Revisiting the structure of (±)-[Co(en)3]I3·H2O – X-ray crystallographic and second-Harmonic results

Abstract As described in the Introduction, we became interested in the existing literature for the crystallization behavior of (±)-[Co(en)3]I3·H2O and the absolute configuration of its enantiomers because of our project on the historical sequence of chemical studies leading Werner to formulate his Theory of Coordination Chemistry. In so doing, we discovered a number of interesting facts, including the possibility that the published “Pbca” structure of the (±)-[Co(en)3]I3·H2O was incorrect, and that it really crystallizes as a kryptoracemate in space group P212121. Other equally interesting facts concerning the crystallization behavior of [Co(en)3]I3·H2O are detailed below, together with an explanation why Platon incorrectly selects, in this case, the space group Pbca instead of the correct choice, P212121. As for the Flack parameter, (±)-[Co(en)3]I3·H2O provides an example long sought by Flack himself – a challenging case, differing from the norm. For that purpose, data sets (for the pure enantiomer and for the racemate) were collected at 100 K with R-factors of 4.24 and 2.82%, respectively, which are ideal for such a test. The fact that Pbca is unacceptable in this case is documented by the results of Second-Harmonic Generation experiments. CCDC nos: 1562401 for compound (I) and 1562403 for compound (II).