Werners Theory: Primary and Secondary Valency Guide

In the 1890s chemists were puzzled by a family of cobalt compounds. CoCl3 combined with ammonia gave a series: CoCl3.6NH3, CoCl3.5NH3, CoCl3.4NH3 and CoCl3.3NH3, all with the same elements but different colours and behaviours. Adding silver nitrate to the first precipitated all three chlorides, to the second only two, and to the third only one. Alfred Werner explained these mysteries in 1893 with coordination theory, proposing that metals have two kinds of valency. The idea won him the 1913 Nobel Prize and founded modern inorganic chemistry.

Werner's theory matters because it is the grammar of coordination compounds. Every formula you write for a complex, every prediction of how many ions a compound releases in solution, and the entire naming system of coordination chemistry rest on his distinction between primary and secondary valency. Conductivity and precipitation experiments in school labs still follow his logic directly.

Key Concepts

• Primary valency: Equals the oxidation state of the metal. It is satisfied by anions (negative ions), is ionizable (the anions split off in solution), and has no directional preference. In CoCl3 the primary valency of cobalt is 3.
• Secondary valency: Equals the coordination number, the number of groups directly bound to the metal. It is satisfied by neutral molecules or anions, is non-ionizable (these groups stay attached in solution), and is directional, pointing to fixed geometries such as octahedral.
• Coordination sphere: Written in square brackets, it contains the metal plus everything satisfying secondary valency. In [Co(NH3)6]Cl3 the six ammonias are inside the sphere and the three chlorides are outside it.
• The cobalt ammine series decoded: CoCl3.6NH3 is [Co(NH3)6]Cl3 (3 ionizable Cl, 3 moles of AgCl precipitate); CoCl3.5NH3 is [Co(NH3)5Cl]Cl2 (2 ionizable Cl); CoCl3.4NH3 is [Co(NH3)4Cl2]Cl (1 ionizable Cl); CoCl3.3NH3 is [Co(NH3)3Cl3] (0 ionizable Cl, a neutral complex that does not conduct).
• Conductivity evidence: Molar conductivity grows with the number of ions released: [Co(NH3)6]Cl3 gives 4 ions, [Co(NH3)5Cl]Cl2 gives 3, [Co(NH3)4Cl2]Cl gives 2, and [Co(NH3)3Cl3] gives none, matching Werner's formulas exactly.
• Modern restatement: Primary valency is now called oxidation number and secondary valency the coordination number, but Werner's structural conclusions were all correct.

How It Works

Werner's method was detective work with precipitation. Take an unknown like CoCl3.4NH3 and add excess silver nitrate: each free chloride ion forms one mole of white AgCl precipitate, but chlorides locked inside the coordination sphere do not react. If 1 mole of complex yields 1 mole of AgCl, exactly one chloride is ionizable, so the other two must be bonded directly to cobalt inside the sphere, giving [Co(NH3)4Cl2]Cl. Conductivity measurements then confirm the total ion count. By combining both tests for each member of the series, Werner mapped every atom to its correct position without ever seeing a molecule, and his directional secondary valency even predicted the existence of geometrical isomers, which were later found.

Worked Example

Question: A coordination compound with formula CoCl3.5NH3 gives 2 moles of AgCl precipitate per mole with excess AgNO3, and its solution conducts electricity corresponding to 3 ions. Deduce its structural formula.

1. Count ionizable chlorides: 2 moles of AgCl means 2 of the 3 chlorides are free ions outside the coordination sphere.
2. Place the third chloride: The remaining chloride must satisfy secondary valency, so it sits inside the sphere bonded to cobalt: [Co(NH3)5Cl] with 2 Cl- outside.
3. Check the ion count: [Co(NH3)5Cl]2+ plus 2 Cl- gives 3 ions total, matching the conductivity data.
4. State both valencies: Primary valency (oxidation state) of Co: x + 5(0) + (-1) = +2, so x = +3. Secondary valency (coordination number): 5 + 1 = 6, octahedral. Final formula: [Co(NH3)5Cl]Cl2.

Common Misconceptions

• Primary valency means the first bonds formed: No. Primary and secondary describe the type of valency (ionizable oxidation state vs directional coordination number), not a time order. Both are satisfied simultaneously in the real compound.
• All chlorides in the formula behave the same: Werner's whole point was that they do not. Only chlorides outside the square brackets precipitate with AgNO3; inner-sphere chlorides are covalently bound to the metal and stay silent in the test.
• Werner's theory is obsolete: The language changed (oxidation state, coordination number) but every structural prediction survived. Modern crystal structures confirm the octahedral [Co(NH3)6]3+ ion exactly as Werner drew it.

Real-World Applications

• Naming complexes: IUPAC names like pentaamminechloridocobalt(III) chloride are built directly from Werner's sphere concept: ligands inside, counter ions outside, oxidation state in Roman numerals.
• Qualitative analysis: School and lab tests for ions (AgCl precipitation, conductivity checks) still use Werner's logic to distinguish free ions from coordinated ones.
• Drug design: Cisplatin, the anticancer drug cis-[Pt(NH3)2Cl2], was understood and developed through coordination theory; its trans isomer, predicted by the same geometry rules, is inactive.
• Industrial catalysts: Knowing which ligands sit in the coordination sphere lets chemists design catalysts like Wilkinson's catalyst, where substrates must enter the sphere to react.

Werner's theory split valency into an ionizable primary part (the oxidation state) and a directional secondary part (the coordination number), and proved it with precipitation and conductivity data from the cobalt ammine series. That single distinction explains complex formulas, isomerism and solution behaviour, and it remains the foundation every coordination chemist builds on.