NEUTRON-DIFFRACTION STUDY OF [PT(EN)(2)][PT(EN)(2)I-2](CLO4)(4) AT 20K - STRUCTURE AND EVIDENCE OF A NEW PHASE-TRANSITION

The structure at 20K of fully deuterated [Pt(en)(2)]-[Pt(en)(2)I-2](Cl O4)(4), where en=ethylenediamine, has been determined by neutron diffr action on a single crystal. It is shown that the symmetry is monoclini c with cell parameters a = 16.650(7), b = 5.760(2), c = 14.751(2) Angs trom and beta = 99.19(1)degrees, that is, with respect to the previous ly reported room-temperature structures, a doubling of the c parameter is observed. The space group is C2/c and the refinements lead to an R value of 0.0374. All the bond lengths (C-C, C-N, C-D, N-D and Cl-O) a nd angles are consistent with the expected values and the main differe nce with respect to the previously reported room-temperature structure s results in an ordering of the stacking of the ethylenediamines. The iodines are distributed over two sites, which gives two Pt-I distances , 2.712(3) Angstrom, attributed to the Pt-IV-I bond, and 3.048(3) Angs trom, attributed to Pt-II-I. Actually, very weak diffraction peaks tha t provide evidence for an additional doubling of the a and b parameter s are also observed and a space group C2 is therefore proposed. This c ould result from a very slight ordering of the chains with respect to one another, but, due to the weakness of such signals and the large nu mber of atoms to be considered, no reliable refinement has been obtain ed in C2. On the other hand, on heating the crystal the diffraction pe aks with I odd greatly decrease in intensity between 150 and 160K, whi ch unambiguously argues for the existence of a phase transition presum ably connected to an order-disorder of the ethylenediamines.