The results of positron lifetime and ultrasonic velocity measurements in dilute aqueous dimethylsulfoxide (DMSO) solutions show the usual extreme characteristic for polar organic solutes. The results obtained suggest that strong structural changes occur in the system, in the region of low DMSO concentrations. Liquid clathrate hydrates forming in the systeni are supposed, with unusual stoichiometry and untypical geometry of the hydrates.PACS numbers: 61.25. Em, 78.70.Bj It is a well established fact now that the thermodynamic properties of aqueous solutions of apolar compounds or molecules having both polar and polar components (polar organic molecules) at low concentrations are predominantly determined by hydrophobic interactions which result in an enhancement of water structure. The stabilization of water stucture also gets support by NMR [1] and infrared spectroscopy [2], by X-ray diffractions [3] as well as by studies on molecular dynamics [4].Because of the fact that many of these organic compounds form clathrate hydrates in the solid state, attempts to explain the characteristic behaviour of liquid solutions in terms of clathrate-like structures seem to be more legitimate than interpretations based on other concepts.

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From a suspension of AsPh4ReO 4 or PPh4ReO 4 in various solutions such as ethanol, methanol, acetone, chloroform, water, benzene, formic acid and others, the red compounds containing oxopentachlororhenate(VI) anions were obtained on reaction with gaseous HCI. AsPh4ReOCI 5 and PPh4ReOC15 crystallize in the space group P4/n, with similar axial lengths. From solutions of these complexes in ethanol the yellow complexes AsPhaReOC14 or PPh4ReOCI 4 could be obtained. The crystal structure of AsPh4ReOCI4 has been determined by single-crystal diffractometry. The structure was solved by the heavy-atom technique and refined by the block-diagonal least-squares method to an R = 0.059 for 1134 reflexions. In the squarepyramidal [ReOCI4I-anion the Re and O atoms lie on a fourfold axis, with Re-O and Re-CI 1.63 (3) and 2-344 (4) A respectively. The AsPh~ cation has 4 ($4) crystallographic symmetry, with As-C 1.91 (1) A and two independent C-As-C angles of 107-2 (5) and 110.6 (5) °. [Crystal data for AsPh4ReOCI4: space group P4/n, a = 13.117 (6), c ---7.358 (4) A.]

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