{"id":19383,"date":"2018-02-16T08:41:45","date_gmt":"2018-02-16T08:41:45","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=19383"},"modified":"2018-02-25T08:41:05","modified_gmt":"2018-02-25T08:41:05","slug":"hypervalent-helium-not","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19383","title":{"rendered":"Hypervalent Helium – not!"},"content":{"rendered":"
\n

Last year, this article[1]<\/a><\/span> attracted a lot of attention as the first example of molecular helium in the form of Na2<\/sub>He. In fact, the helium in this species has a calculated\u2021<\/sup> bond index of only 0.15 and it is better classified as a sodium electride with the ionisation induced by pressure and the presence of helium atoms. The helium is neither valent, nor indeed hypervalent (the meanings are in fact equivalent for this element). In a separate blog posted in 2013, I noted a cobalt carbonyl complex containing a hexacoordinate hydrogen in the form of hydride, H–<\/sup>. A comment appended<\/a> to this blog insightfully asked about the isoelectronic complex containing He instead of H–<\/sup>. Here, rather belatedly, I respond to this comment!<\/p>\n

The complex\u00a0[HCo6<\/sub>(CO)15<\/sub>]–<\/sup> has a calculated bond index at the hydrogen of 0.988 and a calculated NMR chemical shift of 21.6 ppm (\u03c9B97XD\/Def2-TZVPPD calculation) compared to a measured value of 23.2 ppm. Despite being six-coordinate, the hydride has a bond index that does not exceed one (it is not hypervalent).<\/p>\n

So here is the neutral helium analogue.\u00a0The He bond index emerges as 0.71<\/span><\/strong> at the geometry of the hydride complex. Compare this with the bond index of 0.15 calculated for\u00a0Na2<\/sub>He and it would be fair to say that at this geometry, the helium in\u00a0[HeCo6<\/sub>(CO)15<\/sub>] would have a greater claim to be a molecular compound. Back in 2010, extrapolating from a series of posts here, I had speculated[2]<\/a><\/span> about other molecular species of He, including the di-cation below. This has a He bond index of 0.54, rather less than that in\u00a0[HeCo6<\/sub>(CO)15<\/sub>] but much more than in Na2<\/sub>He. It is also vibrationally stable.\"\"<\/p>\n

But now, [HeCo6<\/sub>(CO)15<\/sub>] goes “pear-shaped” (why do pears have such a bad press<\/a>?). I started a process of optimizing the geometry of this complex (ωB97Xd\/Def2-TZVPPD). Slowly, the He started to creep out of the centre of the complex and emerge from the cavity. After about 100 steps it reached the geometry shown below, at which point the Wiberg bond index has dropped to 0.62 and still going down. I think it might take a few more steps to be completely expelled, but I have stopped the geometry optimisation at this stage.<\/p>\n

\"\"<\/p>\n

So helium appears not to be valent in\u00a0[HeCo6<\/sub>(CO)15<\/sub>]. However, I have yet to try Ne, which is both larger and softer. I will post results here.<\/p>\n

\n

\u2021<\/sup>All data at 10.14469\/hpc\/3587<\/a>.<\/p>\n

References<\/h2>\n
  1. X. Dong, A.R. Oganov, A.F. Goncharov, E. Stavrou, S. Lobanov, G. Saleh, G. Qian, Q. Zhu, C. Gatti, V.L. Deringer, R. Dronskowski, X. Zhou, V.B. Prakapenka, Z. Kon\u00f4pkov\u00e1, I.A. Popov, A.I. Boldyrev, and H. Wang, \"A stable compound of helium and sodium at high pressure\", Nature Chemistry<\/i>, vol. 9, pp. 440-445, 2017. https:\/\/doi.org\/10.1038\/nchem.2716<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"The rational design of helium bonds\", Nature Chemistry<\/i>, vol. 2, pp. 390-393, 2010. https:\/\/doi.org\/10.1038\/nchem.596<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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 pressure and the presence of […]<\/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],"tags":[557,1432,237,1395,200,1450,1960,1679,1431,2330,1834,2331],"ppma_author":[2661],"class_list":["post-19383","post","type-post","status-publish","format-standard","hentry","category-hypervalency","tag-chemical-bonding","tag-chemical-elements","tag-chemical-shift","tag-chemistry","tag-helium","tag-hydride","tag-hydrogen","tag-hypervalent-molecule","tag-matter","tag-metal-hydrides","tag-reducing-agents","tag-transition-metal-hydride"],"yoast_head":"\nHypervalent Helium - not! - 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=19383\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Hypervalent Helium - not! - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Last year, this article attracted a lot of attention as the first example of molecular helium in the form of Na2He. 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It was a very rare computed example of a system where the added two electrons populate the higher valence shells known as Rydberg orbitals as an alternative to populating the C-F antibonding \u03c3-orbital to produce CH3- and F-. The net\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":19472,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19472","url_meta":{"origin":19383,"position":2},"title":"What are the highest bond indices for main group and transition group elements?","author":"Henry Rzepa","date":"March 4, 2018","format":false,"excerpt":"A bond index (BI) approximately measures the totals of the bond orders at any given atom in a molecule. Here I ponder what the maximum values might be for elements with filled valence shells. Following Lewis in 1916 who proposed that the full valence shell for main group elements should\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":984,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=984","url_meta":{"origin":19383,"position":3},"title":"Uncompressed Monovalent Helium","author":"Henry Rzepa","date":"October 3, 2009","format":false,"excerpt":"Quite a few threads have developed in this series of posts, and following each leads in rather different directions. In this previous post the comment was made that coordinating a carbon dication to the face of a cyclopentadienyl anion resulted in a monocation which had a remarkably high proton affinity.\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"Inventing the Helium bond","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/10\/C4-He.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1103,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1103","url_meta":{"origin":19383,"position":4},"title":"Hypervalency: a reality check","author":"Henry Rzepa","date":"October 5, 2009","format":false,"excerpt":"We have seen in the series of posts on the topic of hypervalency\u00a0how the first row main group elements such as\u00a0Be, B, C and N can sustain apparent hypercoordination and arguably hypervalency. The latter is defined not so much by expanding the total valence shell of electrons surrounding the hypervalent\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"YOCVIV: Crystal structure of hexacoordinate boron","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/10\/BBr.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":19194,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19194","url_meta":{"origin":19383,"position":5},"title":"Identification of a simplest hypervalent hydrogen fluoride anion.","author":"Henry Rzepa","date":"December 8, 2017","format":false,"excerpt":"An article with the title shown above in part recently appeared. 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":[]}],"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\/19383","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=19383"}],"version-history":[{"count":13,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19383\/revisions"}],"predecessor-version":[{"id":19432,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19383\/revisions\/19432"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=19383"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=19383"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=19383"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=19383"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}