{"id":3956,"date":"2011-04-25T08:19:10","date_gmt":"2011-04-25T08:19:10","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=3956"},"modified":"2013-03-12T08:02:10","modified_gmt":"2013-03-12T08:02:10","slug":"beryllocene-and-uranocene-the-8-18-and-32-electron-rules","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3956","title":{"rendered":"Beryllocene and Uranocene: The 8, 18 and 32-electron rules."},"content":{"rendered":"
\n

In discussing ferrocene in the previous post, I mentioned Irving Langmuir’s 1921 postulate that filled valence shells in what he called\u00a0complete<\/em> molecules would have magic numbers of 2, 8, 18 or 32 electrons (deriving from the sum of terms in 2[1+3+5+7]). The first two dominate organic chemistry of course, whilst the third is illustrated by the transition series, ferrocene being an example of such. The fourth case is very much rarer, only one example ever having\u00a0been suggested[1]<\/a><\/span>, it\u00a0deriving from the actinides. In this post, I thought I would augment ferrocene (an 18-electron example) with beryllocene (an 8-electron example) and then speculate about 32-electron metallocenes.<\/p>\n

\"\"

Cp*-beryllocene. ELF analysis. Click for 3D.<\/p><\/div>The crystal structure of (nonamethyl)bis-cyclopentadienyl beryllium [2]<\/a><\/span>\u00a0illustrates the octet rule directly. Be is ionised to Be2+<\/sup>, the charge balanced by two cyclopentadienyl anions. The octet is formally filled by donation of six electrons from one Cp* anion, and only two from the other, filling the s and p shells of the metal (the 1 and 3 in the sum alluded to earlier). The ELF analysis suggests the molecule is less ionic than ferrocene. ELF disynaptic basis are located for all five Be-C bonds on the \u03b7-5 ring, and only one for the \u03b7-1 ring. The latter basin contains 1.87 electrons (a conventional electron pair bond), whilst the five former range range from 0.57 to 0.68 electrons, adding to 5.02. The formal octet is thus not entirely filled, but in this sense, it is less ionic than ferrocene. (See DOI 10042\/to-8371<\/a> for details of the calculation).<\/p>\n

 <\/p>\n

Uranocene is a rather different beast. The ligands are not cyclopentadienyl, but cyclo-octatetraenyl. Uranium has a radon core, and a 5f3<\/sup>, 6d1<\/sup> and 7s2<\/sup> valence shell(s) electron configuration. Ionised to U4+<\/sup>, formally the 5f, 6d and 7p shells are all empty; a total of 14 + 10 + 6 electrons would be required to achieve a 32-electron filled shell , or 30 additional electrons. The two COT ligands, as di-anions (achieving aromaticity) could provide only 20. So uranocene (Cambridge refcode URACEN10, DOI\u00a010.1021\/ic50111a034<\/a>) is far from the holy-grail of a 32-electron complete<\/em> molecule.<\/p>\n

\"\"

Uranocene. AIM analysis. Click for 3D<\/p><\/div>The QTAIM analysis of the electron density (the molecule itself is a triplet spin state) shows only six bonds from each COT ligand to the metal. The ELF analysis shows NO U-C disynaptic basins, unlike either beryllocene or ferrocene (the features surrounding the U derive from pseudopotential used for the calculation). This indicates that uranocene is the most ionic of the three metallocenes.<\/p>\n

 <\/p>\n

\"\"

Uranocene. ELF analysis. Click for 3D<\/p><\/div>Could a molecule be contrived that might achieve (a formal) 32-electron filled 5f,6d,7p<\/em> valence shell? One would probably need a ligand contributing 14<\/strong> rather than 10<\/strong> electrons whilst keeping the size of the ring manageable, quite a challenge. There may not be enough space for three 10-electron ligands. So, no examples of 32-electron metallocenes just yet then!<\/p>\n

