cis-trans Isomerism among the Octahedral Diaquabis(optically active C-substituted ethylenediamine)nickel(II) Complexes and Their Thermal Reaction Products. An Explanation for a Variety in Colors and Stereochemistry of Lifschitz Complexes.

The solid-phase thermal reactions of trans-diaquabis(diamine)nickel(II) complexes (trans-[Ni(diamine)(2)(H2O)(2)]X-2) were investigated by means of TG/DTA and DSC, and high-temperature electronic spectrometry, where the diamine is an optically active diamine such as (1S,2S)-1,2-diphenyl-1,2-ethanediamine or (S)-4-methyl-1,2-pentanediamine, and X is Cl-, Br-, or NO3-. Several complexes were peculiarly transformed into cis-[NiX2(diamine)(2)] upon thermal deaquation-anation, and then isomerized to trans-[NiX2(diamine)(2)] upon further heating. The results were in contrast to the reactions of the complexes with the corresponding racemic diamines which underwent a simple deaquation-anation, retaining an original trans configuration. The differences must come from a slight difference in the conformation of diamine ligands between trans-[Ni(R-diamine)(S-diamine)(H2O)(2)]X-2 which is obtained for the rac-diamines and trans-[Ni(R- or S-diamine)(2)(H2O)(2)]X-2 which is obtained for the optically active diamines.