{"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":"2013-03-26T13:38:26","modified_gmt":"2013-03-26T13:38:26","slug":"a-very-short-history-of-shared-electron-bonds","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=9973","title":{"rendered":"A (very) short history of shared-electron bonds."},"content":{"rendered":"
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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

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  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
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    \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

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    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,"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":[6],"tags":[1021,1024,1528,1031,1032,1035,1037],"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"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-2AR","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9973","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=9973"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9973\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9973"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9973"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9973"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}