Q.One mole of a purple coloured complex of CoCl₃ and NH₃ on treatment with excess of AgNO₃ produces two moles of AgCl. Write the formula of the complex. (C.N. of Co is 6)

Werner made the first systematic attempt to explain the nature of bonding in coordination compounds, resting on four postulates. First, unlike an ordinary metal salt, the metal in a coordination 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, outer (ionisation) sphere -- generally anions -- which separate as simple free ions in solution. Third, the secondary valency is satisfied by the coordination (inner) sphere -- anions or neutral molecules that stay non-ionisably bonded to the metal; critically, 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 three-dimensional space: a coordination number of six always produces an octahedral geometry, while a coordination number of four produces either a square planar or a tetrahedral geometry. The theory's standing illustration is [Co(NH3)6]Cl3: its coordination (secondary) sphere is the octahedral [Co(NH3)6]3+ unit, and its ionisation (primary) sphere is the three separately-ionisable Cl- counter ions. This structure is confirmed experimentally in a specific, repeatable way: dissolving the compound gives NO positive chemical test for free Co3+ or free NH …