{"id":30890,"date":"2026-03-08T15:00:21","date_gmt":"2026-03-08T15:00:21","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30890"},"modified":"2026-03-08T15:17:13","modified_gmt":"2026-03-08T15:17:13","slug":"valence-bond-representations-with-ve-charges-on-adjacent-atoms-an-odd-titanium-complex-analysed","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30890","title":{"rendered":"Valence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed."},"content":{"rendered":"
\n

A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene,[1]<\/a><\/span> and commented on the odd nature of the N=N double bond formed in this process.[2]<\/a><\/span>. Odd, because the valence bond representation of this dimer (1<\/strong> below[3]<\/a><\/span>) has two formally positive adjacent nitrogen atoms. An energy decomposition analysis (NEDA[4]<\/a><\/span>) of species 1<\/strong> showed an unusually small\u00a0negative interaction energy of -27.6 kcal\/mol between the two nitrosobenzene fragments (typical \u0394E values ~-130 to -180 kcal\/mol[5]<\/a><\/span>), commensurate with the facile equilibrium between two monomers and the dimer[6]<\/a><\/span> A little later I went on to speculate upon a similar theme for the more hypothetical nitric oxide dimer, a species 2<\/strong> which again has two adjacent +ve charges[7]<\/a><\/span> and even a smaller +ve NEDA for the triple bond! You can imagine discussing these results with organic chemists, who would normally shrink from placing two (formal) positive charges on adjacent atoms.<\/p>\n

\"\"<\/a><\/p>\n

Browsing (as one does) the CSD crystal structure database, I came across a molecule shown as representation 5<\/strong> above.[8]<\/a><\/span>. This rang a small alarm bell – why was the central nitrogen atom there shown as neutral? To balance the only +ve charge (on the pyridinium cation), the Ti had a single -ve charge. Representation 3<\/strong> installs a second\u00a0+ve charge on the second nitrogen, just as with 1<\/strong> and 2<\/strong>. The ligand in question (PyN2<\/sub>-1<\/sup>) has an overall charge of -1, and together with the other three negatively charged ligands results in TiIV<\/sup>. The total formal count around the Ti is 6 (from Cp-1<\/sup>) + 2×2 (2Cl-1<\/sup>) + 6 (PyN2<\/sub>-1<\/sup>), making 16e, a fairly normal count for many Ti species and only two short of a filled valence shell of 18e. Alternative representation 4<\/strong> shows only one +ve and one -ve charge in the molecule, but now the Ti formal valence shell has only 14e.<\/p>\n

\"\"<\/p>\n

I decided firstly to find out if there was any supporting data for the N\u2261Ti triple bond as shown in 3<\/strong> and 5<\/strong>.\u00a0A search of the CSD database for species with a Ti-N bond (of any order or type) produces the following plot.<\/p>\n

\"\"<\/p>\n

There is a distinct cluster in the region 1.7-1.8\u00c5, which we may assume corresponds to the shortest TiN bonds, presumed to be triple. Two more diffuse clusters are in the region 1.9 – 2.0 (double bonds) and 2.0-2.3\u00c5 (single and other bonds).\u00a0The crystal structure of HOPSUA<\/a> shows as 1.735\u00c5 and hence appears to be in the triple (3<\/strong>) rather than the double bond (4<\/strong>) region. Moreover the measured TiNN angle is 165\u00b0 whereas 4<\/strong> might be expected to be more highly bent. The N-N bond length is 1.362\u00c5.<\/p>\n

MN15L\/Def2-TZVPP and R2<\/sup>-SCAN-3c calculations give respectively TiN 1.784\u00c5\/1.806\u00c5\u00a0and NNTi 147\u00b0\/152\u00b0.[9]<\/a><\/span> The observed NN distance of 1.362\u00c5 in HOPSUA and containing an N(+)-N(+) motif\u00a0(calculated N-N 1.337\u00c5\/1.303\u00c5) compares with the distribution below in the CSD, where the main features are ~1.3\u00c5 (double), 1.4\u00c5 (aromatic) and ~1.58\u00c5 (single). So the NN bond order in HOPSUA is rather less than double.<\/p>\n

\"\"<\/p>\n

A NEDA (natural energy decomposition analysis)[9]<\/a><\/span> shows that the interaction energy of a neutral singlet pyridine with a singlet NTiCl2<\/sub>Cp fragment (a metal nitrido complex[10]<\/a><\/span>) is -89 kcal\/mol. The charge transfer (CT) component is large (-677.4 kcal\/mol) because combining two fragments, each deploying a lone pair of electrons to form less than a double bond requires transferring electrons out of this region. This overall interaction energy is larger than for nitrosobenzene, but it is still unusually small for a bond interaction and indeed perhaps a feature of systems which have two (formal) repulsive positive charges on adjacent atoms. For completeness, the interaction energy for the two fragments PyN2<\/sub> and TiCl2<\/sub>Cp both as doublet states is -83.36 kcal\/mol, with a much smaller charge transfer component of -307.4 kcal\/mol compared to the NN bond. Despite the apparent disparity between the bond orders of the\u00a0NN and NTi bonds, they appear to have similar interaction energies! The quartet-quartet decomposition is -184.5, whilst the ionic decompositions [singlet C5H5TiCl2(-) and C5H5N2(+)] vs<\/em> [singlet C5H5TiCl2(+) and C5H5N2(-)] are respectively -210.6 and -244.6 kcal\/mol.<\/p>\n

Both 1<\/strong> and 2<\/strong> were previously mooted as systems where adjacent atoms bear formal +ve charges,\u00a0and characterised by their unusually low interaction energies emerging from an energy decomposition analysis. We can now add the known system 3<\/strong> to this class, albeit its interaction energy being somewhat higher than 1<\/strong> or 2<\/strong>. There are probably many more of this type out there.<\/p>\n

