X-RAY CRYSTAL-STRUCTURES OF 2 2,3,4,5,6-HEXAMETHYLBICYCLO[2.1.1]HEX-2-EN-5-YLIUM IONS

The crystal structures of a 6-chloro-substituted and of a 6-hydrogen-s ubstituted example for the title compound (1 and endo-2) were determin ed at low temperatures, because earlier published NMR results of Hogev een and Olah indicated the presence of a three center, two electron bo nd (3c, 2e bond) in such cations. While 1.BCl4 is (also due to the abs ence of heavy atoms like Sb) a rather precise structure (R1 = 4.37%, s igma(C-C) less than or equal to 0.003 Angstrom; 1 has a crystallograph ic mirror plane), endo-2.CF3SO3 has a disordered anion, and the cation endo-2 was refined with an enforced mirror plane (R1 12.33%). A third structure (endo-2.CF3SO3-SbF5) is mentioned but, due to cation and an ion disorder, not discussed. The structure of 1 shows an unusually str ong 3c, 2e bond (C2-C3 = 1.406(1), C2-C5 = C3-C5 = 1.741(2) A), and it s displacement parameters agree with a static structure in the crystal , i.e., pairs of equilibrating ions can be excluded. The results of a new refinement of the earlier published crystal structure of the 7-nor bornenyl ion 5, which has a weaker 3c, 2e bond, are enclosed for compa rison. Detailed experimental procedures are given.