{"id":10801,"date":"2013-07-05T07:06:06","date_gmt":"2013-07-05T06:06:06","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10801"},"modified":"2014-01-17T07:47:24","modified_gmt":"2014-01-17T07:47:24","slug":"is-cli6-hypervalent","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10801","title":{"rendered":"Is CLi6 hypervalent?"},"content":{"rendered":"
\n

A comment<\/a> made on the previous post on the topic of hexa-coordinate carbon cited an article entitled “Observation of hypervalent CLi6<\/sub>\u00a0by Knudsen-effusion mass spectrometry<\/i>“[1]<\/a><\/span> by Kudo as a amongst the earliest of evidence that such species can exist (in the gas phase). It was a spectacular vindication of the earlier theoretical prediction[2]<\/a><\/span>,[3]<\/a><\/span> that such 6-coordinate species are\u00a0stable with respect to dissociation to CLi4<\/sub>\u00a0and Li2<\/sub>.<\/p>\n

The terminology describes these lithium carbides as effectively\u00a0hypervalent<\/strong>; Kudo in the abstract of his 1992 article uses the more explicit phrase “carbon can expand its octet of electrons to form this relatively stable molecule<\/em>“. We are taught early on in chemistry that the carbon octet is due to double occupation of four molecular orbitals formed using linear combinations derived from the relatively low energy 2s\/2p carbon atomic orbital basis. Octet expansion on carbon must therefore involve to some degree the next atomic shell (3s\/3p), which is normally regarded as too high in energy to be capable of significant population for carbon. But use of the 3s\/3p shell seems at first sight inevitable. If one constructs an octahedral complex CLi6<\/sub>\u00a0surely ten electrons must be involved in bonding, four from the carbon and six from the equivalent lithiums? The 3s\/3p carbon population must therefore be ~2 electrons, and we can truly describe a molecule where carbon has of necessity expanded its octet of electrons to ten as hypervalent. Or can we?<\/p>\n

How does a quantitative (\u03c9B97XD\/6-311++G(d) ) calculation[4]<\/a><\/span> reveal this effective hypervalency?\u00a0<\/p>\n

    \n
  1. The octahedral geometry is indeed a stable minimum, with the lowest vibrational wavenumber being 194 cm-1<\/sup>.<\/span><\/li>\n
  2. It also checks out as clearly a closed shell species, stable to open shell perturbations.<\/li>\n
  3. An NBO analysis reveals the Rydberg population (those 3s\/3p atomic orbitals) to be only 0.09 electrons.<\/li>\n
  4. It partitions the electrons into 13.97 for the 1s cores of the seven atoms, 7.67 “valence-Lewis” (i.e. shared covalent) and a mysterious 2.27 (valence, non-Lewis).<\/li>\n<\/ol>\n

    We now have a problem. One of the standard methods for partitioning electrons has isolated two of our ten electrons and placed them, with small partial occupancy, into unshared “lone pairs”, located as it happens on the lithium atoms (shown below for one of these partial lone “pairs”). The carbon is NOT hypervalent, and it has NOT expanded its octet.<\/p>\n

    \"Click

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

    So I tried another procedure, deliberately chosen to be rather different from the orbital-based NBO formalism. This is analysis of the ELF, or electron localisation function, and represents an attempt to derive the result based on a function related to the electron density. The red spheres shown below are the centroids of the twelve<\/strong> ELF basins located:<\/p>\n

    \"Click

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

    Each of these (equivalent) basins has an electron population of ~0.81, making ~9.7 electrons in total. Each lithium sits on a square arrangement of four of these basins, and so has access to ~3.2 valence electrons. How do we interpret the situation for carbon however? Does its valence shell contain an expanded 9.7 electrons? Well, not necessarily. You can see that each of the basins has a three-centre relationship between the one carbon and TWO lithiums. These electrons contribute not just to C-Li bonding, but also to Li…Li bonding. So these 9.7 electrons contribute in part to bonding that does NOT involve the carbon. We can see this in the (Wiberg) bond orders, 0.254 for the C-Li interaction, and 0.116 for adjacent Li…Li interactions (such an explanation was also suggested<\/a> for why II7<\/sub>\u00a0has no expanded octet at the central iodine).\u00a0In fact, the origins of this effect were first clearly identified in the theoretical analysis of 1983[3]<\/a><\/span>: “the extra electrons beyond the usual octet are involved with metal-metal bonding rather than with interactions of the metals with the central atoms<\/em>“.<\/p>\n

    It is nice to see that despite the passage of 30 years, and despite the introduction of many new ways of analysing the wavefunctions and hence the bonding of molecules, the essential original interpretation[3]<\/a><\/span> remains robustly correct!\u00a0<\/p>\n

