Synthetic Metals, 2 7 (1988) A209-A217 A209 "The submitted manuscript has been authored by a contractor of the U.S, Government under contact No. W-31-109-ENG-38, Accordingly, the U.S, Government retains a nonexclusive, royalty- free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes." CHARGE-TRANSFER SALTS DERIVED FROM MET: SYNTHESIS. STRUCTURE AND PROPERTIES OF (MET)(~I04, (MET~PF6 and fMET~(Re04).2.._ MARK A. BENO*, URS GEISER, ARAVINDA M. KINI, HAU H. WANG, K. DOUGLAS CARLSON, MITZI M. MILLER, THOMAS J. ALLEN, JOHN A. SCHLUETER, ROGER B. PROKSCH and JACK M. WILLIAMS Chemistry and Materials Science Divisions Argonne National Laboratory Argonne, Illinois 60439-4831 (U.S.A.) ABSTRACT Three charge transfer salts derived from the MET (4,5-methylenedithio-4',5'- ethylenedithiotetrathiafulvalene) donor molecule have been synthesized: (MET)CIO4 and isostructural (MET)PF6 crystallize in the monoclinic space group C 2/m, Z = 2, with unit cell dimensions a = 13.842(3) A, b = 10.255(3) A, c = 5.842(1) A, !3 = 104.31(2) ° , Vc = 803.6(3) A 3 and a = 14.146(4) A, b = 10.521(2) A, c = 5.9o1(1) A, I~ = 105.02(2) ° , Vc = 848.2(3) A 3, respectively, and (MET)3(ReO4)2 is triclinic, P'I, Z = 1, with unit cell dimensions a = 8.007(6) A, b = 9.814(6) A, c = 15.526(9) A, ct = 90.90(5) ° , 13 = 90-85(6) °, T = 110.58(5) °, Vc = 1142(2) A3. Single crystal electrical conductivity and ESR measurements show that these salts all exhibit semiconducting behavior. A structural phase transition is observed in the 1:1 (MET)CIO4 salt at _=_ 180 K and is indicated in the 1:1 (MET)PF 6 salt on the basis of low temperature ESR data. The packing of the donor molecules in the two 1:1 salts is similar to that previously reported in (BEDSe-TSeF)PF6. The structure type observed for (MET)3(PF6)2 is most closely related that observed in 3:1 MT salts. INTRODUCTION Numerous synthetic metals (synmetals) based on the organic donor BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene or simply ET, 1) have been synthesized with transport properties that range from semiconducting to metallic to superconducting [1]. In the ET system, we have shown that the anion size [2] and anion-CH2 hydrogen interactions [3,4] are critical factors that control the packing of the ET donor molecule and thus determine the transport properties of the charge-transfer salt. Changes in the donor-to-anion hydrogen bonding interactions which accompany variations in anion size have been correlated to the lattice softness and phonon frequencies and therefore, to the superconducting T c values [4]. 0379-6779/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands