ON "a, b - ISOMERISM" OF Cu(NH3 )2 Br2
Dušan Miklos, Michal Dunaj - Jurco, Vladimír Jorik, Milan Hrubo
Department of Inorganic Chemistry ,Slovak
Technical University Bratislava 812 37 Bratislava, Slovak
Republic
miklos@cvt.stuba.sk
Keywords: crystallochemistry, Cu-complexes, isomerism,
double salts
In late fifties Cu(NH3)2Br2 has been extensively studied as a part of the study of preparation, structure and properties of Cu-complexes. Based on the Debye-Scherrer diffraction patterns two modifications have been detected [1]. According to the Peyron reaction scheme ([CuBr4]2- with ammonia) a powder preparate has been obtained which was designated as (monoclinic) a-isomer, while application of the Jörgensen reaction scheme (thermal decomposition of an undefined product [Cu(NH3)n]Br2, n > 2) resulted in powder form of the (cubic) b-isomer. Also a "crystalline preparate" has been prepared which was identified as the b-form [2]. The structure of the "crystalline preparate" which was believed to be "b-Cu(NH3)2Br2" was solved from single crystal diffraction data [3], yielding a cubic structural model with statistically occupied positions for the Cu-atom, and that of the a-form from powder diffraction data [4]. Nevertheless, observed differences in powder diffraction patterns and properties of the obtained preparations have not been explained satisfactorily. Moreover, the conclusions have not been supported by reliable results of chemical analysis and density measurements.
We decided, therefore, to verify previous results by systematic synthesis and characterization of both modifications, prepared by different reaction ways.
A series of samples of the b-form in the system xNH4Br.Cu(NH3)2Br2 (x - rational number) has been prepared , indicating that x may vary continuously. Chemical analysis and observed linear dependence of the Cu-content vs density have been used to determine compositions of the actual samples. Corresponding density measurements on crystals of the same sample batch, of which the single crystal used for the structure analysis in [3] comes from, have shown that the actual composition of the crystal corresponds to 7.4NH4Br.Cu(NH3)2Br2. Refinement of single crystal data from a sample having the composition 4.7NH4Br.Cu(NH3)2Br2 has also been reported [5]. We have refined [6] structures of other two members of the series, showing low content of NH4Br, 0.6NH4Br.Cu(NH3)2Br2 and 1.6NH4Br.Cu(NH3)2Br2, with essentially the same structure model as in [3], but taking into account also space groups of lower symmetry. Refinement of the Cu occupation factors has confirmed the results of chemical analysis, corresponding to the above given formulae. Thus, none of the known structure refinements for single crystals of the "b-form" confirmed originally assumed composition Cu(NH3)2Br2.
Three powder samples of the expected a-form of Cu(NH3)2Br2 have been prepared by three different ways (from an aqueous solution of the system Cu(II) - NH3 - Br- - HCO3-, from an acetone solution, and by thermal decomposition of 3CuBr2.10NH3). All three samples have shown typical "monoclinic" diffraction patterns. Rietveld refinement of data for these three samples has shown that (a) for the samples which have been prepared from the aqueous and from the acetone solution a statistical structural model, different from that of [4], had to be adopted, and (b) for the sample which was prepared by thermal decomposition, the best result has been obtained, assuming a mechanical mixture of Cu(NH3)2Br2 and NH4Br, yielding the overall composition 0.11NH4Br + Cu(NH3)2Br2 which is in agreement with chemical analysis.
Thus, the existence of the original structural model of the a-form of Cu(NH3)2Br2 has not been confirmed, rather a statistical model should be adopted. The assumed -form appears to be a disordered structure of xNH4Br.Cu(NH3)2Br2 with varying x. As these two forms represent substances having different and possibly varying compositions, one cannot speak about isomerism of Cu(NH3)2Br2.