1. Double salts, coordination compounds and Werner’s theory
Double salt (potash alum, carnallite, Mohr’s salt): exists only as a solid; dissociates completely into simple ions in water. Coordination compound (K₄[Fe(CN)₆], [Cu(NH₃)₄]SO₄): the complex ion stays intact in water.
Werner (1898): primary valency — ionisable, satisfied by anions, = oxidation state; secondary valency — non-ionisable, satisfied by ligands, = coordination number, fixed for a metal and directed in space (6 → octahedral; 4 → tetrahedral or square planar).
Evidence: AgNO₃ precipitates only Cl⁻ outside the sphere; conductivity depends on number of ions: [Co(NH₃)₆]Cl₃ (4 ions, 3 AgCl), [CoCl(NH₃)₅]Cl₂ (3, 2), [CoCl₂(NH₃)₄]Cl (2, 1), [CoCl₃(NH₃)₃] (0, 0).
| Formula | Ions | mol AgCl |
|---|---|---|
| [Co(NH₃)₆]Cl₃ | 4 | 3 |
| [CoCl(NH₃)₅]Cl₂ | 3 | 2 |
| [CoCl₂(NH₃)₄]Cl | 2 | 1 |
| [CoCl₃(NH₃)₃] | 0 | 0 |
Key terms
- Double salt
A compound of two salts that exists only in the solid and gives all its simple ions in water.
- Primary valency
Ionisable valency of the metal, equal to its oxidation state.
- Secondary valency
Non-ionisable valency, equal to the coordination number and directed in space.
2. Ligands, coordination number and oxidation state
Ligand: Lewis base donating an electron pair to the central atom (Lewis acid). Denticity: unidentate (NH₃, Cl⁻), didentate (en, C₂O₄²⁻), tridentate (dien), tetradentate (trien), pentadentate (EDTA³⁻), hexadentate (EDTA⁴⁻). Chelating ligand: polydentate ligand forming a ring; chelates are extra stable (chelate effect). Ambidentate: NO₂⁻/ONO⁻, SCN⁻/NCS⁻, CN⁻/NC⁻.
Coordination number = number of donor atoms. Oxidation state: charge on complex = OS + sum of ligand charges. Homoleptic (one kind of ligand) vs heteroleptic.
Key terms
- Chelate effect
Greater stability of complexes with chelating ligands than of similar complexes with unidentate ligands.
- Ambidentate ligand
A ligand that can bind through either of two different donor atoms.
- Coordination number
The number of donor atoms bonded to the central metal.
3. IUPAC nomenclature
- Cation first, then anion. 2. Ligands in alphabetical order (ignore di-, tri-), then metal. 3. di/tri/tetra; bis/tris/tetrakis for ligands with numbers in their names (en). 4. Anionic ligands end in -o (chlorido, cyanido, hydroxido, oxalato); neutral: aqua, ammine, carbonyl, nitrosyl. 5. Metal OS in Roman numerals. 6. Anionic complex: metal ends in -ate (ferrate, cuprate, argentate, aurate, plumbate, stannate, zincate, cobaltate).
Examples: [CrCl₂(H₂O)₄]Cl — tetraaquadichloridochromium(III) chloride; Na₂[Zn(OH)₄] — sodium tetrahydroxidozincate(II); [Fe(CO)₅] — pentacarbonyliron(0); K₃[Al(C₂O₄)₃] — potassium trioxalatoaluminate(III).
| Ligand | Name |
|---|---|
| H₂O | aqua |
| NH₃ | ammine |
| CO | carbonyl |
| Cl⁻ | chlorido |
| CN⁻ | cyanido |
| NO₂⁻ (N) | nitrito-N |
Key terms
- Ammine
The name for NH₃ as a ligand.
- -ate ending
Used for the metal when the complex ion is an anion.
4. Isomerism
Structural: ionisation ([Pt(NH₃)₄Cl₂]Br₂ / [Pt(NH₃)₄Br₂]Cl₂); linkage ([Mn(CO)₅(SCN)] / [Mn(CO)₅(NCS)]); coordination ([Cu(NH₃)₄][PtCl₄] / [Pt(NH₃)₄][CuCl₄]); hydrate (three forms of CrCl₃·6H₂O).
Geometrical: cis/trans in square planar MA₂B₂ and octahedral MA₂B₄; fac/mer in MA₃B₃; none in tetrahedral complexes.
Optical: non-superimposable mirror images (d, l): [M(AA)₃] and cis-[MA₂(AA)₂]; trans forms have a mirror plane. [CoCl₂(en)₂]⁺ and [Cr(C₂O₄)₂(H₂O)₂]⁻ each have 3 stereoisomers.
Key terms
- Linkage isomerism
Isomerism due to an ambidentate ligand binding through different donor atoms.
- Enantiomers
Non-superimposable mirror-image isomers that rotate plane-polarised light in opposite directions.
Exam tips
Common mistakes
- Counting ligands instead of donor atoms for the coordination number.
- Writing "amine" for NH₃ or using -ate for cationic complexes.
- Alphabetising by prefixes (di-, tetra-).
- Claiming geometrical isomers for tetrahedral complexes, or optical activity for trans isomers.
How marks are usually awarded
- Werner formula (2 marks): ionisable Cl⁻ from AgCl data; formula with correct CN.
- Names/formulas (1 mark each): order, oxidation state and -ate ending all correct.
- Isomerism (2–3 marks): type named and illustrated with formulas or diagrams.