Q.When excess of AgNO$_3$ is added to a complex, one mole of AgCl is precipitated. The formula of complex is _____.

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 …