Coordination sphere, polyhedron, oxidation number and types of complexes
The bracketed part is the coordination sphere; its shape is the coordination polyhedron. Work out the metal's oxidation number from the charges of the ligands and the complex.
Coordination sphere and counter ions
The central atom and the ligands attached to it are written inside square brackets. Together they form the coordination sphere. Ions outside the bracket are counter ions, and they are ionisable. In K₄[Fe(CN)₆], the coordination sphere is [Fe(CN)₆]⁴⁻ and the counter ions are four K⁺. In [Co(NH₃)₆]Cl₃, the counter ions are three Cl⁻.
Coordination polyhedra
| Coordination number | Shape (polyhedron) | Examples |
|---|---|---|
| 2 | Linear | [Ag(NH₃)₂]⁺ |
| 4 | Tetrahedral | [Ni(CO)₄], [NiCl₄]²⁻ |
| 4 | Square planar | [PtCl₄]²⁻, [Ni(CN)₄]²⁻ |
| 5 | Trigonal bipyramidal | [Fe(CO)₅] |
| 6 | Octahedral | [Co(NH₃)₆]³⁺, [Fe(CN)₆]³⁻ |
Working out oxidation numbers
Find the oxidation number of the metal in (a) K₃[Fe(C₂O₄)₃] (b) [CoCl(NH₃)₅]Cl₂ (c) [Cu(CN)₄]³⁻ (d) [Ni(CO)₄].
- Charges to use: C₂O₄²⁻ is −2; Cl⁻ and CN⁻ are −1; NH₃ and CO are 0; K⁺ is +1. Let the metal's oxidation number be x.
- (a) Three K⁺ outside, so the complex ion is [Fe(C₂O₄)₃]³⁻. x + 3(−2) = −3, so x = +3.
- (b) Two Cl⁻ outside, so the complex ion is [CoCl(NH₃)₅]²⁺. x + (−1) + 5(0) = +2, so x = +3.
- (c) x + 4(−1) = −3, so x = +1.
- (d) x + 4(0) = 0, so x = 0.
Fe(III), Co(III), Cu(I) and Ni(0).
Homoleptic vs heteroleptic complexes
Activity: prepare potassium ferric oxalate, K₃[Fe(C₂O₄)₃]·3H₂O
This green complex is potassium trioxalatoferrate(III). It is **homoleptic**: three didentate oxalate ions give Fe³⁺ a coordination number of 6. Oxalic acid and potassium hydroxide give the oxalate ions in the beaker, and ferric chloride supplies Fe³⁺.
- 1Dissolve oxalic acid in hot water (about 40 °C) in a small beaker.About 3 g of oxalic acid in 12.5 mL of water.
- 2Add potassium hydroxide (about 3.8 g) a little at a time, stirring until each lot dissolves.KOH turns the acid into oxalate ions. The reaction gives out heat, so add it slowly.
- 3FeCl₃ + 6KOH + 3H₂C₂O₄ → K₃[Fe(C₂O₄)₃] + 3KCl + 6H₂OAdd ferric chloride (about 2.5 g) with constant stirring until it dissolves. The solution turns green.
- 4Filter, then concentrate the green filtrate in a porcelain dish on a water bath.Stop well before it is dry; otherwise KCl also crystallises.
- 5Cool the solution slowly. Green crystals of K₃[Fe(C₂O₄)₃]·3H₂O separate.The complex is sensitive to light, so keep it away from sunlight.
- 6★Filter the crystals, wash them with a 1 : 1 mixture of cold water and alcohol, dry them between filter papers and record the yield.
★ key step
In haemoglobin, each Fe²⁺ is held by four N atoms of the haem ring and one N from the protein. The sixth position binds the oxygen you breathe, completing an octahedral coordination sphere.
1 marks: “Write the oxidation number of Pt in [PtCl₆]²⁻.” x + 6(−1) = −2, so x = +4.
2 marks: “Define homoleptic and heteroleptic complexes with one example each.” Homoleptic: only one kind of ligand, [Co(NH₃)₆]³⁺. Heteroleptic: more than one kind, [CoCl₂(NH₃)₄]⁺.
2 marks: “What is a coordination polyhedron? Name the polyhedron in [Co(NH₃)₆]³⁺ and in [Ni(CO)₄].” The spatial arrangement of the ligand donor atoms around the central atom. Octahedral and tetrahedral.
Inside the bracket is the sphere; its shape is the polyhedron. Oxidation number: metal + ligand charges = charge on the bracket.