Complement activation induces excessive T cell cytotoxicity in severe COVID-19.

Severe COVID-19 is linked to both dysfunctional immune response and unrestrained immunopathology, and it remains unclear whether T cells contribute to disease pathology. Here, we combined single-cell transcriptomics and single-cell proteomics with mechanistic studies to assess pathogenic T cell functions and inducing signals. We identified highly activated CD16+ T cells with increased cytotoxic functions in severe COVID-19. CD16 expression enabled immune-complex-mediated, T cell receptor-independent degranulation and cytotoxicity not found in other diseases. CD16+ T cells from COVID-19 patients promoted microvascular endothelial cell injury and release of neutrophil and monocyte chemoattractants. CD16+ T cell clones persisted beyond acute disease maintaining their cytotoxic phenotype. Increased generation of C3a in severe COVID-19 induced activated CD16+ cytotoxic T cells. Proportions of activated CD16+ T cells and plasma levels of complement proteins upstream of C3a were associated with fatal outcome of COVID-19, supporting a pathological role of exacerbated cytotoxicity and complement activation in COVID-19.

Severe COVID-19 Is Marked by a Dysregulated Myeloid Cell Compartment.

Coronavirus disease 2019 (COVID-19) is a mild to moderate respiratory tract infection, however, a subset of patients progress to severe disease and respiratory failure. The mechanism of protective immunity in mild forms and the pathogenesis of severe COVID-19 associated with increased neutrophil counts and dysregulated immune responses remain unclear. In a dual-center, two-cohort study, we combined single-cell RNA-sequencing and single-cell proteomics of whole-blood and peripheral-blood mononuclear cells to determine changes in immune cell composition and activation in mild versus severe COVID-19 (242 samples from 109 individuals) over time. HLA-DRhiCD11chi inflammatory monocytes with an interferon-stimulated gene signature were elevated in mild COVID-19. Severe COVID-19 was marked by occurrence of neutrophil precursors, as evidence of emergency myelopoiesis, dysfunctional mature neutrophils, and HLA-DRlo monocytes. Our study provides detailed insights into the systemic immune response to SARS-CoV-2 infection and reveals profound alterations in the myeloid cell compartment associated with severe COVID-19.

Multimodal Profiling of Peripheral Blood Identifies Proliferating Circulating Effector CD4+ T Cells as Predictors for Response to Integrin α4ß7-Blocking Therapy in Inflammatory Bowel Disease.

BACKGROUND & AIMS: Despite the success of biological therapies in treating inflammatory bowel disease, managing patients remains challenging due to the absence of reliable predictors of therapy response. METHODS: In this study, we prospectively sampled 2 cohorts of patients with inflammatory bowel disease receiving the anti-integrin α4ß7 antibody vedolizumab. Samples were subjected to mass cytometry; single-cell RNA sequencing; single-cell variable, diversity, and joining sequencing; serum proteomics; and multidimensional flow cytometry to comprehensively assess vedolizumab-induced immunologic changes in the peripheral blood and their potential associations with treatment response. RESULTS: Vedolizumab treatment led to substantial alterations in the abundance of circulating immune cell lineages and modified the T-cell receptor diversity of gut-homing CD4+ memory T cells. Through integration of multimodal parameters and machine learning, we identified a significant increase in proliferating CD4+ memory T cells among nonresponders before treatment compared with responders. This predictive T-cell signature demonstrated an activated T-helper 1/T-helper 17 cell phenotype and exhibited elevated levels of integrin α4ß1, potentially making these cells less susceptible to direct targeting by vedolizumab. CONCLUSIONS: These findings provide a reliable predictive classifier with significant implications for personalized inflammatory bowel disease management.

Heteroatom-Based Diradical(oid)s.

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

Spatially Resolved Multi-Omics Single-Cell Analyses Inform Mechanisms of Immune Dysfunction in Pancreatic Cancer.

