Werner's coordination theory

In the late 1800s chemists were puzzled. They could make a series of compounds from cobalt chloride and ammonia — CoCl3 with six, five, or four ammonias attached — and these solids had very different colors and behaviors, yet on paper they all looked like the same atoms stuck together. Worse, cobalt was 'supposed' to bond to only three chlorines, so why did extra ammonia molecules cling to it at all? The accepted rules of valence simply could not explain these stubborn, well-behaved compounds.

Alfred Werner's answer (1893) was bold: a metal atom has two kinds of combining power at once. It has its ordinary valence, which we now call the oxidation state, satisfied by ions like chloride. But it also has a second, separate capacity to hold a fixed number of groups directly around itself in space — and that number, for cobalt(III), is six. So in CoCl3·6NH3 all six ammonia molecules sit directly on the cobalt forming the ion [Co(NH3)6]3+, with the three chlorides held loosely outside as counter-ions. Werner predicted how many chlorides would be 'free' (and so precipitate with silver, or carry current in solution) in each member of the series, and experiment matched him exactly.

This was the birth of coordination chemistry. The idea that a central metal sits at the heart of a fixed geometric arrangement of attached groups — and that these directly attached groups are chemically distinct from outer ions — underlies every complex, every catalyst, and every metal site in biology that the rest of this field describes. Werner even resolved a complex into its mirror-image forms, proving the geometry was real three-dimensional structure and not a guess. He received the 1913 Nobel Prize for it.