References<\/h2>\n
  1. H. Rzepa, \"Mechanism of the dimerisation of Nitrosobenzene.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.28849<\/a>\n\n<\/li>\n
  2. H. Rzepa, \"The mysterious N=N double bond in nitrosobenzene dimer.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.29383<\/a>\n\n<\/li>\n
  3. D.A. Dieterich, I.C. Paul, and D.Y. Curtin, \"Structural studies on nitrosobenzene and 2-nitrosobenzoic acid. Crystal and molecular structures of cis-azobenzene dioxide and trans-2,2'-dicarboxyazobenzene dioxide\", Journal of the American Chemical Society<\/i>, vol. 96, pp. 6372-6380, 1974. https:\/\/doi.org\/10.1021\/ja00827a021<\/a>\n\n<\/li>\n
  4. C.R. Landis, R.P. Hughes, and F. Weinhold, \"Bonding Analysis of TM(cAAC)<sub>2<\/sub> (TM = Cu, Ag, and Au) and the Importance of Reference State\", Organometallics<\/i>, vol. 34, pp. 3442-3449, 2015. https:\/\/doi.org\/10.1021\/acs.organomet.5b00429<\/a>\n\n<\/li>\n
  5. H. Rzepa, \"Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.29410<\/a>\n\n<\/li>\n
  6. K.G. Orrell, V. \u0160ik, and D. Stephenson, \"Study of the monomer\u2010dimer equilibrium of nitrosobenzene using multinuclear one\u2010 and two\u2010dimensional NMR techniques\", Magnetic Resonance in Chemistry<\/i>, vol. 25, pp. 1007-1011, 1987. https:\/\/doi.org\/10.1002\/mrc.1260251118<\/a>\n\n<\/li>\n
  7. H. Rzepa, \"The even more mysterious N\u2261N triple bond in a nitric oxide dimer.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.29429<\/a>\n\n<\/li>\n
  8. M. Retb\u00f8ll, Y. Ishii, and M. Hidai, \"Synthesis and Reductive N\u2212N Bond Cleavage of Neutral and Cationic Titanium (1-Pyridinio)imido Complexes\", Organometallics<\/i>, vol. 18, pp. 150-155, 1998. https:\/\/doi.org\/10.1021\/om980661t<\/a>\n\n<\/li>\n
  9. H. Rzepa, \"Valence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed.\", 2026. https:\/\/doi.org\/10.14469\/hpc\/15774<\/a>\n\n<\/li>\n
  10. Carroll, Maria E.., Pinter, Balazs., Carroll, Patrick J.., and Mindiola, Daniel J.., \"CCDC 1420931: Experimental Crystal Structure Determination\", 2015. https:\/\/doi.org\/10.5517\/cc1jplh9<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene, and commented on the odd nature of the N=N double bond formed in this process.. Odd, because the valence bond representation of this dimer (1 below) has two formally positive adjacent nitrogen atoms. An energy decomposition analysis […]<\/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":"federated","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":[],"ppma_author":[2661],"class_list":["post-30890","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nValence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed. - 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=30890\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Valence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene, and commented on the odd nature of the N=N double bond formed in this process.. Odd, because the valence bond representation of this dimer (1 below) has two formally positive adjacent nitrogen atoms. 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One of the properties of this molecule is that the equilibrium between the monomer and dimer can be detected,\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":29626,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29626","url_meta":{"origin":30890,"position":1},"title":"Hydrogenating the even more mysterious N\u2261N triple bond in a nitric oxide dimer.","author":"Henry Rzepa","date":"August 25, 2025","format":false,"excerpt":"Previously I looked at some of the properties of the mysterious dimer of nitric oxide \u00a01 - not the known weak dimer but a higher energy form with a \"triple\" N\u2261N bond. This valence bond isomer of the weak dimer was some 24 kcal\/mol higher in free energy than the\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":28849,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","url_meta":{"origin":30890,"position":2},"title":"Mechanism of the dimerisation of Nitrosobenzene.","author":"Henry Rzepa","date":"June 14, 2025","format":false,"excerpt":"I am in the process of revising my annual lecture to first year university students on the topic of \"curly arrows\". I like to start my story in 1924, when Robert Robinson published the very first example as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":30890,"position":3},"title":"Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.","author":"Henry Rzepa","date":"August 13, 2025","format":false,"excerpt":"In the previous post, I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. Here\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":29665,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29665","url_meta":{"origin":30890,"position":4},"title":"Alternative reactions of the N\u2261N “triple bond” in a nitric oxide dimer: forming the trimer N3O3.","author":"Henry Rzepa","date":"September 3, 2025","format":false,"excerpt":"In the previous post I mooted the possibility that a high energy form of the dimer of nitric oxide 1 might nonetheless be able to be detected using suitable traps (such as hydrogenation or cycloaddition). However, an interesting alternative is that this species could be trapped by nitric oxide itself.\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":29429,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29429","url_meta":{"origin":30890,"position":5},"title":"The even more mysterious N\u2261N triple bond in a nitric oxide dimer.","author":"Henry Rzepa","date":"August 18, 2025","format":false,"excerpt":"Previously, I pondered about the strange N=N double bond in nitrosobenzene dimer as a follow up to commenting on the curly arrow mechanism of the dimerisation. By the same curly arrow method, one can produce the below, showing how the simpler nitric oxide radical could potentially dimerise to a species\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":[]}],"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\/30890","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=30890"}],"version-history":[{"count":69,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/30890\/revisions"}],"predecessor-version":[{"id":30970,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/30890\/revisions\/30970"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=30890"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=30890"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=30890"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=30890"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}