BACKGROUND & AIMS: As pancreatic ductal adenocarcinoma (PDAC) continues to be recalcitrant to therapeutic interventions, including poor response to immunotherapy, albeit effective in other solid malignancies, a more nuanced understanding of the immune microenvironment in PDAC is urgently needed. We aimed to unveil a detailed view of the immune micromilieu in PDAC using a spatially resolved multimodal single-cell approach. METHODS: We applied single-cell RNA sequencing, spatial transcriptomics, multiplex immunohistochemistry, and mass cytometry to profile the immune compartment in treatment-naïve PDAC tumors and matched adjacent normal pancreatic tissue, as well as in the systemic circulation. We determined prognostic associations of immune signatures and performed a meta-analysis of the immune microenvironment in PDAC and lung adenocarcinoma on single-cell level. RESULTS: We provided a spatially resolved fine map of the immune landscape in PDAC. We substantiated the exhausted phenotype of CD8 T cells and immunosuppressive features of myeloid cells, and highlighted immune subsets with potentially underappreciated roles in PDAC that diverged from immune populations within adjacent normal areas, particularly CD4 T cell subsets and natural killer T cells that are terminally exhausted and acquire a regulatory phenotype. Differential analysis of immune phenotypes in PDAC and lung adenocarcinoma revealed the presence of extraordinarily immunosuppressive subtypes in PDAC, along with a distinctive immune checkpoint composition. CONCLUSIONS: Our study sheds light on the multilayered immune dysfunction in PDAC and presents a holistic view of the immune landscape in PDAC and lung adenocarcinoma, providing a comprehensive resource for functional studies and the exploration of therapeutically actionable targets in PDAC.

Prophylactic antibodies inhibit spike-specific T and B cell responses after COVID-19 vaccination.

Active and passive immunization is used in high-risk patients to prevent severe courses of COVID-19, but the impact of prophylactic neutralizing antibodies on the immune reaction to the mRNA vaccines has remained enigmatic. Here we show that CD4 T and B cell responses to Spikevax booster immunization are suppressed by the therapeutic antibodies Casirivimab and Imdevimab. B cell and T cell responses were significantly induced in controls but not in antibody-treated patients. The data indicates that humoral immunity, i. e. high levels of antibodies, negatively impacts reactive immunity, resulting in blunted cellular responses upon boosting. This argues for temporal separation of vaccination efforts; with active vaccination preferably applied before prophylactic therapeutic antibody treatment.

A Phosphorus-Centred, Zirconocene-Bridged Tetraradical: Synthesis, Structure and Application as Molecular Double Switch.

Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi)=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zirconcene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570 nm), this tetraradical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5 h. Upon irradiation at 415 nm, the reverse reaction is observed, fully recovering tetraradical 5. Through single-crystal-to-single-crystal transformation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light.

Tricyanomethane or Dicyanoketenimine-Silylation makes the Difference.

Pseudohalides such as tricyanomethanide, [C(CN)3]-, are well known in chemistry, biochemistry and industrial chemistry. The protonated species HC(CN)3, a classic hydrogen pseudohalide Brønsted acid, is a very strong acid with a pKa value of -5. However, HC(CN)3 is difficult to handle as it tends to decompose rapidly or, more precisely, to oligo- and polymerize. Therefore, silylated pseudohalide compounds with the [Me3Si]+ as the "big organometallic proton" have become interesting, exhibiting similar chemical properties but better kinetic protection. Here, the stepwise silylation of the pseudohalide anion [C(CN)3]- is reported, forming the heavier homologue of HC(CN)3, namely [Me3Si][C(CN)3], and in presence of two additional [Me3Si]+ cations even the dicationic species [(Me3Si-NC)3C]2+ as stable [B(C6F5)4]- salt. Surprisingly, in contrast to the protonated species HC(CN)3, in which the proton is bound to the central carbon atom of [C(CN)3]-, silylation of the [C(CN)3]- anion occurs at one of the three terminal nitrogen atoms, thus forming the long-sought dicyanoketenimine [Me3Si-NC-C(CN)2]. All further silylation steps take place exclusively on the terminal N atoms of the three CN groups and not on the central carbon atom, until the intriguing, highly symmetrical dication, [(Me3Si-NC)3C]2+, is finally generated. The experimental data are supported by quantum chemical calculations in terms of thermodynamics and chemical bonding.