    References<\/h2>\n
    1. H. Kudo, \"Observation of hypervalent CLi6 by Knudsen-effusion mass spectrometry\", Nature<\/i>, vol. 355, pp. 432-434, 1992. https:\/\/doi.org\/10.1038\/355432a0<\/a>\n\n<\/li>\n
    2. E.D. Jemmis, J. Chandrasekhar, E.U. Wuerthwein, P.V.R. Schleyer, J.W. Chinn, F.J. Landro, R.J. Lagow, B. Luke, and J.A. Pople, \"Lithiated carbocations. The generation, structure, and stability of CLi5+\", Journal of the American Chemical Society<\/i>, vol. 104, pp. 4275-4276, 1982. https:\/\/doi.org\/10.1021\/ja00379a051<\/a>\n\n<\/li>\n
    3. P.V.R. Schleyer, E.U. Wuerthwein, E. Kaufmann, T. Clark, and J.A. Pople, \"Effectively hypervalent molecules. 2. Lithium carbide (CLi5), lithium carbide (CLi6), and the related effectively hypervalent first row molecules, CLi5-nHn and CLi6-nHn\", Journal of the American Chemical Society<\/i>, vol. 105, pp. 5930-5932, 1983. https:\/\/doi.org\/10.1021\/ja00356a045<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      A comment made on the previous post on the topic of hexa-coordinate carbon cited an article entitled “Observation of hypervalent CLi6\u00a0by Knudsen-effusion mass spectrometry” by Kudo as a amongst the earliest of evidence that such species can exist (in the gas phase). It was a spectacular vindication of the earlier theoretical prediction, that such 6-coordinate […]<\/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":[7,4],"tags":[24,145,1097,1095,1096,74],"ppma_author":[2661],"class_list":["post-10801","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-energy","tag-gas-phase","tag-knudsen","tag-low-energy-2s2p-carbon","tag-metal-metal-bonding","tag-pence"],"yoast_head":"\nIs CLi6 hypervalent? - 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=10801\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Is CLi6 hypervalent? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A comment made on the previous post on the topic of hexa-coordinate carbon cited an article entitled “Observation of hypervalent CLi6\u00a0by Knudsen-effusion mass spectrometry” by Kudo as a amongst the earliest of evidence that such species can exist (in the gas phase). It was a spectacular vindication of the earlier theoretical prediction, that such 6-coordinate […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10801\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-07-05T06:06:06+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2014-01-17T07:47:24+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/CLi6-Lp.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":"Is CLi6 hypervalent? - 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=10801","og_locale":"en_GB","og_type":"article","og_title":"Is CLi6 hypervalent? - Henry Rzepa's Blog","og_description":"A comment made on the previous post on the topic of hexa-coordinate carbon cited an article entitled “Observation of hypervalent CLi6\u00a0by Knudsen-effusion mass spectrometry” by Kudo as a amongst the earliest of evidence that such species can exist (in the gas phase). 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Given the apparent similarity of HF1- to CH3F1- and CH3F2-, the latter of which I introduced on this blog previously,\u00a0I thought it of interest to apply my analysis to HF1-. The authors conclude that \"the F atom of HF\u2212\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":10839,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10839","url_meta":{"origin":10801,"position":1},"title":"Hexacoordinate hydrogen.","author":"Henry Rzepa","date":"July 8, 2013","format":false,"excerpt":"A feature of a blog which is quite different from a journal article is how rapidly a topic might evolve.\u00a0Thus I started a few days ago with the theme of dicarbon (C2), identifying a metal carbide that showed C2 as a ligand, but which also entrapped a single carbon in\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/6-H.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":783,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=783","url_meta":{"origin":10801,"position":2},"title":"Capturing penta-coordinate carbon! (Part 1).","author":"Henry Rzepa","date":"September 22, 2009","format":false,"excerpt":"The bimolecular nucleophilic substitution reaction at saturated carbon is an icon of organic chemistry, and is better known by its mechanistic label, SN2. It is normally a slow reaction, with half lives often measured in hours. This implies a significant barrier to reaction (~15-20 kcal\/mol) for the transition state, shown\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"The Sn2 transition state","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/09\/sn2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":15823,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15823","url_meta":{"origin":10801,"position":3},"title":"Real hypervalency in a small molecule.","author":"Henry Rzepa","date":"February 21, 2016","format":false,"excerpt":"Hypervalency is defined as a molecule\u00a0that contains one or more main group elements formally bearing more than eight\u00a0 electrons in their\u00a0 valence shell. One example of a molecule so characterised was CLi6 where the description \"\u201ccarbon can expand its octet of electrons to form this relatively stable molecule\u201c was used.\u2026","rel":"","context":"In "Bond slam"","block_context":{"text":"Bond slam","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2237"},"img":{"alt_text":"CH4","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/02\/CH4.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":19307,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19307","url_meta":{"origin":10801,"position":4},"title":"Are diazomethanes hypervalent molecules? An attempt into more insight by more “tuning” with substituents.","author":"Henry Rzepa","date":"December 26, 2017","format":false,"excerpt":"Recollect the suggestion\u00a0that diazomethane has hypervalent character. When I looked into this, I came to the conclusion that it probably was mildly hypervalent, but on carbon and not nitrogen. Here I try some variations with substituents to see what light if any this casts. I have expanded the resonance forms\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\/2017\/12\/H2CNCCN_ELF-1024x258.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":19383,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19383","url_meta":{"origin":10801,"position":5},"title":"Hypervalent Helium – not!","author":"Henry Rzepa","date":"February 16, 2018","format":false,"excerpt":"Last year, this article attracted a lot of attention as the first example of molecular helium in the form of Na2He. In fact, the helium in this species has a calculated\u2021 bond index of only 0.15 and it is better classified as a sodium electride with the ionisation induced by\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"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","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\/10801","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=10801"}],"version-history":[{"count":20,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10801\/revisions"}],"predecessor-version":[{"id":11947,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10801\/revisions\/11947"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10801"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10801"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10801"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10801"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}