Werner's Theory of Coordination Compounds

Swiss chemist Alfred Werner was the first to propose a theory of coordination compounds explaining their observed behaviour, put forward in 1893 -- remarkably, before the electron itself had even been discovered (by J.J. Thomson in 1896), using nothing but simple reaction chemistry, work for which he won the 1913 Nobel Prize in Chemistry as the first inorganic chemist to do so. The puzzle Werner set out to explain: cobalt(III) chloride and ammonia both have their ordinary valences fully satisfied on their own, yet they react to form a family of distinctly coloured complexes -- CoCl₃.6NH₃ (yellow), CoCl₃.5NH₃ (purple), trans-CoCl₃.4NH₃ (green) and cis-CoCl₃.4NH₃ (violet) -- that precipitate different numbers of moles of AgCl per mole of complex when treated with excess silver ion. Werner's postulates: (1) most elements exhibit two kinds of valence, primary valence (in modern terms, the metal's oxidation state) and secondary valence (in modern terms, its coordination number), and every element tends to satisfy both; for cobalt here, the primary valence is 3 and the secondary valence is 6. (2) The primary valence is positive in most cases (zero in certain cases) and is always satisfied by negative ions -- in CoCl₃.6NH₃ the primary valence +3 is satisfied by three Cl⁻ ions. (3) The secondary valence is satisfied by negative ions, neutral molecules, positive ions, or a combination of these -- in CoCl₃.6NH₃ the secondary valence 6 is satisfied by six neutral NH₃ molecules, while in CoCl₃.5NH₃ it is satisfied by five NH₃ molecules plus one Cl⁻ ion. (4) There are two spheres of attraction around the metal atom/ion: an inner sphere, the coordination sphere, whose groups are firmly attached to the metal, and an outer sphere, the ionisation sphere, whose groups are loosely bound and can be separated into free ions on dissolving the complex. (5 …