Rational Design of a Phosphorus-Centered Disbiradical.

Phosphorus-centered disbiradicals, in which the radical sites exist as individual spin doublets with weak spin-spin interaction have not been known so far. Starting from monoradicals of the type [⋅P(µ-NTer)2 P-R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [⋅P(µ-NTer)2 P⋅] (1) was treated with 1,6-dibromohexane, affording the brominated species {Br[P(µ-NTer)]2 }2 C6 H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {⋅[P(µ-NTer)]2 }2 C6 H12 (4) featuring a large distance between the radical phosphorus sites in the solid state and formally the highest biradical character observed in a P-centered biradical so far, approaching 100 %. EPR spectroscopy revealed a three-line signal in solution with a considerably larger exchange interaction than would be expected from the molecular structure of the single crystal. Quantum chemical calculations revealed a highly dynamic conformational space; thus, the two radical sites can approach each other with a much smaller distance in solution. Further reduction of 4 resulted in the formation of a potassium salt featuring the first structurally characterized P-centered distonic radical anion (5- ). Moreover, 4 could be used in small molecule activation.

Rational Design of Persistent Phosphorus-Centered Singlet Tetraradicals and Their Use in Small-Molecule Activation.

Biradicals are important intermediates in the process of bond formation and breaking. While main-group-element-centered biradicals have been thoroughly studied, much less is known about tetraradicals, as their very low stability has hampered their isolation and use in small-molecule activation. Herein, we describe the search for persistent phosphorus-centered tetraradicals. Starting from an s-hydrindacenyl skeleton, we investigated the introduction of four phosphorus-based radical sites linked by an N-R unit and bridged by a benzene moiety. By varying the size of the substituent R, we finally succeeded in isolating a persistent P-centered singlet tetraradical, 2,6-diaza-1,3,5,7-tetraphospha-s-hydrindacene-1,3,5,7-tetrayl (1), in good yields. Furthermore, it was demonstrated that tetraradical 1 can be utilized for the activation of small molecules such as molecular hydrogen or alkynes. In addition to the synthesis of P-centered tetraradicals, the comparison with other known tetraradicals as well as biradicals is described on the basis of quantum mechanical calculations with respect to its multireference character, coupling of radical electrons, and aromaticity. The strong coupling of radical electrons enables selective discrimination between the first and the second activations of small molecules, which is shown by the example of H2 addition. The mechanism of hydrogen addition is investigated with parahydrogen-induced hyperpolarization NMR studies and DFT calculations.

Resident memory CD4+ T lymphocytes mobilize from bone marrow to contribute to a systemic secondary immune reaction.

Resident memory T lymphocytes (TRM ) of epithelial tissues and the Bm protect their host tissue. To what extent these cells are mobilized and contribute to systemic immune reactions is less clear. Here, we show that in secondary immune reactions to the measles-mumps-rubella (MMR) vaccine, CD4+ TRM are mobilized into the blood within 16 to 48 h after immunization in humans. This mobilization of TRM is cognate: TRM recognizing other antigens are not mobilized, unless they cross-react with the vaccine. We also demonstrate through methylome analyses that TRM are mobilized from the Bm. These mobilized cells make significant contribution to the systemic immune reaction, as evidenced by their T-cell receptor Vß clonotypes represented among the newly generated circulating memory T-cells, 14 days after vaccination. Thus, TRM of the Bm confer not only local, but also systemic immune memory.

Access to Benzo- and Naphtho-Azaphospholes via C-H Bond Activation of Aryl-Substituted Isonitriles.

