Transition metals

Complexes, colour, catalysis, reactions of aqueous ions

Four characteristic properties

Transition metals share four characteristic properties: variable oxidation states, coloured ions, catalytic activity and complex formation. If a question asks what makes them special, this quartet is the answer.

Complexes

A ligand donates a lone pair to the metal ion, forming a co-ordinate bond, and the co-ordination number is the number of these bonds. [Cu(H₂O)₆]²⁺ is octahedral with a co-ordination number of 6.

Bidentate ligands such as ethanedioate and ethane-1,2-diamine form two co-ordinate bonds each, while hexadentate EDTA⁴⁻ forms six. Swapping monodentate for multidentate ligands is entropy-driven — this is the chelate effect, favourable because more particles are 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 and jump between split d-levels, with ΔE = hν. Change the ligand, oxidation state or co-ordination number and the colour changes, because the size of the split changes.

Catalysis

Heterogeneous catalysts work in a different phase from the reactants: Fe in the Haber process and V₂O₅ in the Contact process. Homogeneous catalysts share the reactants' phase, like Fe²⁺ catalysing S₂O₈²⁻ + I⁻. Autocatalysis is when a product catalyses its own reaction — Mn²⁺ in the MnO₄⁻/C₂O₄²⁻ reaction, which speeds up as product forms.

3.2.5Ligand substitution — the three patterns

Pattern one: similar-size ligands swap with no change in number. [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O, and the colour goes pale to deep blue.

Pattern two: a bigger ligand comes in and the co-ordination number drops 6 → 4. [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O — blue to yellow-green, and the shape becomes tetrahedral.

Pattern three: a multidentate ligand replaces monodentate ones (the chelate effect). [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O turns 2 particles into 7, so ΔS is strongly positive while ΔH ≈ 0 (same number of similar bonds), making ΔG negative.