 <\/p>\n

References<\/h2>\n
  1. 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
  2. M.M. Conejo, R. Fern\u00e1ndez, D. del R\u00edo, E. Carmona, A. Monge, and C. Ruiz, \"Synthesis and structural characterization of Be(\u03b7<sup>5<\/sup>-C<sub>5<\/sub>Me<sub>5<\/sub>)(\u03b7<sup>1<\/sup>-C<sub>5<\/sub>Me<sub>4<\/sub>H). Evidence for ring-inversion leading to Be(\u03b7<sup>5<\/sup>-C<sub>5<\/sub>Me<sub>4<\/sub>H)(\u03b7<sup>1<\/sup>-C<sub>5<\/sub>Me<sub>5<\/sub>)\", Chem. Commun.<\/i>, pp. 2916-2917, 2002. https:\/\/doi.org\/10.1039\/b208972f<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    In discussing ferrocene in the previous post, I mentioned Irving Langmuir’s 1921 postulate that filled valence shells in what he called\u00a0complete molecules would have magic numbers of 2, 8, 18 or 32 electrons (deriving from the sum of terms in 2[1+3+5+7]). The first two dominate organic chemistry of course, whilst the third is illustrated by […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_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}},"categories":[4],"tags":[514,513,144,518,157,74,515],"ppma_author":[2661],"class_list":["post-3956","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-8-electron-rule","tag-beryllocene","tag-cambridge","tag-irving-langmuir","tag-metal","tag-pence","tag-uranocene"],"yoast_head":"\nBeryllocene and Uranocene: The 8, 18 and 32-electron rules. - 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=3956\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Beryllocene and Uranocene: The 8, 18 and 32-electron rules. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"In discussing ferrocene in the previous post, I mentioned Irving Langmuir’s 1921 postulate that filled valence shells in what he called\u00a0complete molecules would have magic numbers of 2, 8, 18 or 32 electrons (deriving from the sum of terms in 2[1+3+5+7]). The first two dominate organic chemistry of course, whilst the third is illustrated by […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3956\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-04-25T08:19:10+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-03-12T08:02:10+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/04\/beryllocene-elf.jpg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Beryllocene and Uranocene: The 8, 18 and 32-electron rules. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3956","og_locale":"en_GB","og_type":"article","og_title":"Beryllocene and Uranocene: The 8, 18 and 32-electron rules. - Henry Rzepa's Blog","og_description":"In discussing ferrocene in the previous post, I mentioned Irving Langmuir’s 1921 postulate that filled valence shells in what he called\u00a0complete molecules would have magic numbers of 2, 8, 18 or 32 electrons (deriving from the sum of terms in 2[1+3+5+7]). 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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\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":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/04\/ferrocene-aim.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":23588,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23588","url_meta":{"origin":3956,"position":1},"title":"Two new reality-based suggestions for molecules with a metal M\u2a78C quadruple bond.","author":"Henry Rzepa","date":"May 8, 2021","format":false,"excerpt":"Following from much discussion over the last decade about the nature of C2, a diatomic molecule which some have suggested sustains a quadruple bond between the two carbon atoms, new ideas are now appearing for molecules in which such a bond may also exist between carbon and a transition metal\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/05\/Screenshot-702-300x63.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":30548,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30548","url_meta":{"origin":3956,"position":2},"title":"Molecules of the year 2025: Benzene-busting inverted sandwich.","author":"Henry Rzepa","date":"January 1, 2026","format":false,"excerpt":"Sandwich compounds are the colloquial term used for molecules where a metal atom such as an iron dication is \"sandwiched\" between two carbon-based rings as ligands, most commonly cyclopentadienyl anion (the \"bread\") as in e.g. Ferrocene - a molecule first discovered in 1951. 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":9218,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9218","url_meta":{"origin":3956,"position":3},"title":"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh.","author":"Henry Rzepa","date":"January 19, 2013","format":false,"excerpt":"The transient \u03c0-complex formed during the \"[5,5]\" sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity? I showed previously that\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":"pi-QTAIM","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/pi-QTAIM.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17142,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17142","url_meta":{"origin":3956,"position":4},"title":"Molecule of the year (month\/week)?","author":"Henry Rzepa","date":"December 12, 2016","format":false,"excerpt":"Chemical and engineering news (C&EN) is asking people to vote for their molecule of the year from six highlighted candidates. This reminded me of the history of internet-based \"molecules of the moment\". It is thought that the concept originated in December 1995 here at Imperial and in January 1996 at\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/12\/ferris.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10937,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10937","url_meta":{"origin":3956,"position":5},"title":"VSEPR Theory: A closer look at chlorine trifluoride, ClF3.","author":"Henry Rzepa","date":"July 27, 2013","format":false,"excerpt":"Valence shell electron pair repulsion theory is a simple way of rationalising the shapes of many compounds in which a main group element is surrounded by ligands. ClF3 is a good illustration of this theory. The standard application of VSEPR theory to this molecule is as follows: Central atom: chlorine\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"VSEPR","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/VSEPR.jpg?resize=350%2C200","width":350,"height":200},"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","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3956","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=3956"}],"version-history":[{"count":4,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3956\/revisions"}],"predecessor-version":[{"id":9777,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3956\/revisions\/9777"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3956"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3956"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3956"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=3956"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}