{"id":18806,"date":"2017-09-16T20:38:03","date_gmt":"2017-09-16T19:38:03","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=18806"},"modified":"2017-09-22T10:19:30","modified_gmt":"2017-09-22T09:19:30","slug":"the-di-anion-of-dilithium-not-the-star-trek-variety-another-hyper-bond","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18806","title":{"rendered":"The di-anion of dilithium (not the Star Trek variety): Another “Hyper-bond”?"},"content":{"rendered":"
\n

Early in 2011, I wrote<\/a> about how the diatomic molecule Be2<\/sub> might be persuaded to improve upon its normal unbound state (bond order\u00a0~zero) by a double electronic excitation to a strongly bound species. I yesterday updated this post with further suggestions and one of these inspired this follow-up.<\/p>\n

The standard molecular orbital diagram for Be2<\/sub>\u00a0below shows two electrons in both the 2s\u00a0\u03a3g<\/sub> and\u00a0\u03a3u<\/sub> levels, the first being considered bonding<\/strong> and the second antibonding<\/strong>. By exciting the two electrons from the\u00a0\u03a3u<\/sub> into the \u03a0u<\/sub> MO to form a triplet, one converts one antibonding occupancy into two bonding occupancies, in the process changing the total formal bond order from zero to two.<\/p>\n

 <\/p>\n

\"\"<\/a>

The triplet excited state of diberyllium<\/p><\/div>\n

You can see the results of my playing with these ideas both in my appended comments<\/a> to the original post and the table below. This shows that the calculated bond order for the excited triplet state of Be2<\/sub> is actually closer to 1.50 rather than to two, but definitely not zero!<\/p>\n\n\n\n\n\n\n\n\n
System<\/th>\nWiberg bond order<\/th>\nBond length<\/th>\nFAIR Data<\/th>\n<\/tr>\n
Be2<\/sub> singlet<\/td>\n0.15<\/td>\n2.805<\/td>\n10.14469\/hpc\/3082<\/a><\/td>\n<\/tr>\n
Be2<\/sub> excited triplet<\/td>\n1.50<\/td>\n1.785<\/td>\n10.14469\/hpc\/3075<\/a><\/td>\n<\/tr>\n
Be2<\/sub>2+<\/sup><\/td>\n1.00<\/td>\n2.135<\/td>\n10.14469\/hpc\/3076<\/a><\/td>\n<\/tr>\n
Be2<\/sub>2-<\/sup> triplet<\/td>\n0.89<\/td>\n2.242<\/td>\n10.14469\/hpc\/3074<\/a><\/td>\n<\/tr>\n
Be2<\/sub>2-<\/sup>\u00a0excited singlet<\/td>\n3.00<\/td>\n1.817<\/td>\n10.14469\/hpc\/3083<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The games above represent isoelectronic substitutions and here I try one more, namely that Li2<\/sub>2-<\/sup> is isoelectronic with Be2<\/sub>. Unlike the latter, there is no need to force an electronic excitation (\u03c9B97XD\/Def2-QZVPPD\/SCRF=water) to achieve the required occupancies with\u00a0Li2<\/sub>2-<\/sup>.<\/p>\n\n\n\n\n
System<\/th>\nWiberg bond order<\/th>\nBond length<\/th>\nFAIR Data<\/th>\n<\/tr>\n
Li2<\/sub>2-<\/sup> triplet<\/td>\n1.501<\/td>\n2.381<\/td>\n10.14469\/hpc\/3087<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

I also checked what crystal structures could tell us about Li-Li bonds and it seems 2.38\u00c5 is about as short as they get. \"\"<\/p>\n

At this point, the NBO analysis of the Li2<\/sub>2-\u00a0<\/sup>localised orbitals alerted me to another feature, which is that the Rydberg occupancy amounted to 2.18e. This in turn reminded me of the previous post<\/a> which dealt with such occupancy in another small molecule, CH3<\/sub>F2-<\/sup>, but here the Rydberg occupancy involved the 3s\/3p AOs of the carbon and the fluorine. With\u00a0Li2<\/sub>2-<\/sup> triplet, it is of the lithium 2p AO (2.18e) and only a tiny occupancy of 3d (0.03). By definition, for alkali metals such as Li the normal valence shell is just 2s, whereas 2p occupancy is considered a Rydberg state; a hypervalent state if you will. So\u00a0Li2<\/sub>2-<\/sup> triplet has a Li-Li hyper-bond!\u2021<\/sup> Of course, by this definition most Li compounds are then hypervalent, since many have populated 2p shells.<\/p>\n

