1893

Coordination Compounds

Alfred Werner (1866–1919), Victor L. King (1886–1958)

The difficulty of understanding the structure of compounds like Prussian blue was proof that there was something different about the chemistry of metals. They formed complex molecules, which seemed to be made up of metal atoms interacting with other molecules, called ligands. Different elements seemed to pick up two, four, six, or eight ligands at a time, in patterns that were very hard to interpret. Swiss chemist Alfred Werner threw some much-needed light on the subject with his theories of coordination, first published in an 1893 paper. Metals, he found, attracted these ligands according to their oxidation state, and in specific patterns in space. The ligands (things like ammonia and other amines, cyanide, chloride, nitrate, and others) could be arranged in a square, a shape like two pyramids base to base, or toward what would be the six faces of a cube centered around the metal, among others. This accounted for why there were metal complexes known with the exact same formulas but different properties (color, solubility, reactivity, etc.): their ligands were arranged differently around the metal. We would now call the metal a Lewis acid, and each ligand a Lewis base, since it is an electron pair on each ligand that is coordinating with the metal. And we now know that coordination chemistry is a fundamental property of metallic elements, which shows up in everything from minerals in the Earth’s crust to drug molecules (see Cisplatin).

These arrangements in space suggested that some metal complexes could have chirality, meaning they could exist in right-handed and left-handed forms. Werner’s lab spent over a decade on the problem, which his American student Victor L. King finally solved by preparing chiral cobalt complexes in 1907. King had been greeted for months by fellow students in Zurich with the question “Does it rotate yet?” referring to the rotation of polarized light that a chiral compound produces. Werner himself was reportedly so overjoyed at the news that he stopped near strangers on the street to tell them about the discovery. (See Noble Gas Compounds for another example of the same problem!)

These are various possible coordination compounds. The metal atom sits in the middle, with the ligands arranged around it in defined geometries.