Alfred Werner and the Semi-factual Story of Coordination Chemistry

In 1913, a Swiss chemist named Alfred Werner was awarded the Noble Prize in Chemistry for his work on what would be called

Vanadium rainbow.

coordination chemistry, which would lead to a new understanding of how chemicals bond together. Coordination theory describes the nature of bonding in transitional metals and the formation of complexes, which at the time seemed to follow bizarre and unpredictable patterns. Atoms in groups 1-7A followed somewhat predictable patterns in their bonding as shown by classical experiments. For example, atoms in group I took on +1 charges and bonded once to other more negatively charged atoms like the group 7A halogens, but it remained a mystery how the transition metals bonded and why they had so many oxidation states.  Vanadium, for example, produces a wonderful rainbow of oxidation states when potassium permanganate is added to a Vanadium II solution.  Over time it separates into different states: V +2 is violet, V +3 is green, VO +2 is blue, VO2 +1 is pale yellow, MnO2 is brown and MnO4 –1 is pink.  In Werner’s time, the shape of salts like (CoCl3 * 6 NH3) were still undetermined and throughout much of the 1800’s one popular theory emerged: chain theory. It was supported by some of the most powerful chemist-sorcerers at the time, including Werner’s chief rival: S.M. Jorgensen.

Jorgensen believed that the ligands in compounds like (CoCl3 * 4 NH3) were arranged in chains, that is, bonded to each other in some fashion. The main point being that the atoms would follow known valence rules at the time, especially Kekule’s principle, which abstracted the number of times a compound could bond from known chemical reactions. Though useful, it ran into problems when trying to describe why atoms with larger electronic configurations bonded in so many different arrangements. Transitional metals in particular confounded these rule sets.

Werner, however, proposed a different theory that relied on the concept that cobalt (in the above compound) could have more than the three bonds predicted by Kekule’s theory and that the ligands would be centered around cobalt in an octahedrally arrangement, rather than in chains. According to his theory, a compound like the above due to its structure would have two possible conformations: a cis isomer (with chlorine atoms on adjacent vertices) and a more stability favored trans isomer (with the chlorine atoms on opposite side of one another). Interestingly, the two are identifiable by their color with the trans compound being green and cis being a delightful purple color

Naturally, this caused controversy amongst chemists and the debate began. At the time only the structurally favored green trans compound had been synthesized, while the more difficult cis compound was thought to be non-existent.  Cis compounds are generally less stable and in this case it is due to repulsions between the electronegative chlorine atoms positioned close to each other.  Whenever Werner published results that seemingly confirmed his theory, Jorgensen was there to propose a counter theory in favor of the more popular chain theory.   Chain theory had strength in the fact that there are many possibilities in the way ligands can be arranged in that manner.  Eventually, Werner was able to prove his case conclusively through a variety of methods like optical resolution of the compounds and electrical conductivity measurements. The capstone, as the story goes, was his synthesis of the elusive purple cis isomer of [Co (NH3)4 Cl3] and sending a sample through the mail to Jorgensen.  The flurry of high fives and  chest bumps went unabated for three months afterward and was actually seismically measured in Sweden.

Werner used this clever method to synthesize his purple cis isomer. By adding HCL at 0C, carbon dioxide is released and chlorine atoms in solution replace the oxygen atoms lost. (Note: picture does not show positive charge on Cobalt atom)

Transition metal like many of the blue colored above are used in a variety of reactions ranging from biological (Zn, Co, Cu, etc) to industrial (Os, V, Pb, Pt, etc).

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