Atomically Precise Insights into Metal–Metal Bonds Using Comparable Endo-Units of Sc ₂ and Sc ₂ C ₂ Article Swipe

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Yang‐Rong Yao , Xiang‐Mei Shi , Shan-Yu Zheng , Zuo‐Chang Chen , Su‐Yuan Xie , Rong‐Bin Huang , Lan‐Sun Zheng · YOU? · · 2020 · Open Access · · DOI: https://doi.org/10.31635/ccschem.020.202000681

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Atomically Precise Insights into Metal–Metal Bonds Using Comparable Endo-Units of Sc2 and Sc2C2 Yang-Rong Yao, Xiang-Mei Shi, Shan-Yu Zheng, Zuo-Chang Chen, Su-Yuan Xie, Rong-Bin Huang and Lan-Sun Zheng Yang-Rong Yao State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Xiang-Mei Shi State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Shan-Yu Zheng State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Zuo-Chang Chen State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Su-Yuan Xie *Corresponding author: E-mail Address: [email protected] State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Rong-Bin Huang State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 and Lan-Sun Zheng State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 https://doi.org/10.31635/ccschem.020.202000681 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The precise identification of metal–metal bonds is critical to fully understanding the nature of metal–metal bonding but remains a fundamental challenge. Herein, we show the essence of Sc–Sc bonds with a metal–metal distance of 3.36 Å in a C3v(8)-C82 fullerene cage using crystallography. To elucidate the metal–metal bond formation, a metallofullerene Sc2C2@C3v(8)-C82 with a Sc2C2 endo-unit encapsulated inside the same fullerene[82] cage was studied as a unique control molecule. Theoretical calculations reveal that each Sc atom donates one electron to the Sc–Sc bond. The existence of the Sc–Sc bond in Sc2@C3v(8)-C82 versus Sc2C2@C3v(8)-C82 is thoroughly investigated. Download figure Download PowerPoint Introduction The nature of metal–metal bonds, one of the important topics in inorganic and organometallic chemistry,1–3 has attracted widespread attention due to continued discoveries of various novel metal–metal bonds among the main group,4,5 transition,6,7 lanthanide,8–10 and actinide11–13 elements. Metal–metal bonding can be primarily evaluated in terms of the intermetallic distance in the crystal structure, such as the famous Re–Re quadruple bond.14 However, intermetallic distance is not a precise parameter because surrounding factors, including molecular charges, bridging ligands, and crystal packing, also influence the distance and strength of metal–metal bonds.15 Although some characterization techniques, such as X-ray photoelectron spectroscopy, electron spin resonance, and theoretical calculations, have been developed in the past decades,16 a complete understanding of the nature of these bonds remains challenging. For example, the bond order of the Ga–Ga triple bond is still under debate17 because various conclusions, such as a double bond or a nonclassical triple bond, have been reached using different theoretical methods.18–21 Therefore, the precise identification of metal–metal bonds remains one of the most fundamental questions in chemistry. To precisely understand the nature of metal–metal bonds, studying the bonding of a metal dimer in a fullerene cage is a promising alternative, as it provides an isolated internal space, allowing the exclusion of other external factors that could influence the bonding of the endo-metal dimer. However, only a few cases have been found experimentally or theoretically after considerable efforts to explore endo-metal–metal bonds in fullerene cages.22–36 An unexpected challenge in determining endo-metal–metal bonding in fullerene cages by X-ray crystallography is posed by severe disorder in the crystal structures, especially for metallic endo-clusters, and there can be up to tens of disordered sites when determining endo-metal atoms in a fullerene cage. Therefore, searching for an example system with less metallic disorder is the key to precisely elucidating the nature of endo-metal–metal bonds. Herein, we report a precise case of an endohedral metallofullerene (EMF) containing a Sc–Sc bond in a C3v(8)-C82 (the number 8 is specified by the Fowler–Manolopoulos spiral code37) fullerene cage, namely, Sc2@C3v(8)-C82. Furthermore, another endohedral fullerene, Sc2C2@C3v(8)-C82, containing a bridging C2 endo-unit between two nonbonding Sc atoms, has also been crystallized for comparison. Different from previously reported structures rendering severe disorder,38,39 only a crystallographic mirror-related counterpart is found in this crystal. The geometries of the two structures have been thoroughly investigated, proving the formation of a Sc–Sc bond in Sc2@C3v(8)-C82. In addition, theoretical calculations have been performed to understand