A REVERSIBLE NONDISRUPTIVE PHASE-TRANSITION SHOWN BY THE ZINC IODIDE DIMETHYLFORMAMIDE COMPLEX ZNI2(DMF)(2)

At 228 K crystals of ZnI2(dmf)(2) show a reversible phase transition w hich does not disrupt the lattice. Above the transition temperature th e space group is C2/c and the cell contains eight symmetrically equiva lent molecules. Cooling to below the transition temperature has little effect on the cell parameters or on the Zn- and I-atom positions, but the space group is now P2(1)/n and the asymmetric unit comprises two conformationally different molecules. These arise from cooperative rot ations of either ca +25 or -43 degrees about the Zn-O bond of one of t he dmf ligands in the high-temperature form. This displacive transitio n involves large movements of some atoms. The corresponding chloride a nd bromide are isomorphous with the higher temperature C2/c form, but it is only with the iodide that the weaker intermolecular forces permi t the unusual phase change. The transition has been followed by differ ential scanning calorimetry, which gives an enthalpy change of 1.44 (5 ) kJ mol(-1).