Differently substituted phenyl isonitriles (with C-H bonds in ortho-position) and naphthyl isonitriles were reacted with the cyclic biradical [⋅P(µ-N-Ter)2 P⋅] (1). Insertion of the isonitrile formed a cyclic five-membered biradical [⋅P(NTer)2 C(R)P⋅] (2R, R=phenyl, naphthyl) in the first step, followed by C-H activation at the aryl substituent, resulting in novel azaphospholes (5R), which could be isolated and fully characterized. The formation of the azaphospholes can be prevented by the addition of a second equivalent of isonitrile, which causes the blocking of the radical centers in 2R by adduct formation (3R). Quantum mechanical calculations showed that a significant increase in the aromaticity of the benzo- and naphtho-azaphospholes is one of the driving forces for the activation process leading to the formation of thermodynamically favored azaphospholes. Targeted activation of C-H bonds using biradical systems represents a new synthetic approach to generate benzo- and naphtho-azaphospholes.

On the Dynamic Behavior of Pacman Phosphanes─A Case of Cooperativity and Redox Isomerism.

In solution, the Pacman chlorophosphane (2Cl) shows fast exchange of the endo/exo-orientation of the two P-Cl bonds in the molecule featuring cooperativity. Experimental and quantum mechanical investigations of the inversion on the phosphorus(III) centers reveal a crucial role of chloride ions in the dynamic process. To confirm the results, the homologous Pacman halogen-phosphanes 2X were prepared by halogen exchange reactions (X = F, Br, and I). Besides accelerated dynamic behavior for the heavier analogues, significant differences in the molecular structure are caused by the halogen exchange reactions, including the formation of an endo-endo substituted Pacman fluorophosphane as well as dicationic species by phosphorus halogen bond dissociation. The latter process can be regarded as redox isomerism since two PIII atoms in 2X become PV centers in the dications.

Synthesis of Stable Potassium Phosphinophosphides and Reaction with Organosilyl Halides and Chlorophosphanes.

The synthesis of sterically demanding 2,6-bis(2,4,6-trimethylphenyl)phenyl (Ter)-stabilized and H-substituted diphosphanes TerHP-PR2 (4a-4c) via conversion of the phosphide TerPHK (2) with secondary chlorophosphanes ClPR2 (3a-3c, where R = iPr, Ph, and tBu, respectively) is described. The diphosphanes 4a-4c were deprotonated using KH in tetrahydrofuran, selectively yielding the potassium phosphinophosphides K[TerP-PR2] (5a-5c). These phosphinophosphides are stable in solution as well as in the solid state and can be further functionalized via salt-metathesis reactions. Reaction with organosilyl halides selectively yields the silylated diphosphanes Ter(SiR12R2)P-P(iPr)2 (6a and 6b, where R1 = R2 = CH3 and R1 = CH3, R2 = Ph, respectively), whereas conversion with chlorophosphanes selectively yields the triphosphanes R12P-P(Ter)-P(iPr)2 (7a and 7b, where R = iPr and Ph, respectively).

Peripheral Blood Immune Cell Composition After Autologous MSC Infusion in Kidney Transplantation Recipients.

Tacrolimus is the backbone of immunosuppressive agents to prevent transplant rejection. Paradoxically, tacrolimus is nephrotoxic, causing irreversible tubulointerstitial damage. Therefore, infusion of mesenchymal stromal cells (MSC) 6 and 7 weeks post-transplantation was assessed to facilitate withdrawal of tacrolimus in the randomized phase II TRITON trial. Here, we performed detailed analysis of the peripheral blood immune composition using mass cytometry to assess potential effects of MSC therapy on the immune system. We developed two metal-conjugated antibody panels containing 40 antibodies each. PBMC samples from 21 MSC-treated patients and 13 controls, obtained pre-transplant and at 24 and 52 weeks post-transplantation, were analyzed. In the MSC group at 24 weeks, 17 CD4+ T cell clusters were increased of which 14 Th2-like clusters and three Th1/Th2-like clusters, as well as CD4+FoxP3+ Tregs. Additionally, five B cell clusters were increased, representing either class switched memory B cells or proliferating B cells. At 52 weeks, CCR7+CD38+ mature B cells were decreased. Finally, eight Tc1 (effector) memory cytotoxic T cell clusters were increased. Our work provides a comprehensive account of the peripheral blood immune cell composition in kidney transplant recipients after MSC therapy and tacrolimus withdrawal. These results may help improving therapeutic strategies using MSCs with the aim to reduce the use of calcineurin inhibitors. Clinical Trial Registration: ClinicalTrials.gov, identifier NCT02057965.

