{"id":9973,"date":"2013-03-26T13:38:26","date_gmt":"2013-03-26T13:38:26","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9973"},"modified":"2014-01-17T07:42:21","modified_gmt":"2014-01-17T07:42:21","slug":"a-very-short-history-of-shared-electron-bonds","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9973","title":{"rendered":"A (very) short history of shared-electron bonds."},"content":{"rendered":"
\n

The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. N. Lewis suggested a model for single and double bonds that involved sharing either 2 or 4 electrons between a pair of atoms[1]<\/a><\/span>. We tend to think of such (even electron) bonds in terms of their formal bond order (an integer), recognising that the actual bond order (however defined) may not fulfil this value. I thought I would very (very) briefly review the history of such bonds.<\/p>\n

    \n
  1. 1916:<\/strong> G. N. Lewis[1]<\/a><\/span> proposed a model for carbon involving\u00a0a cube with one electron at each corner, thus making an octet.\u2021<\/sup> A single bond<\/strong> would be created by two atoms sharing a common edge (= 2 shared electrons), and a double bond<\/strong>\u00a0by sharing a common face (= 4 shared electrons). The recognition that the formal bond order of two could be partitioned into one electron pair of\u00a0\u03c3 symmetry and one of \u03c0 was not achieved until ~1929 (by H\u00fcckel). It is also now recognised that whilst most bonds of order 1 are of type \u03c3, a rare few can be\u00a0\u03c0 (these are called homo or “suspended” bonds).<\/li>\n
  2. 1916:<\/strong>\u00a0Lewis also speculates about a rather less well-known model comprising “eight electrons in which pairs are symmetrically placed about a center gives … the model of the tetrahedral carbon atom<\/em>.” He then points out that two tetrahedra, attached by one, two or three corners each would represent the single, the double and the triple bond<\/strong>. The latter “represents the highest possible degree of union between two atoms<\/em>“. He chooses acetylene as an example, representing it as H:C:::C:H and two “tautomers” (we would now call them valence bond isomers)\u2020<\/sup> with lower bond orders, these being what we now call a bis-carbene and a biradical:\"Lewis\"<\/li>\n
  3. 1965:<\/strong> It took a remarkable wait of 49 years (a span which encompasses the development and maturity of quantum mechanics) to extend the “highest possible degree of union” to the quadruple bond, identified by Cotton in the previously known compound [Re2<\/sub>Cl8<\/sub>]2-<\/sup>.[2]<\/a><\/span>.
    \n
  4. 2005:<\/strong> Another 40 years elapsed before quintuple or “fivefold” bonding was discovered by Power[5]<\/a><\/span> in ArCrCrAr. There has been a bit of a race since to discover the shortest example of this genre.
    \n
    \"Click

    Click for 3D<\/p><\/div>\n<\/li>\n

  5. 2013:<\/strong> Unlike the lower bond orders, where direct structural data for larger molecules is available, speculation about sextuple bonds is limited largely to theoreticians, who have been at it for quite a while. The latest thinking is summarised here[6]<\/a><\/span> (also doi: 10.1039\/C2CP43559D<\/a>).\u00a0The current best candidates for a sextuple bond include Mo2<\/sub> and W2<\/sub>.<\/li>\n
  6. What is the limit of the formal integer bond order? I do not believe anyone thinks that septuple or octuple bonds (formal or otherwise) will be discovered (or even speculated upon) any time soon, but there is no fundamental law which would prohibit them.[7]<\/a><\/span> Quite possibly if we get beyond element 120 in the periodic table, examples might emerge!<\/li>\n<\/ol>\n
    \n

    \u2021<\/sup>A formula for predicting the filled electron shells is 2(N+1)2<\/sup>, which gives the values 2, 8, 18, 32[8]<\/a><\/span>,[9]<\/a><\/span> 50. It is also, as it happens, the rule for\u00a03D aromaticity in clusters.<\/p>\n

