Chemistry · Coordination Compounds · NEET
Primary valence is the positive charge (oxidation state) of the metal ion. It is satisfied by negative ions such as Cl-, and these ions sit OUTSIDE the coordination sphere, so they can leave the complex in water (ionisable, non-directional). Secondary valence is the coordination number of the metal, satisfied by ligands bonded directly to the metal INSIDE the square brackets. These do not ionise and they fix the shape of the complex. Short version: primary = ionisable charge (outside), secondary = coordination number (inside).
Yes, in modern language the primary valence equals the oxidation state of the central metal. In [Co(NH3)6]Cl3 the primary valence of Co is 3, and cobalt is in the +3 oxidation state, balanced by three Cl- outside the sphere. This matters for NEET because a common question asks for the oxidation state of the metal, which is exactly the primary valence.
Yes. Secondary valence is Werner's old name for the coordination number, which is the number of donor atoms (ligands) directly bonded to the metal inside the sphere. In [Co(NH3)6]Cl3 the secondary valence is 6, because six NH3 molecules are attached to Co. Secondary valence is directional and gives the complex its geometry (6 = octahedral, 4 = tetrahedral or square planar).
Only ions that ionise (break free in water) can react with Ag+. Chloride held by secondary valence is bonded directly to the metal inside the coordination sphere, so it stays locked to the metal and does not become a free Cl- ion. Only the Cl- satisfying primary valence (outside the brackets) is free, so only that chloride precipitates as AgCl. This single idea is the key to the whole CoCl3.xNH3 PYQ series.
Everything written inside the square brackets [ ] is inside the coordination sphere (secondary valence, does not ionise). Everything written outside the brackets is a counter ion (primary valence, ionises in water). Example: in [Co(NH3)5Cl]Cl2, one Cl is inside (bonded to Co, will not give AgCl) and two Cl are outside (free, give 2 AgCl).
Werner said secondary valences ARE directional (they point in fixed directions and decide the geometry, like octahedral or square planar). Primary valences are non-directional (they are just the ionic charge and do not fix any shape). This is why coordination number, not charge, controls the shape of the complex.
The correct order of the stoichiometries of AgCl formed when AgNO3 in excess is treated with the complexes CoCl3.6NH3, CoCl3.5NH3, CoCl3.4NH3 respectively is:
Out of the following complex compounds, which compound will have the minimum conductance in solution?
A 1:3 electrolyte in aqueous solution is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) Metals show two valences: primary (ionisable, equals oxidation state) and secondary (non-ionisable, equals coordination number). 2) Primary valence is satisfied by negative ions outside the sphere. 3) Secondary valence is satisfied by ligands inside the sphere and is directional, so it fixes the geometry (6 = octahedral, 4 = square planar or tetrahedral).
The primary valence of Co is 3 (satisfied by three Cl- outside the sphere, which is also the +3 oxidation state). The secondary valence is 6 (satisfied by six NH3 ligands inside the sphere), so the coordination number is 6 and the shape is octahedral.
Yes. In [Co(NH3)5Cl]Cl2 one Cl- is bonded to Co inside the sphere, so it satisfies a secondary valence AND helps balance the +3 charge (a primary valence). The other two Cl- outside satisfy the rest of the primary valence and ionise to give 2 AgCl.
It is the base of the whole Coordination Compounds chapter. Almost every year NEET asks a question about AgCl formation, molar conductance, or number of ions, and all of them are solved by one rule: only ions outside the coordination sphere (primary valence) ionise, ions inside (secondary valence) do not.
