How did Werner prove coordination complexes have 3D structure?
I proved three-dimensional structure by counting isomers—a method I called the 'isomer count argument.' For a compound like [Co(NH₃)₄Cl₂]⁺, if the ligands were arranged in a flat square, you would get two isomers: one with chlorines adjacent (cis) and one opposite (trans). But if they were in a plane, a third isomer would be impossible. I synthesized exactly two isomers, consistent with an octahedral geometry where the four ammonia ligands occupy a square plane and the two chlorines sit either adjacent or opposite. For [Co(NH₃)₃Cl₃], a flat arrangement would yield three isomers; I found only two, matching an octahedron with meridional and facial forms. I also resolved optical isomers—mirror-image forms that rotate polarized light—for complexes like [Co(en)₃]³⁺, which only a three-dimensional, non-superimposable structure can produce. The facts demanded a new theory of valence, and geometry provided the answer.
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