{"id":18839,"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":"2017-09-16T20:38:03","modified_gmt":"2017-09-16T19:38:03","slug":"the-di-anion-of-dilithium-not-the-star-trek-variety-another-hyper-bond","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=18839","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!‡<\/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

‡<\/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,"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":[2250,2251,1414,1416,2262,2264,164,2269,2275,2278,1640,1494,2290,1500,2293,1610],"class_list":["post-18839","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"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-4TR","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18839","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=18839"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18839\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=18839"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=18839"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=18839"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}