Werner proposed that metal ions have two types of valencies: primary valence (ionisable, equals oxidation state) and secondary valence (non-ionisable, equals coordination number, fixed and directed in space). In [Co(NH3)6]Cl3, primary valence = 3 (satisfied by 3 Cl-) and secondary valence = 6 (satisfied by 6 NH3 in octahedral geometry). The coordination entity [Co(NH3)6]3+ retains its identity in solution. Key point: coordination number counts donor atoms, not ligands — EDTA is one ligand but contributes 6 to the coordination number.