Infinite ribbons with only very short intermolecular ClCl contacts, and not dipole–dipole interactions, characterize the crystal packing of ClF (depicted on the right). The explanation for this surprising finding is that chlorine in ClF may act simultaneously as an electron donor and acceptor, as shown by the experimental Laplacian charge density maps and ab initio calculations for the CIF dimer.

the same type of structure (Fig. 2) for the anion 2 as mentioned above for the case of the molybdenum compounds. The oxygen atom bridges all three titanium atoms and each pair of Ti atoms is linked via a side-on coordinated disulfide unit. The Ti-Ti distances of 314 to 318 pm are markedly greater than the Mo-Mo distances in the molybdenum clusters-as to be expected for a Ti'" compound, [**I Hetero n-Systems, Part 15. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen 1ndustrie.-Part 14: [S]. Angew. Chem. Inr. Ed. Engl. 27 11988) No. 2 0 VCH Verlogsgesellschqfr mbH, 0-6940 Weinheim, 1988 0570-0833/88/0202-0295 $ 02.50/0

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lo] 2a : Crystal suitable for X-ray diffraction grown from dichloromethane/ ethanol under nitrogen. Triclinic P i , a = 1052.4(4), b = 1801.0(3), c = 1038.9(5) pm, a = 103.26 (3),y=76.11(2)", V = 1 . 7 0 5 6 2~ lo6 pm', 2 = 2 ; /I(MoK,)=71.069 pm; R=0.0398, R,, =0.0409 for 3455 unique reflections [2<20<46', 1>3o(I)], measured on a CAD4 diffractometer and solved using SHELX76. Further details of the crystal structure investigation are available on request from the Director of the tinue, albeit in a weaker form, in the case of 1,2-dihydrocyclobutabenzene 2aC2l and more markedly so in the case of 1,2,4,5-tetrahydrodicyclobuta(a,d)benzene 3.13. 41 So far X-ray structure analyses have been reported only for derivatives of 2a (2b-2d),[51 while in the case of 3 only one structure determination (from the year 1969)I6I exists, whose standard deviations d o not permit an interpretation of the bond lengths in the benzene ring. 2a, R' = ~2 = H 2b, R', R2 = 0 &:: 2 c , R' = H, R2 = CI, Br, I R' 2d. R' = H, R2 = ONO,The substituents in 2b-2d give rise to reductions in symmetry and electron shifts that are not observed in the parent compound (cf. also the situation in l"]). We have therefore carried out an X-ray structure analysis of 2a (m.p. ca. -75°C) at -170°C and calculated the X-X deformation electron density ; single crystals prepared by a Fig. 1. a) 2a with atomic labeling and important distances [A] and angles ["I (standard deviations 0.001 A and 0.1 resp.) averaged for mm2 symmetry (corrected values according to Schomaker and Trueblood [I31 in brackets): 86.5. b) X-X-deformation electron densities of 2a in the plane of the-molecule. Contour interval 0.025 eA', negative electron densities dashed, intervals 0.05 eA3. c) Bond lengths [A] and angles ["I of 2a, calculated in each case linearly via the maximum of the deformation electron densities (estimated standard deviations about I in the last decimal place):

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