The main postulates are : (i) In coordination compounds metals show two types of linkages(valencies)-primary and secondary. (ii) The primary valencies are normally ionisable and are satisfied bynegative ions. (iii) The secondary valencies are non ionisable. These are satisfied byneutral molecules or negative ions. The secondary valency is equal to thecoordination number and is fixed for a metal. (iv) The ions/groups bound by the secondary linkages to the metalhave characteristic spatial arrangements corresponding to differentcoordination numbers.