    \u2020<\/sup>A bis-carbene form, whilst not appropriate for carbon, may indeed become more realistic as one proceeds down column 14 of the periodic table. Thus [10]<\/a><\/span>, where Ar-Sn\u2261Sn-Ar has a C-Sn-Sn bond angle of 125\u00b0.<\/p>\n

    \"Click

    Click for 3D.<\/p><\/div>\n

    Or perhaps an even better example[11]<\/a><\/span> with a C-Sn-Sn angle of 98\u00b0. There is also an example of C-Pb-Pb[12]<\/a><\/span> with an angle of 94\u00b0.<\/p>\n

    \n

    References<\/h2>\n
    1. G.N. Lewis, \"THE ATOM AND THE MOLECULE.\", Journal of the American Chemical Society<\/i>, vol. 38, pp. 762-785, 1916. https:\/\/doi.org\/10.1021\/ja02261a002<\/a>\n\n<\/li>\n
    2. F.A. Cotton, \"Metal-Metal Bonding in [Re<sub>2<\/sub>X<sub>8<\/sub>]<sup>2-<\/sup> Ions and Other Metal Atom Clusters\", Inorganic Chemistry<\/i>, vol. 4, pp. 334-336, 1965. https:\/\/doi.org\/10.1021\/ic50025a016<\/a>\n\n<\/li>\n
    3. R.S. Mulliken, \"Note on Electronic States of Diatomic Carbon, and the Carbon-Carbon Bond\", Physical Review<\/i>, vol. 56, pp. 778-781, 1939. https:\/\/doi.org\/10.1103\/physrev.56.778<\/a>\n\n<\/li>\n
    4. S. Shaik, H.S. Rzepa, and R. Hoffmann, \"One Molecule, Two Atoms, Three Views, Four Bonds?\", Angewandte Chemie International Edition<\/i>, vol. 52, pp. 3020-3033, 2013. https:\/\/doi.org\/10.1002\/anie.201208206<\/a>\n\n<\/li>\n
    5. T. Nguyen, A.D. Sutton, M. Brynda, J.C. Fettinger, G.J. Long, and P.P. Power, \"Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers\", Science<\/i>, vol. 310, pp. 844-847, 2005. https:\/\/doi.org\/10.1126\/science.1116789<\/a>\n\n<\/li>\n
    6. F. Ruip\u00e9rez, M. Piris, J.M. Ugalde, and J.M. Matxain, \"The natural orbital functional theory of the bonding in Cr<sub>2<\/sub>, Mo<sub>2<\/sub>and W<sub>2<\/sub>\", Phys. Chem. Chem. Phys.<\/i>, vol. 15, pp. 2055-2062, 2013. https:\/\/doi.org\/10.1039\/c2cp43559d<\/a>\n\n<\/li>\n
    7. G. Frenking, and R. Tonner, \"The six-bond bound\", Nature<\/i>, vol. 446, pp. 276-277, 2007. https:\/\/doi.org\/10.1038\/446276a<\/a>\n\n<\/li>\n
    8. 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
    9. 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
    10. A.D. Phillips, R.J. Wright, M.M. Olmstead, and P.P. Power, \"Synthesis and Characterization of 2,6-Dipp<sub>2<\/sub>-H<sub>3<\/sub>C<sub>6<\/sub>SnSnC<sub>6<\/sub>H<sub>3<\/sub>-2,6-Dipp<sub>2<\/sub> (Dipp = C<sub>6<\/sub>H<sub>3<\/sub>-2,6-Pr<sup>i<\/sup><sub>2<\/sub>):\u2009 A Tin Analogue of an Alkyne\", Journal of the American Chemical Society<\/i>, vol. 124, pp. 5930-5931, 2002. https:\/\/doi.org\/10.1021\/ja0257164<\/a>\n\n<\/li>\n
    11. Y. Peng, R.C. Fischer, W.A. Merrill, J. Fischer, L. Pu, B.D. Ellis, J.C. Fettinger, R.H. Herber, and P.P. Power, \"Substituent effects in ditetrel alkyne analogues: multiple vs. single bonded isomers\", Chemical Science<\/i>, vol. 1, pp. 461, 2010. https:\/\/doi.org\/10.1039\/c0sc00240b<\/a>\n\n<\/li>\n
    12. L. Pu, B. Twamley, and P.P. Power, \"Synthesis and Characterization of 2,6-Trip<sub>2<\/sub>H<sub>3<\/sub>C<sub>6<\/sub>PbPbC<sub>6<\/sub>H<sub>3<\/sub>-2,6-Trip<sub>2<\/sub> (Trip = C<sub>6<\/sub>H<sub>2<\/sub>-2,4,6-<i>i<\/i>-Pr<sub>3<\/sub>):\u2009 A Stable Heavier Group 14 Element Analogue of an Alkyne\", Journal of the American Chemical Society<\/i>, vol. 122, pp. 3524-3525, 2000. https:\/\/doi.org\/10.1021\/ja993346m<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. N. Lewis suggested a model for single and double bonds that involved sharing either 2 or 4 electrons between a pair of atoms. We tend to think of such (even electron) bonds […]<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[1016,1030,2651,1018,1019,1015,1017],"ppma_author":[2661],"class_list":["post-9973","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-double-bond","tag-high-energy","tag-historical","tag-quadruple-bond","tag-quintuple-bond","tag-single-bond","tag-triple-bond"],"yoast_head":"\nA (very) short history of shared-electron bonds. - 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=9973\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A (very) short history of shared-electron bonds. