Serendipity: Werner's argument that the `two-only forms' (green and violet) of [CoCl2(en)2]+ salts demanded his octahedral model was correct. True?

Abstract

Chemists of the late 19th century, including Alfred Werner, prepared salts containing either green or violet cations of composition [CoCl2(en)2]+ (en is ethylenediamine, C2H8N2); we now refer to these as trans-dichloro and cis-dichloro species. We have discovered a third salt, purple in color, containing cations of the same elemental composition and whose asymmetric unit composition is [CoCl2(en)2]2Cl2·3H2O, in which the cobalt cations are a cis:trans dichloro pair. Such a discovery would undermine Werner's argument that if only two forms can be prepared, his octahedral theory was proven. Probably because his students never examined their crystals under a microscope, they failed to observe the `third' species, thereby ruining Werner's argument since he relied strictly on color to identify them. That was fortunate since our purple salt would have led him to abandon, or certainly delay, his momentous discovery. Our crystals consist of a 1:1 mixture of the cis and trans cations, thereby sharing the same elemental analysis and conductivity as the single salts, but not their crystal structure, inasmuch as X-ray diffraction had not even been discovered then. Serendipitously, our discovery would have been a great boon to his theoretical acumen, while his `two-color' argument may have doomed him.