Werner's theory and key terms

A coordination compound has a central metal atom or ion with molecules or ions bonded around it. Take [Co(NH₃)₆]Cl₃. The cobalt sits in the middle, six ammonia molecules are bonded to it, and the whole group is written in square brackets. The three chloride ions sit outside the brackets.

The key words:

• The bracketed group is the coordination entity, or coordination sphere. The ions outside it are counter ions.
• The molecules or ions bonded to the metal are ligands. Each ligand gives a pair of electrons to the metal; the metal, being positive, is happy to take them.
• The coordination number is the number of donor atoms bonded to the metal: 6 in [Co(NH₃)₆]³⁺.
• The shape the ligand atoms make around the metal (octahedral, tetrahedral, square planar) is the coordination polyhedron.

Ligands are sorted by how many atoms they donate through, their denticity: like giving with one hand or with two.

• Unidentate: one donor atom (NH₃, H₂O, Cl⁻, CN⁻).
• Didentate: two donor atoms. Ethane-1,2-diamine (en, H₂NCH₂CH₂NH₂) gives through both nitrogens; oxalate (C₂O₄²⁻) through two oxygens.
• Polydentate: many donor atoms. EDTA⁴⁻ is hexadentate, with six.
• Ambidentate: one ligand that can bond through either of two different atoms, but only one at a time. NO₂⁻ can bond through N or through O; SCN⁻ through S or through N.

Werner's theory (1898) explained these compounds with two kinds of valence:

• The primary valence is ionisable: it equals the metal's oxidation state and is satisfied by the counter ions.
• The secondary valence is non-ionisable: it equals the coordination number and is satisfied by the ligands.

In water, only the ions outside the brackets separate; the coordination entity stays whole. Adding AgNO₃ precipitates only the free chloride as AgCl, which lets you count what is outside. For example, CrCl₃·6H₂O gives 2 mol of AgCl per mole, so only two Cl⁻ are outside. The sixth place inside goes to one Cl⁻ plus five water molecules: [Cr(H₂O)₅Cl]Cl₂·H₂O.

Compare a double salt such as Mohr's salt, FeSO₄·(NH₄)₂SO₄·6H₂O. In water it splits completely into its simple ions and gives every ion's test, Fe²⁺ included. A complex ion, like [Fe(CN)₆]⁴⁻, keeps its identity in water and does not give the test for free Fe²⁺.