{"id":3908,"date":"2011-04-17T15:02:25","date_gmt":"2011-04-17T15:02:25","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=3908"},"modified":"2021-01-25T18:10:54","modified_gmt":"2021-01-25T18:10:54","slug":"ferrocene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3908","title":{"rendered":"Ferrocene"},"content":{"rendered":"
\n

\n\tThe structure of ferrocene was famously analysed by Woodward and Wilkinson in 1952[1]<\/a><\/span>,[2]<\/a><\/span>, symmetrically straddled in history by Pauling<\/a> (1951) and Watson and Crick<\/a> (1953). Quite a trio of Nobel-prize winning molecular structural analyses, all based on a large dose of intuition. The structures of both proteins and DNA succumbed to models built from simple Lewis-type<\/a> molecules with covalent (and hydrogen) bonds; ferrocene is intriguingly similar and yet different. Similar because e.g. carbon via<\/em> four electron pair<\/strong> bonds. He did not (in 1916) realise that 8<\/strong> = 2(1 + 3), and that the next in sequence would be 18<\/strong> = 2(1 + 3 + 5). That would have to wait for quantum mechanics, and of course inorganic chemists now call it the 18-electron rule <\/strong>(for an example of the 32-electron rule, or 2+6+10+14<\/strong>, as first suggested by Langmuir in 1921[3]<\/a><\/span> (see also here[4]<\/a><\/span>).
\n<\/lewis><\/p>\n

\n\tIron has an argon core, and 6 (of a maximum of 10) 3d electrons, 2 (of a maximum of 2) 4s, and 0 (of a maximum of 6) 4p electrons. Or, 8 = 2 + 0 + 6 rather than 18 = 2 + 6 + 10. So Woodward and Wilkinson argued that sharing a further 10 electrons would bring iron up to, in effect, a Lewis shell (albeit one using not just s and p shells, but d shells too). These 10 electrons would be provided by two cyclopentadienyl radicals. Thence a Nobel prize (for Wilkinson)! But wait!\n<\/p>\n

\"\"

QTAIM analysis for Ferrocene. Green=bcp, red=rrp, blue=ccp.<\/p><\/div>\n

\n\tFirstly, let me adjust slightly the counting above. Rather than starting with neutral Fe, we ionise it to Fe2+<\/sup>. Rather than starting with two cyclopentadienyl radicals, let us use two aromatic cyclopentadienyl anions. But now, Lewis' idea of covalency via<\/em> shared electron pair bonds struggles. If 18 electrons really are being deployed (12 from the cyclopentadienyl anions, 6 from the Fe2+<\/sup>), does that imply 9 shared electron-pair bonds? How might the bonds in ferrocene be represented? This matters. Since the 1970s, the idea of searching for molecules via<\/em> what is called its connectivity (a simple index which ignores bond order, and simply specifies whether two atoms are connected by a bond, any kind of bond, or not) has revolutionised searching for molecules. Think of CAS, Pubchem, REAXYS, CCDC, and SMILES and InChI. So is it useful to try to partition 9 bonds into ferrocene (it kind of difficult, since it has five-fold symmetry)? Indeed, most students trying to search for a ferrocene in any of the aforementioned databases will scratch their head over this one. The normal solution is to draw 10 bonds<\/strong> from the iron, one to each of the ten carbon atoms (and the databases accept this as a valid search query). But what can that mean?\n<\/p>\n

\n\tTo find out, I show here a QTAIM and an ELF analysis of the bonds <\/em>(in italics, since we do not know if these conform to Lewis covalent bonds or not). The QTAIM is shown above, and it shows a bond-critical point along all ten of the Fe…C regions. The electron density, ρ(r) at each of these is 0.083 au. In truth, this is rather low, even for a single bond. The Laplacian ∇2<\/sup>ρ(r) at each of these points is +0.29, which is in what Hiberty and Shaik<\/a> have called the charge-shift category (i.e. NOT a covalent bond). The Laplacian isosurface is shown below, contoured at 0.25, and you can see each of the bond-critical points for the ten Fe-C regions is shrouded in blue (a positive Laplacian). So, NOT a pure covalent two-electron bond of the Lewis variety then(?).\n<\/p>\n

\n\t

\"\"

Electronic Laplacian for ferrocene. Click for 3D<\/p><\/div>How about how many electrons<\/strong> are there in these Fe-C bonds<\/em>? Enter that other method known as ELF (also the subject of many blog posts here, go searching if you want to find out more).;\n<\/p>\n

