{"id":4666,"date":"2011-08-03T17:09:24","date_gmt":"2011-08-03T17:09:24","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=4666"},"modified":"2011-08-06T07:05:29","modified_gmt":"2011-08-06T07:05:29","slug":"extreme-chemical-intimacy-the-xe2c60-ion-pair","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4666","title":{"rendered":"Extreme chemical intimacy: the Xe2@C60 ion-pair."},"content":{"rendered":"
\n

Unusual bonds are always intriguing, and the Xe-Xe bond is no exception. It was first reported (10.1002\/anie.199702731<\/a>) for the species Xe2<\/sub>+<\/sup>. Sb4<\/sub>F21<\/sub>– <\/sup> and its length (3.09\u00c5) was claimed as “unsurpassed in length in main\u00a0group chemistry by any other element -element bond”. Krapp and Frenking<\/a> then creatively tweaked the bond (in a computer). The counterion was replaced by C60<\/sub>, and the two xenon atoms placed inside! Buckyballs have a fascinating ability to absorb electrons, up to six in fact, from whatever is placed inside the cavity, and so this assembly acts as a rather intriguing ion-pair. So the issue reduces to how many electrons does C60<\/sub> manage to scavenge from two Xenon atoms, and what is the nature of any resulting bonding formed between these two atoms?<\/p>\n

Before taking a look at that question, I note first a previous post<\/a> in which I speculate upon the ultimate chemical bond, a\u00a05f-\u03c6 bond between two uranium atoms inside C60<\/sub>. The U2<\/sub> atoms have indeed been stripped of the full complement of six electrons, and the resulting U2<\/sub>6+<\/sup> ion is claimed<\/a> to support a\u00a0\u03c6 bond. But back to Xe.\u00a0Krapp and Frenking conclude (from NBO charges) that ~1 electron has been stripped out of Xe2<\/sub>, and the resulting (calculated) Xe-Xe bond length is shortened to 2.49\u00c5, which is actually less than calculated for an isolated Xe2<\/sub>2<\/span>+<\/sup> ion (2.75\u00c5). This is an unusual (and it has to be said hypothetical) example of a compressed (thermodynamically unstable) bond which would certainly “explode” if released from its prison, reclaiming the electron in the process. The system is also of interest given the unusual nature of the (charge shift) bonding in F2<\/sub>, Cl2<\/sub>, Br2<\/sub><\/a> and also what the effect of injecting electrons into an aromatic periphery of carbon atoms would be (adding two electrons nominally subverts<\/a> an aromatic<\/em> 4n+2 rule into an antiaromatic<\/em> 4n one).<\/p>\n

I decided to take another look at Xe2<\/sub>@C60<\/sub> because since the original study of this species in 2007, computational methodology has evolved to (a) allow the effect of dispersion forces to be included and (b) if the outer skeleton does indeed absorb electrons, a (continuum) solvation correction may also influence the properties.<\/a> This was done as a \u03c9B97XD\/cc-pVDZ\/SCRF=water calculation<\/a>, with aug-cc-pVDZ-pp on Xe.<\/p>\n

\"\"

Xe2@C60 showing charge distribution. Click for 3D.<\/p><\/div>The result is shown with the \u00a0atoms colour-coded for charge distribution (red = -ve, green = +ve), showing the two six-membered rings opposite the two \u00a0Xe atoms have accumulated electrons, at the expense of the Xe and the central carbon atoms. Appropriately, these two \u00a0end rings are also de-aromatised, with ring bonds of \u00a01.498, 1.424, 1.473, 1.486, 1.473, 1.424\u00c5, compared with\u00a01.451\/1.388\u00c5 for pure C60<\/sub> with nothing inside.\u00a0The results of a QTAIM analysis of the electron density \u03c1(r) show a Xe-Xe bond critical point (\u03bdXe-Xe<\/sub> 401 cm-1<\/sup>) with \u03c1 0.114, and \u22072<\/sup> -0.0013 for a length \u00a0of 2.43\u00c5. ELF shows a \u00a0Xe-Xe basin integrating to 0.81 electrons. All of this is pretty much in accord with\u00a0Krapp and Frenking<\/a>‘s original very comprehensive study. So we see how the effect of pressure can induce (1-electron) bonds to form between atoms which would normally be considered impossible as bonding partners. It is also an unusual example where ionisation is accompanied by covalent bond formation (normally, ionization is at the expense of a covalent bond).<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Unusual bonds are always intriguing, and the Xe-Xe bond is no exception. It was first reported (10.1002\/anie.199702731) for the species Xe2+. Sb4F21– and its length (3.09\u00c5) was claimed as “unsurpassed in length in main\u00a0group chemistry by any other element -element bond”. Krapp and Frenking then creatively tweaked the bond (in a computer). The counterion was […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[7,4],"tags":[642,644,643],"ppma_author":[2661],"class_list":["post-4666","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-endohedral-complexes","tag-extreme-chemical-intimacy","tag-ultimate-chemical-bond"],"yoast_head":"\nExtreme chemical intimacy: the Xe2@C60 ion-pair. - 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=4666\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Extreme chemical intimacy: the Xe2@C60 ion-pair. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Unusual bonds are always intriguing, and the Xe-Xe bond is no exception. It was first reported (10.1002\/anie.199702731) for the species Xe2+. Sb4F21– and its length (3.09\u00c5) was claimed as “unsurpassed in length in main\u00a0group chemistry by any other element -element bond”. Krapp and Frenking then creatively tweaked the bond (in a computer). 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It was first reported (10.1002\/anie.199702731) for the species Xe2+. Sb4F21– and its length (3.09\u00c5) was claimed as “unsurpassed in length in main\u00a0group chemistry by any other element -element bond”. Krapp and Frenking then creatively tweaked the bond (in a computer). 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