Even if use of the term hyper-bond to describe\u00a0Li2<\/sub>2-<\/sup> triplet is rather artificial, this example does reveal the games one can play with the first row elements Li-B (see table above). Given that most introductory text books on bonding normally only explain the diatomics formed from N-Ne (occasionally including C), I might suggest that these earlier elements are equally instructive and fun to play with.<\/p>\n

\n

\u2021<\/sup> This species is 36.0 kcal\/mol higher in free energy than two separated Li–<\/sup> anions.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Early in 2011, I wrote about how the diatomic molecule Be2 might be persuaded to improve upon its normal unbound state (bond order\u00a0~zero) by a double electronic excitation to a strongly bound species. I yesterday updated this post with further suggestions and one of these inspired this follow-up. The standard molecular orbital diagram for Be2\u00a0below […]<\/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":[2248,2247,1402,1395,2250,2244,40,2245,2252,2249,1630,1439,2246,1512,2251,1560],"ppma_author":[2661],"class_list":["post-18806","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-be-be-double-bond","tag-be-be-triple-bond","tag-chemical-bond","tag-chemistry","tag-cs-cs-double-bond","tag-diatomic-molecule","tag-free-energy","tag-general-chemistry","tag-k-k-double-bond","tag-li-li-double-bond","tag-molecular-geometry","tag-oxygen","tag-provincestate-be2","tag-quantum-chemistry","tag-rb-rb-double-bond","tag-stereochemistry"],"yoast_head":"\nThe di-anion of dilithium (not the Star Trek variety): Another "Hyper-bond"? - 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=18806\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The di-anion of dilithium (not the Star Trek variety): Another "Hyper-bond"? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Early in 2011, I wrote about how the diatomic molecule Be2 might be persuaded to improve upon its normal unbound state (bond order\u00a0~zero) by a double electronic excitation to a strongly bound species. 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The standard molecular orbital diagram for Be2\u00a0below […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18806\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2017-09-16T19:38:03+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2017-09-22T09:19:30+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/01\/Be2b.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":"The di-anion of dilithium (not the Star Trek variety): Another \"Hyper-bond\"? - 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=18806","og_locale":"en_GB","og_type":"article","og_title":"The di-anion of dilithium (not the Star Trek variety): Another \"Hyper-bond\"? - Henry Rzepa's Blog","og_description":"Early in 2011, I wrote about how the diatomic molecule Be2 might be persuaded to improve upon its normal unbound state (bond order\u00a0~zero) by a double electronic excitation to a strongly bound species. 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This was unusual because unlike the original metallocenes, the metal atom was not symmetrically disposed between the two cyclopentadienyl faces. Now diberyllocene\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/06\/Screenshot-21-1024x596.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":3462,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3462","url_meta":{"origin":18806,"position":2},"title":"Shorter is higher: the strange case of diberyllium.","author":"Henry Rzepa","date":"January 21, 2011","format":false,"excerpt":"Much of chemistry is about bonds, but sometimes it can also be about anti-bonds. It is also true that the simplest of molecules can have quite subtle properties. 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The blog poses the question whether any bonds can be constructed which use a fourth type of component, the \u03c6.","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"Elements in Groups 5\/15 of the Periodic Table.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/08\/periodic-table-V.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/08\/periodic-table-V.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/08\/periodic-table-V.jpg?resize=525%2C300 1.5x"},"classes":[]},{"id":23712,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23712","url_meta":{"origin":18806,"position":5},"title":"What does a double \u03c3-bond along a bond axis look like?","author":"Henry Rzepa","date":"May 10, 2021","format":false,"excerpt":"Introductory chemistry will tell us that a triple bond between say two carbon atoms comprises just one bond of \u03c3-axial symmetry and two of \u03c0-symmetry. 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