IGLV3-21*01 is an inherited risk factor for CLL through the acquisition of a single-point mutation enabling autonomous BCR signaling.

The prognosis of chronic lymphocytic leukemia (CLL) depends on different markers, including cytogenetic aberrations, oncogenic mutations, and mutational status of the immunoglobulin (Ig) heavy-chain variable (IGHV) gene. The number of IGHV mutations distinguishes mutated (M) CLL with a markedly superior prognosis from unmutated (UM) CLL cases. In addition, B cell antigen receptor (BCR) stereotypes as defined by IGHV usage and complementarity-determining regions (CDRs) classify ∼30% of CLL cases into prognostically important subsets. Subset 2 expresses a BCR with the combination of IGHV3-21-derived heavy chains (HCs) with IGLV3-21-derived light chains (LCs), and is associated with an unfavorable prognosis. Importantly, the subset 2 LC carries a single-point mutation, termed R110, at the junction between the variable and constant LC regions. By analyzing 4 independent clinical cohorts through BCR sequencing and by immunophenotyping with antibodies specifically recognizing wild-type IGLV3-21 and R110-mutated IGLV3-21 (IGLV3-21R110), we show that IGLV3-21R110-expressing CLL represents a distinct subset with poor prognosis independent of IGHV mutations. Compared with other alleles, only IGLV3-21*01 facilitates effective homotypic BCR-BCR interaction that results in autonomous, oncogenic BCR signaling after acquiring R110 as a single-point mutation. Presumably, this mutation acts as a standalone driver that transforms IGLV3-21*01-expressing B cells to develop CLL. Thus, we propose to expand the conventional definition of CLL subset 2 to subset 2L by including all IGLV3-21R110-expressing CLL cases regardless of IGHV mutational status. Moreover, the generation of monoclonal antibodies recognizing IGLV3-21 or mutated IGLV3-21R110 facilitates the recognition of B cells carrying this mutation in CLL patients or healthy donors.

Deep phenotypical characterization of human CD3+ CD56+ T cells by mass cytometry.

CD56+ T cells are a group of pro-inflammatory CD3+ lymphocytes with characteristics of natural killer cells, being involved in antimicrobial immune defense. Here, we performed deep phenotypic profiling of CD3+ CD56+ cells in peripheral blood of normal human donors and individuals sensitized to birch-pollen or/and house dust mite by high-dimensional mass cytometry combined with manual and computational data analysis. A co-regulation between major conventional T-cell subsets and their respective CD3+ CD56+ cell counterparts appeared restricted to CD8+ , MAIT, and TCRγδ+ T-cell compartments. Interestingly, we find a co-regulation of several CD3+ CD56+ cell subsets in allergic but not in healthy individuals. Moreover, using FlowSOM, we distinguished a variety of CD56+ T-cell phenotypes demonstrating a hitherto underestimated heterogeneity among these cells. The novel CD3+ CD56+ subset description comprises phenotypes superimposed with naive, memory, type 1, 2, and 17 differentiation stages, in part represented by a phenotypical continuum. Frequencies of two out of 19 CD3+ CD56+ FlowSOM clusters were significantly diminished in allergic individuals, demonstrating less frequent presence of cells with cytolytic, presumably protective, capacity in these donors consistent with defective expansion or their recruitment to the affected tissue. Our results contribute to defining specific cell populations to be targeted during therapy for allergic conditions.

