Werner’s theory and key terms

The essentials

Alfred Werner proposed that metals show two kinds of valency. Primary valency is ionisable and corresponds to the oxidation state; it is satisfied by negative ions. Secondary valency is non-ionisable and equals the coordination number; it is satisfied by neutral molecules or negative ions and has a fixed spatial arrangement. For example, CoCl₃·6NH₃ is [Co(NH₃)₆]Cl₃ and gives 3 mol AgCl with excess AgNO₃, while CoCl₃·5NH₃ is [Co(NH₃)₅Cl]Cl₂ and gives 2 mol.

Key points

• Ligand: an ion or molecule bound to the central atom by donating electron pairs. Unidentate (Cl⁻, NH₃, H₂O), didentate (ethane-1,2-diamine "en", oxalate C₂O₄²⁻), polydentate (EDTA⁴⁻ is hexadentate).
• Chelate: a complex where a di- or polydentate ligand binds through two or more donor atoms forming a ring; chelates are more stable.
• Ambidentate ligand: can bind through two different atoms, e.g. NO₂⁻ (through N or O), SCN⁻ (through S or N).
• Coordination number: number of ligand donor atoms directly bonded to the metal. In [Co(en)₃]³⁺ it is 6.
• Coordination sphere: central atom plus ligands, written in square brackets; ions outside are counter ions.
• Coordination polyhedron: the spatial arrangement of ligands: octahedral, tetrahedral, square planar.
• Homoleptic complexes have only one kind of ligand ([Co(NH₃)₆]³⁺); heteroleptic have more than one ([Co(NH₃)₄Cl₂]⁺).
• Double salts such as Mohr's salt and potash alum dissociate completely in water, whereas complex ions like [Fe(CN)₆]⁴⁻ remain intact.

Exam tips

• Find the oxidation number of the metal by balancing charges: in K₄[Fe(CN)₆], Fe is +2.

Common mistakes

• Taking coordination number as the number of ligands rather than the number of donor atoms.