Ferrocene: Ironclad History or Rashomon Tale?**Pierre Laszlo and Roald Hoffmann*A critical stance is essential to science. Proving otherpeople wrong is a favorite private and public satisfactionÐthere is nothing some scientists like better. But, excess zealdiscounted, doubt serves as a powerful impulse to theadvancement of knowledge. We document it here with thediscovery of the structure of ferrocene, a story which alsoplays up the virtue of the spoken word.[1]We base it on variouspublished (and most fragmentary, if not mutually inconsis-tent!) accounts, supplemented with some very helpful corre-spondence from colleagues.Two groups independently reported at the very end of 1951preparation of a compound C10H10Fe from cyclopentadiene.Kealy and Pauson published their contribution in theDecember 15, 1951 edition of Nature, while Miller, Tebboth,and Tremaines account of their independent (and apparentlyearlier) preparation of the identical compound, with the sameªtraditionalº formulation (what would today be called a di-s-complex), appeared a little later in the Journal of the ChemicalSociety.[2]Lets turn to the reception of those two papers in theChemistry Department at Harvard University. As soon as itarrived in the library, during the first couple of weeks inJanuary 1952, R. B. Woodward apparently saw the Kealy andPauson piece. And he had a hunch that the structure waswrong. He set a graduate student, Myron Rosenblum,preparing not only ferrocene, but also analogues from othertransition metals and cyclopentadiene. For this Rosenblumneeded ruthenium trichloride. He went to Geoffrey Wilkin-son, who had recently taken up an Assistant professorship atHarvard, to ask for a sample of ruthenium. Wilkinson ªratherbrusquely wanted to knowº what Rosenblum needed it for.[3]Wilkinson writes:[4]ª...Mike Rosenblum came into my laboratory asking if Ihad got any ruthenium. I cant remember what I said, exceptthat I think it was along the lines of let me tell you what youwant it for. ºClearly, Wilkinson had already also seen the Kealy andPauson paper and had decided to study this problematicstructure of ferrocene.[4]Wilkinson continues:ªHowever, the upshot was that Woodward and I had lunch atthe Harvard Faculty Club on Monday[5]and sorted thingsout. The possibility that the C5H5ring in the iron compoundcould possibly undergo Friedel ± Crafts or other aromaticreactions simply had not dawned on me, but other than thestructure, this seemed to be Bobs main interest, whereasmine was to go on to other transition metals.ºOne can only bemoan the fact that Woodward left noaccount of the discovery, and apply all the reservations thescience historian has painfully had to learn of the constructionof history (yes, even by the very participants) after the fact.Myron Rosenblum offers a some what different perspectiveon the directions taken by the protagonists:[3]ªWoodward certainly had an interest in extending themetallocene series vertically along the periodic table andpossibly horizontally as well, since I set up four reactionssimultaneously on January 31st. These involved the reac-tions of NiCl2, CoCl2,CrCl3, and RuCl3with cyclopenta-dienylmagnesium bromide. From my research notebook Isee that I had actually made two attempts to prepareanhydrous RuCl3on January 16th, five days prior to my firstpreparation of ferrocene itself. But in any event, Woodwardand Wilkinson had apparently reached a private agreementwhich apparently left this initiative (extending the ferrocenefamily) to Geoff.ºWoodwards contribution to the ferrocene story (and dareone identify this as the main contribution, too?) was the divina-tion of the sandwich structure, and a beautiful piece of insightabout its aromaticity. Of the latter Rosenblum writes:[3]ªOn a Thursday evening, March 13th (1952), as we weretaking our seats before that evenings seminar, we werechatting about some of the chemistry of this new substance,when [RBW] suggested simply, almost matter-of-factly, thatI might want to try a Friedel ± Crafts reaction on this newsubstance....I put off doing the reaction until Monday,March 17th...the product was a beautiful red crystallinematerial with an enormous carbonyl stretching absorption inthe infrared. ºESSAYAngew. Chem. Int. Ed. 2000, 39, No. 1  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0570-0833/00/3901-0123 $ 17.50+.50/0 123[*] Prof. R. HoffmannDepartment of Chemistry, Cornell UniversityBaker Laboratory, Ithaca, NY 14853-1301 (USA)Fax : ( 1) 607-255-5707E-mail: [email protected]. LaszloLaboratoire de Chimie, Ecole Polytechnique91128 Palaiseau CeÂdex (France)Fax : ( 33) 169333010E-mail: [email protected][**] Akira Kurosawas classic film ªRashomonº depicts a violent event (arape and murder in a forest) as seen by four participants. In hisautobiography (A. Kurosawa, Something Like an Autobiography,Vintage Books ±Random House, New York, 1983, p. 183). Kurosawawrites of the difficulty his co-workers had understanding the complexplot. He told them: ªThis film is like a strange picture scroll that isunrolled and displayed by the ego...you cant understand the script atall, but that is because the human heart itself is impossible tounderstand.ºThe correct ferrocene structure and its aromaticity were mostrapidly published in the form of a communication in Journalof the American Chemical Society ;[6]the coauthors were asidefrom Wilkinson and Woodward, Rosenblum and Mark C.Whiting (a postdoctoral associate in the Woodward group,who joined the research effort).Meanwhile, during the spring of 1952, E. O. Fischer wasindependently studying the structure of ferrocene in hislaboratory at the Technische Hochschule in Munich (with W.Pfab). Basing his conclusions on evidence from X-raycrystallography, and on very reasonable bonding notions, healso concluded that the molecule was best formulated as aniron(ii) atom in-between two cyclopentadienide rings asligands. Fischer and his co-workers immediately synthesizedthe ferrocenium cation, and started exploring cognate mole-cules such as the corresponding cobaltocene.[7]The structures proposed by Wilkinson, Woodward, Rose-nblum, Whiting, Fischer, and Pfab were truly revolutionary.Contemporary chemists were shocked; here is a recentaccount by one of the greatest of structural chemists, J. D.Dunitz:[8]ªOne afternoon, I opened the Library copy of JACS [inCambridge, England] and came across R. B. Woodwardsproposal that the molecule