\n\t \n<\/p>\n

\n\t

\"\"

Centroids of ELF basins for ferrocene. Click for 3D.<\/p><\/div>Yes, there again are C-Fe disynaptic basins! But the integration of the basin is a measly 0.3e (of ~3.1 electrons for all ten Fe-C bonds<\/em>). Of course, ELF can detect the difference between covalency and ionicity (the disynaptic basins vanish for ionic bonds), and the low basin count suggests a fair degree of the latter. The ELF function itself is pretty, and is shown below.\n<\/p>\n

\n\t \n<\/p>\n

\n\t

\"\"

ELF isosurface for ferrocene. Click for 3D<\/p><\/div>We can see that ferrocene has departed a long way from Lewis' model of an electron pair bond, or perhaps even from the covalent bond. But at least we can be assured that the connection table often used for searching for ferrocene and its derivatives is not a fiction! Next, uranocene!<\/p>\n

References<\/h2>\n
  1. G. Wilkinson, M. Rosenblum, M.C. Whiting, and R.B. Woodward, \"THE STRUCTURE OF IRON BIS-CYCLOPENTADIENYL\", Journal of the American Chemical Society<\/i>, vol. 74, pp. 2125-2126, 1952. https:\/\/doi.org\/10.1021\/ja01128a527<\/a>\n\n<\/li>\n
  2. G. Wilkinson, \"The iron sandwich. A recollection of the first four months\", Journal of Organometallic Chemistry<\/i>, vol. 100, pp. 273-278, 1975. https:\/\/doi.org\/10.1016\/s0022-328x(00)88947-0<\/a>\n\n<\/li>\n
  3. I. Langmuir, \"Types of Valence\", Science<\/i>, vol. 54, pp. 59-67, 1921. https:\/\/doi.org\/10.1126\/science.54.1386.59<\/a>\n\n<\/li>\n
  4. J. Dognon, C. Clavagu\u00e9ra, and P. Pyykk\u00f6, \"Towards a 32\u2010Electron Principle: Pu@Pb<sub>12<\/sub> and Related Systems\", Angewandte Chemie International Edition<\/i>, vol. 46, pp. 1427-1430, 2007. https:\/\/doi.org\/10.1002\/anie.200604198<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The structure of ferrocene was famously analysed by Woodward and Wilkinson in 1952,, symmetrically straddled in history by Pauling (1951) and Watson and Crick (1953). Quite a trio of Nobel-prize winning molecular structural analyses, all based on a large dose of intuition. The structures of both proteins and DNA succumbed to models built from simple […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[1176,512,1177,511,2651,516,74,373,517],"ppma_author":[2661],"class_list":["post-3908","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-10-1126","tag-18-electron-rule","tag-54-1386-59","tag-ferrocene","tag-historical","tag-normal-solution","tag-pence","tag-tutorial-material","tag-valid-search-query"],"yoast_head":"\nFerrocene - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3908\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Ferrocene - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The structure of ferrocene was famously analysed by Woodward and Wilkinson in 1952,, symmetrically straddled in history by Pauling (1951) and Watson and Crick (1953). 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An \"inverted\" sandwich is where the carbon\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":15992,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15992","url_meta":{"origin":3908,"position":4},"title":"Celebrating Paul Schleyer: searching for hidden treasures in the structures of metallocene complexes.","author":"Henry Rzepa","date":"April 2, 2016","format":false,"excerpt":"A celebration of the life and work of the great chemist\u00a0Paul von R. Schleyer was held this week in Erlangen, Germany. There were many fantastic talks given by some great chemists describing fascinating chemistry. Here I highlight the presentation\u00a0given by\u00a0Andy Streitwieser on the topic of organolithium chemistry, also a great\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":10448,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10448","url_meta":{"origin":3908,"position":5},"title":"The \u03c0-complex theory of metal-alkene compounds.","author":"Henry Rzepa","date":"May 13, 2013","format":false,"excerpt":"The period 1951--1954 was a golden one for structural chemistry; proteins, DNA, Ferrocene (1952) and the one I discuss here, a bonding model for Zeise's salt (3). In\u00a0\"A review of \u03c0 Complex Theory\",\u00a0 Bull. Soc. Chim. Fr., 1951, 1 8 , C79 (it is not online) M. J. S.\u00a0Dewar\u00a0sets out\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3908","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3908"}],"version-history":[{"count":4,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3908\/revisions"}],"predecessor-version":[{"id":23280,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3908\/revisions\/23280"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3908"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3908"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3908"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=3908"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}