Crystal data (from single-crystal measurements) are reported for 19 different crystalline n-molecular compounds, representing 16 different chemical species (three polymorphic pairs are included). 13 of these molecular compounds have equimolar compositions, five have donor:acceptor ratios of 1:2 and the remaining one has composition (fluorene)3 :( 1,3,5-trinit robenzene)4.

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Welcnzan and Regev 761 1 1369. Pentajluoroethyl Derivatives of Selenium B y N. WELCMAN and (MRs.) H. REGEVSeveral pentafluoroethyl derivatives of selenium have been prepared by the use of the mercurial, (C,F,Se),Hg, or preferably the diselenide, (C,F,) ,Se,, as starting materials. Reactions of these new compounds were studied, and some of their physical properties are described. Pentafluoroethyl selenyl trichloride is stable under excess pressure of chlorine, and the tertiary pentafluoroethylselenyl amine seems to be unstable. PERFLUOROALKYL derivatives of metals and non-metals can be prepared mainly by two types of reaction. (1)The direct reaction between the element and the respective perfluoroiodoalkane.(2) The thermal interaction of the element and a heavy-metal salt of the appropriate perfluoroalkyl acid.Most perfluoroalkyl compounds of many elements, notably those of phosphorus, arsenic, antimony, and sulphur, were prepared by the first meth0d.l Bistrifluoromethyl mono-and di-selenide were also prepared by the interaction of trifluoroiodomethane and selenium.2 Bis-pentafluoroethyl and -heptafluoropropyl mono-and di-selenides, on the other hand, were prepared by the action of mercuric or silver propionate and butyrate, respectively, on selenium metal p ~w d e r . ~ Bis-trifluoromethyl and -pentafluoroethyl diselenide were separated from the respective original reaction mixtures by fractional vacuum distillation, but the corresponding separation of bisheptafluoropropyl diselenide was not fea~ible.~ Bispentafl uoroethyl diselenide reacted readily with the stoicheiometric amount of chlorine to form pentafluoroethylselenyl chloride, and with excess of chlorine it gave pent afluoroethylselenyl trichloride. This is a pale yellow solid, stable only under pressure of chlorine, and on exposure to the atmosphere it gives a colourless, volatile solid, soluble in water to give an acid solution.The reaction between the diselenide and mercury was stoicheiometric and gave the mercurial, (C2F5Se),Hg, a white, crystalline solid, m. p. 3 9 4 0 " . This mercurial reacted at room temperature instantaneously with iodine to give the diselenide, but the reaction (a)

The influence of rr-molecular compound formation on the deviations from planarity of the overcrowded aromatic molecule benzo[clphenanthrene has been studied by determining the crystal structure of benzolclphenanthrene-2,3-dichloro-5,6-dicyanobenzoquinone l a = 10.953 (2), b = 16.090 (3), c -13. 827 (3) A, fl = 122.63 (1)°: space group P2t/c; Z = 4]. The structure was solved by direct methods and refined to R = 0.065 for 3385 observed reflexions. Stacks with alternating donor and acceptor molecules extend in the e direction. The acceptor molecule takes on two orientations, with population ratio'-'4:1. Comparison of the structures of the benzo[clphenanthrene molecule here and in its pure crystals shows that it is significantly less distorted from planarity in the present molecular compound, but that bond lengths and angles remain practically unchanged. The diminished distortion from planarity may be due to the donor-acceptor interaction.

Pyrene : p -benzoquinone (C 16 H 10 :C 6 H 4 O 2 ) crystallizes in the tetragonal system, with cell constants a = 7.698(2), c = 25.570(5) Å, space group P 4 1 and four formula units per unit cell. The structure was solved by direct methods and refined to an R value of 0.048 for 778 observed reflexions. The pyrene and p -benzoquinone molecules are arranged alternately in mixed stacks, with interplanar spacing of 3.49 Å and an angle of 1.1° between molecular planes. The mixed stacks are arranged in sheets parallel to (001), successive sheets being related by the 4 1 axis. This type of arrangement of mixed stacks has not been encountered previously among π -molecular compounds. Neither the stacks nor their arrangement are centrosymmetric; as the component molecules are both centrosymmetric this constitutes a second unusual feature of this crystal structure.