Werner theory of coordination complexes

Werner made the first serious attempt to explain how bonding works in coordination compounds, and his theory rests on four postulates. First, unlike an ordinary metal salt, the metal in a complex possesses TWO distinct kinds of valency: a primary (ionisable) valency and a secondary (non-ionisable) valency. Second, the primary valency is satisfied by the ionisable sphere -- generally anions -- which separate freely as simple ions in solution (the ionisation/outer sphere). Third, the secondary valency is satisfied by the coordination sphere -- entities (anions or neutral molecules) that are non-ionisable and stay bonded to the metal; the number of secondary valencies is fixed for a given metal ion and is exactly equal to its coordination number. Fourth, these secondary valencies have a FIXED spatial arrangement in space around the metal ion: a coordination number of six always gives an octahedral geometry, while a coordination number of four gives either a square planar or a tetrahedral geometry. [Co(NH3)6]Cl3 is the worked illustration throughout: its coordination (secondary) sphere is [Co(NH3)6]3+ (six NH3 bonded directly to cobalt, octahedral), and its ionisation (primary) sphere is the three Cl- counter ions, which separate completely on dissolving. This is confirmed experimentally -- dissolving [Co(NH3)6]Cl3 in water gives NO positive test for free Co3+ or free NH3 (they remain locked inside the intact coordination sphere), yet treating the solution with AgNO3 immediately gives a curdy white precipitate corresponding to all THREE moles of chloride, proving all three Cl- are freely-ionisable counter ions rather than metal-bound ligands. Two further worked 'how many Cl- are ionisable' problems in this section (a purple complex giving 2 moles …