hexadentate: EDTA⁴⁻. Swapping monodentate for multidentate ligands is entropy-driven (chelate effect — more particles released).

Colours to know cold

• [Cu(H₂O)₆]²⁺ pale blue · +NH₃(excess) deep royal blue · Fe²⁺(aq) pale green · Fe³⁺(aq) yellow-brown
• Hydroxide ppts: Cu(OH)₂ blue · Fe(OH)₂ green (browns on standing in air) · Fe(OH)₃ orange-brown
• Colour arises when d-electrons absorb visible light jumping between split d-levels: ΔE = hν. Change the ligand, oxidation state or co-ordination number and the colour changes.

Catalysis

• Heterogeneous: Fe (Haber), V₂O₅ (Contact process). Homogeneous: Fe²⁺ catalysing S₂O₈²⁻ + I⁻. Autocatalysis: Mn²⁺ in MnO₄⁻/C₂O₄²⁻ — the reaction speeds up as product forms.

3.2.5Ligand substitution — the three patterns

• Similar-size ligands, no change in number: [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O (pale → deep blue).
• Bigger ligand, co-ordination drops 6 → 4: [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O (blue → yellow-green, tetrahedral).
• Multidentate replaces monodentate (chelate effect): [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O — 2 particles become 7, ΔS strongly positive, ΔH ≈ 0 (same number of similar bonds), so ΔG negative.