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. 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The measured C-C bond length was ~2.9a\u00c5 and my conclusion was that the C...C region represented more of a weak \"interaction\" than of a bond\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":24612,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24612","url_meta":{"origin":9973,"position":2},"title":"Chasing ever higher bond orders; the strange case of beryllium.","author":"Henry Rzepa","date":"February 7, 2022","format":false,"excerpt":"Ever since the concept of a shared two-electron bond was conjured by Gilbert N. Lewis in 1916, chemists have been fascinated by the related concept of a bond order (the number of such bonds that two atoms can participate in, however a bond is defined) and pushing it ever higher\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/02\/Be2Li6_mo13-150x150.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":10903,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10903","url_meta":{"origin":9973,"position":3},"title":"The butterfly effect in chemistry: bimodal bond angles.","author":"Henry Rzepa","date":"July 18, 2013","format":false,"excerpt":"This potential example of a molecule on the edge of chaos was suggested to me by a student (thanks Stephen!), originating from an inorganic tutorial. It represents a class of Mo-complex ligated by two dithiocarbamate ligands and two aryl nitrene ligands (Ar-N:). I focus on two specific examples, where R=R'\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":"Click for c3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/PNTCMO.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":11741,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11741","url_meta":{"origin":9973,"position":4},"title":"A curly-arrow pushing manual","author":"Henry Rzepa","date":"December 4, 2013","format":false,"excerpt":"I have several times used arrow pushing on these blogs. But since the rules for this convention appear to be largely informal, and there appears to be no definitive statement of them, I thought I would try to produce this for our students. This effort is here shared on my\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"180px-Hunter","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/12\/180px-Hunter.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1691,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1691","url_meta":{"origin":9973,"position":5},"title":"Quintuple bonds","author":"Henry Rzepa","date":"February 16, 2010","format":false,"excerpt":"Climbers scale Mt. Everest, because its there, and chemists have their own version of this. Ever since G. N. Lewis introduced the concept of the electron-pair bond in 1916, the idea of a bond as having a formal bond-order has been seen as a useful way of thinking about molecules.\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/02\/Cr.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\/9973","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=9973"}],"version-history":[{"count":35,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9973\/revisions"}],"predecessor-version":[{"id":11934,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9973\/revisions\/11934"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9973"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9973"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9973"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=9973"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}