Neutron diffraction study of [Pt(en)(2)][Pt(en)(2)I-2](ClO4)(4) at 20 K: Structure and evidence of a new phase transition

The structure at 20K of fully deuterated [Pt(en)(2)]-[Pt(en)(2)I-2](ClO4)(4), where en=ethylenediamine, has been determined by neutron diffraction on a single crystal. It is shown that the symmetry is monoclinic with cell parameters a = 16.650(7), b = 5.760(2), c = 14.751(2) Angstrom and beta = 99.19(1)degrees, that is, with respect to the previously 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) and angles are consistent with the expected values and the main difference with respect to the previously reported room-temperature structures 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) Angstrom, attributed to Pt-II-I. Actually, very weak diffraction peaks that provide evidence for an additional doubling of the a and b parameters are also observed and a space group C2 is therefore proposed. This could 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 number of atoms to be considered, no reliable refinement has been obtained in C2. On the other hand, on heating the crystal the diffraction peaks with I odd greatly decrease in intensity between 150 and 160K, which unambiguously argues for the existence of a phase transition presumably connected to an order-disorder of the ethylenediamines.