Cis-Trans Isomerism among the Octahedral Diaquabis(N,N- or N,N′-dialkylethylenediamine)nickel(II) Complexes and Their Thermal Reaction Products

The thermochemical changes in coordination structure of diaquabis(N,N- or N,N′,-dialkylethylenediamine)nickel(II) complexes ([Ni(H2O)2(diamine)2]X2·nH2O) were reinvestigated by means of TG-DTA and electronic spectroscopy at room and elevated temperatures, where diamine is N,N (or N,N′-dimethylethylenediamine (NN- or NN′-dmen) or N,N-(or N,N′)-diethylethylenediamine (NN- or NN′-deen); X is Cl−, Br−, I−, or NO3−; n is 0, 1, 2, or 3. The diaqua complexes prepared were all trans except for cis-[Ni(H2O)2(NN-dmen)2]I2. The trans complexes with the symmetric (N,N′-dialkylsubstituted) diamines brought about a deaquation-anation upon heating, retaining the original trans configuration, while those with the asymmetric (N,N-dialkylsubstituted) diamines transformed into the cis complexes. In the case of the NN-dmen complexes, cis-halogenoaqua species were isolated as a stable intermediate during the thermal reactions. The complex nitrates changed to trans-dinitrato or cis-mononitrato species depending upon the diamines. The effects of the N-substituents in the diamines and of the anions on the patterns of thermal reactions were also discussed