Either IL-7 activation of JAK-STAT or BEZ inhibition of PI3K-AKT-mTOR pathways dominates the single-cell phosphosignature of ex vivo treated pediatric T-cell acute lymphoblastic leukemia cells.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer arising from lymphoblasts of T-cell origin. While TALL accounts for only 15% of childhood and 25% of adult ALL, 30% of patients relapse with a poor outcome. Targeted therapy of resistant and high-risk pediatric T-ALL is therefore urgently needed, together with precision medicine tools allowing the testing of efficacy in patient samples. Furthermore, leukemic cell heterogeneity requires drug response assessment at the single-cell level. Here we used single-cell mass cytometry to study signal transduction pathways such as JAK-STAT, PI3K-AKT-mTOR and MEK-ERK in 16 diagnostic and five relapsed T-ALL primary samples, and investigated the in vitro response of cells to Interleukin-7 (IL-7) and the inhibitor BEZ-235. T-ALL cells showed upregulated activity of the PI3K-AKT-mTOR and MEK-ERK pathways and increased expression of proliferation and translation markers. We found that perturbation induced by the ex vivo administration of either IL-7 or BEZ-235 reveals a high degree of exclusivity with respect to the phospho-protein responsiveness to these agents. Notably, these response signatures were maintained from diagnosis to relapse in individual patients. In conclusion, we demonstrated the power of mass cytometry single-cell profiling of signal transduction pathways in T-ALL. Taking advantage of this advanced approach, we were able to identify distinct clusters with different responsiveness to IL-7 and BEZ-235 that can persist at relapse. Collectively our observations can contribute to a better understanding of the complex signaling network governing T-ALL behavior and its correlation with influence on the response to therapy.

A Lewis Acid Stabilized Ketenimine in an Unusual Variant of the Electrophilic Aromatic Substitution.

Electrophilic aromatic substitution (EAS) can provide a straightforward approach to the efficient synthesis of functionalized complex aromatic molecules. In general, Lewis acids serve as a beneficial stimulus for the formation of a Wheland complex, the intermediate in the classical SE Ar mechanism of EAS, which is responsible for H/E (E=electrophile) substitution under formal H+ elimination. Herein, we report an unusual variant of EAS, in which a complex molecule such as the tricyanomethane, HC(CN)3 , is activated with a strong Lewis acid (B(C6 F5 )3 ) to the point where it can finally be used in an EAS. However, the Lewis acid here causes the isomerization of the tricyanomethane to the ketenimine, HN=C=C(CN)2 , which in turn directly attacks the aromatic species in the EAS, with simultaneous proton migration of the aromatic proton to the imino group, so that no elimination occurs that is otherwise observed in the SE Ar mechanism. By this method, it is possible to build up amino-malononitrile-substituted aromatic compounds in one step.

Low-Temperature Isolation of a Labile Silylated Hydrazinium-yl Radical Cation, [(Me3 Si)2 N-N(H)SiMe3 ].

The oxidation of silylated hydrazine, (Me3 Si)2 N-N(H)SiMe3 , with silver salts led to the formation of a highly labile hydrazinium-yl radical cation, [(Me3 Si)2 N-N(H)SiMe3 ].+ , at very low temperatures (decomposition > -40 °C). EPR, NMR, DFT and Raman studies revealed the formation of a nitrogen-centered radical cation along the N-N unit of the hydrazine. In the presence of the weakly coordinating anion [Al{OCH(CF3 )2 }4 ]- , crystallization and structural characterization in the solid state were achieved. The hydrazinium-yl radical cation has a significantly shortened N-N bond and a nearly planar N2 Si3 framework, in contrast to the starting material. According to DFT calculations, the shortened N-N bond has a total bond order of 1.5 with a π-bond order of 0.5. The π bond can be regarded as a three-π-electron, two-center bond.