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How did Alfred Werner use isomers to show that cobalt(3+) complexes can have octahedral structures?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

Alfred Werner established the octahedral configuration of hexacoordinate cobalt(3+) complexes by comparing the number and types of isomers he prepared in the laboratory with those theoretically predicted for various geometric arrangements.1

By analyzing these series of compounds, Werner demonstrated that the observed isomerism was consistent with an octahedral structure rather than the rival chain theory proposed by his contemporaries. A pivotal moment in this validation occurred in 1911, when Werner and his student Victor L. King successfully resolved the complex [CoCl(NH3)(en)2]Cl2 into its optical isomers, providing definitive evidence for his coordination theory.1

Werner’s work effectively refuted the Blomstrand-Jørgensen chain theory and proved that stereochemistry is a general phenomenon applicable to inorganic chemistry, not just organic chemistry. His systematic experimental achievements, including the resolution of complex inorganic compounds, ultimately confirmed his theory and earned him the 1913 Nobel Prize for Chemistry.12