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The crystal structure of the monoclinic polymorph of a,a-diferrocenylmethylium tetrafluoroborate ( a = 15.99, b = 12.75, c = 9.365 A, 6 = 90.6', space group P21/a, 4 formula units per cell) has been determined by standard methods using 2648 (222 with Iobsd = 0) intensities measured on a Weissenberg diffractometer using graphite-monochromated Mo Ka radiation. The final R factor was 9.3%. The cation has a transoid conformation, similar to that found for diferrocenyl ketone: the BF4-ions are orientationally disordered over three groups of sites. The ferrocenyl rings are bent at the a-carbon atoms so as to reduce the distances between the two Fe atoms and the exocyclic carbon atom to 2.69 and 2.81 A. Analogous distortions have been reported in the bis[(cyclopentadienyl)dicarbonyliron]propylenium cation (Fe --C+ distances 2.57,2.72 A) and to a lesser extent in other organometallic cations. We conclude that both metal-exocyclic carbon atom interaction, with resulting geometrical distortion of the cation, and delocalization of the positive charge over suitable regions of the cation lead to stabilization of organometallic carbenium cations.The exceptional stability of ferrocenylcarbenium ions has been considered a major factor governing the course of reaction in much of the organic chemistry of the ferrocenyl system leading to an intensive search for the sources of such ~tability.~ Success has not yet been achieved. In the hope that particular features of the geometry of such carbenium ions hold the clue to their stabilization, we have determined the crystal structure of the stable carbenium ion salt a,a-diferrocenylmethylium tetrafluoroborate (I). The results have been briefly r e p~r t e d .~ r (C0)S 1Tv V The other organometallic carbenium ion salts whose crystal structures have been reported are bis(a-cyclobutadienetricarbony1iron)phenylcarbenium tetrafluoroborate (11),5 ferrocenyldiphenylcyclopropenium tetrafluoroborate (III)? and bis[(cyclopentadienyl)dicarbonyliron]propylenium hexafluorophosphate7(IV). Intercomparison of all these results suggests that there are two interrelated factors which stabilize organometallic moiety in the delocalization of the positive charge, the other is the ability of the molecule to undergo geometrical changes which result in greater bonding interac-0002-7863/78/1500-5554$01 .OO/O tion between the metal atom and the formally positive ligand moiety. Experimental SectionThe (air-stable) title compound (see Figure 2 for crystallographic numbering) was synthesized by adaption of techniques developed by Cais and Eisenstadt8 and recrystallized from methanol. A number of salts were studied and the tetrafluoroborate was found to be the most stable and to give the best crystals for d i f f r a~t i o n .~ Two polymorphs were found, one monoclinic reported here in detail (Table I) andoneorthorhombic(a = 18.27(1),b = 1 1 . 8 8 ( 1 ) ,~= 8 . 9 8 ( 2 ) A , V = 1944 A3, dc&d = 1.59 g cm-3 for z = 4, d, , , , = 1.55 g cm-3).Unfortunately the orthorhombic crystal used for photography was lost before t...

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Das aus dem Ferrocenylcarbinol (I) durch Umsetzung mit Tetrafluoroborsäure in Acetanhydrid hergestellte Ferrocenylcarboniumsalz (II) reagiert nach der Fällung mit Tetrachlorkohlenstoff oder Cyclohexan und Umsetzung mit Äthyl‐diisopropylamin über radikalische Zwischenstufen zu den Verbindungen (III)‐(VI), während bei der Fällung des Salzes mit Diäthyläther und Umsetzung mit dem Amin noch die beiden Verbindungen (VII) und (VIII) erhalten werden.

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