the nature of Sc–Sc bond formation. Experimental Methods Experimental details including the synthesis, isolation, UV spectra, and X-ray single-crystal characterizations of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 are presented in the Supporting Information. Soot containing Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 was produced by vaporization of a core-drilled graphite rod filled with a mixture of Sc2O3/graphite powder in an optimized Krätschmer–Huffman arc-discharge reactor under 400 mbar of helium. The synthesized soot was then collected and repeatedly extracted with toluene using an ultrasonic bath. The extracted fullerene solution was analyzed and characterized by mass spectrometry and multiple-stage high-performance liquid chromatography (HPLC), as shown in Supporting Information Figures S1 and S2. The visible-near-infrared (Vis-NIR) absorption spectra of purified Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 dissolved in toluene are shown in Supporting Information Figure S3. The crystals of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 were obtained by slow evaporation from a toluene solution mixed with NiII(OEP) (OEP = 2,3,7,8,12,13,17,18-octaethylporphin dianion). The structures were unambiguously determined by single-crystal X-ray diffraction and refined as Sc2@C3v(8)-C82·NiII(OEP)·2C7H8 and Sc2C2@C3v(8)-C82·NiII(OEP)·2C7H8. The detailed crystallographic information is shown in Supporting Information Table S1. Results and Discussion The space group of both crystals is C2/m, identical to many analogous EMF/NiII(OEP) systems. In this case, the fullerene cage is bisected into two parts by a crystallographic mirror plane, and an intact fullerene cage with occupancy of 0.5 is refined by combining one-half of the cage with the mirror counterpart of the other. For Sc2@C3v(8)-C82, only two Sc sites (Sc82A and Sc82B) with occupancies of 0.5 and their mirror-related counterparts (Sc82Am and Sc82Bm) are found in the crystal ( Supporting Information Figure S4). However, Sc82Am and Sc82Bm can be excluded because the distances between the Sc sites and nearest cage carbons (2.14 and 2.12 Å, respectively) are shorter than those of other Sc-based endohedral fullerenes with ordered crystal structures (generally larger than 2.20 Å).40–43 For Sc2C2@C3v(8)-C82, only three Sc sites are found, including Sc84B, Sc84A, and its mirror-related counterpart, Sc84Am, because Sc84B is located in the crystallographic mirror plane ( Supporting Information Figure S5). Similar to Sc82Am and Sc82Bm, the distance between Sc84Am and the nearest cage carbon is only 2.10 Å so that it can also be excluded. In addition, the pyramidalization angles of the nearest cage carbons of Sc82A, Sc82B, and Sc84A are much larger than those of their mirror-related counterparts due to the metallic extrusion effects. Therefore, the Sc2 and Sc2C2 endo-units can be uniquely determined, providing a good opportunity for enhanced understanding of the essence of endo-metal–metal bonding. The interactions of all components in both crystals are illustrated in Figures 1a and 1b with two orthogonal views. The shortest distances between the benzene rings and the corresponding cage carbons range from 3.13 to 3.29 Å, suggesting noticeable π–π interactions between the solvent molecules and fullerene cages. Interestingly, it can be found in both crystals, except for the endo-units, that not only the cages and NiII(OEP) molecules but also the corresponding solvents show surprisingly similar orientations. Moreover, the toluene molecules over the side site of the fullerenes even share the same angle (79.7°) between the twofold axis of toluene and the plane of the NiII(OEP) molecule, while the toluene molecules over the top site of the fullerenes have a slightly different angle. Figure 1 | Front and side views of (a) Sc2@C3v(8)-C82·NiII(OEP)·2C7H8 and (b) Sc2C2@C3v(8)-C82·NiII(OEP)·2C7H8. To better show the solvent molecules in the crystal structure, the cages are drawn as sticks. Oak Ridge thermal ellipsoid plot (ORTEP) drawings of (c) Sc2@C3v(8)-C82 and (d) Sc2C2@C3v(8)-C82 and their interactions with planar NiII(OEP) molecules. The solvent molecules are omitted for clarity. The thermal ellipsoids are drawn at the 30% probability level. Download figure Download PowerPoint Figures 1c and 1d depict the interactions between Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 and their cocrystallized NiII(OEP) molecule. Obviously, the NiII(OEP) moiety prefers a fixed orientation to form π–π interactions with the cage of C3v(8)-C82. Notably, the sulfide cluster metallofullerene of Sc2[email protected]C3v(8)-C82 with a fully ordered carbon cage also prefers a similar orientation to facilitate contact with the NiII(OEP) moiety.42 Moreover, the endohedral fullerenes of Sc2C2@C3v(8)-C82 and Sc2[email protected]C3v(8)-C82 are widely accepted as (Sc2C2)4+@[C3v(8)C82]4− and (Sc2S)4+@[C3v(8)C82]4−, respectively, indicating uniformity in the formation of the supramolecular interactions between the NiII(OEP) hosts and the [C3v(8)-C82]4− guests. Accordingly, the EMF of Sc2@C3v(8)-C82 can also be described as (Sc2)4+@[C3v(8)C82]4−, suggesting that the Sc2 endo-unit only transfers four, instead of six, electrons, as in (Sc2)6+@[C2v(4059)-C66] to the C3v(8)-C82 cage with the Sc–Sc bond formed by the two Sc atoms. The details of the cluster–cage interactions are illustrated in Figures 2a and 2b. In Sc2@C3v(8)-C82, Sc82A is situated at the vertex of two hexagons (2HC) and a pentagon with the shortest Sc–C distances ranging from 2.21 to 2.40 Å, forming a distorted trigonal bipyramidal configuration. Such a configuration is also observed with Sc84A in Sc2C2@C3v(8)-C82 over the same cage region with a similar Sc–C distance range from 2.18 to 2.35 Å. In contrast, Sc82B in Sc2@C3v(8)-C82 resides close to a pair of carbon atoms, forming a [5,6] bond, and the six shortest Sc–C distances fall over a relatively broad range from 2.24 to 2.51 Å. However, because of the C2 endo-unit, Sc84B atom in Sc2C2@C3v(8)-C82 shifts to the neighboring hexagonal carbocycle. Interestingly, the Sc84A atom, analogous to Sc82A, remains almost fixed during the extrusion of the inserted C2 unit. Figure 2c describes an overlap of the cages with the same orientation shown in Figure 1c and 1d but sectioned along the symmetric plane. Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 are drawn in gray and orange, respectively. The relative shift between Sc82A and Sc84A was measured to be only 0.08 Å, indicating a similar metal–cage interaction. Nevertheless, in the presence of the C2 endo-unit, the Sc84B atom shifts by 0.88 Å from its original position to fit the inner environment, resulting in a nonbonding Sc–Sc distance of 3.88 Å. In contrast, the Sc–Sc distance in Sc2@C3v(8)-C82 is 3.36 Å, shorter than the sum of their van der Waals radii. The unusual behaviors of the metallic endo-units suggest that their bonding mode and relative geometry configuration will vary depending on the local environment, showing unique environmental adaptability. For Sc2@C3v(8)-C82, a direct Sc–Sc bond is formed, transferring only four electrons to stabilize the cage of C3v(8)-C82. Figure 2 | Detailed structures of the encapsulated units of (a) Sc2@C3v(8)-C82 and (b) Sc2C2@C3v(8)-C82 with the closest aromatic fragments of the cages. (c) An overlap of both cages was sectioned along the symmetric plane to show the relative shifts of the corresponding Sc atoms. The cages of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 are colored in gray and orange, respectively, and the cage carbon atoms closest to the Sc atoms are colored in red. Thermal ellipsoids are shown at the 15% probability level to show structural differences. Download figure Download PowerPoint Notably, the cage carbons closest to the Sc atoms in both crystals are forced up by approximately 0.10 Å, compared to the structure without Sc atoms. To further understand the extrusion effects of endo-metals, the pyramidalization angles θp44 for the individual carbons in both cages, which share the same numbering (the complete numberings are shown in Supporting Information Figure S6), were considered and plotted in Figure 3. Generally, the carbon atoms in a fullerene cage obeying the isolated pentagon rule (IPR)45 can be roughly sorted into two types, those at the vertex of three hexagons (3HC) and those at the vertex of one pentagon and 2HC, except in the case of Ih-C60, in which all carbon atoms are equal. In the cages of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82, the pyramidalization angles of the 3HC carbon atoms fall in the ranges of 6.7–9.7° and 6.5–9.6°, respectively, whereas the 2HC carbons have larger pyramidalization ranges of 7.7–11.7° and 7.8–11.9°, respectively. However, the carbon atoms close to the Sc atoms stand out as being more pyramidalized, falling in the range of 9.0–12.8° for Sc2@C3v(8)-C82 and 9.6–13.7° for Sc2C2@C3v(8)-C82. Further analysis confirmed that only the carbon atoms closest to the Sc atoms, including C1, C68, and C69 for Sc2@C3v(8)-C82 and C1, C80, and C81 for Sc2C2@C3v(8)-C82, display pyramidalization angles larger than 12°. In addition, the two curves exhibit macroscopically good similarity with each other. However, the curves are dramatically different when the carbon atoms at the same position (same number) are adjacent to the endohedral Sc atoms for one curve but are not for another curve. The difference in the pyramidalization angle caused by the extrusion effects of the neighboring metal reaches as high as 3.4°, which is in good agreement with the previously observed 0.10 Å extrusion shown in Figure 2c. As a result, the endo-metal atoms not only transfer electrons to the carbon cage but also distort the adjacent cage carbon atoms, forming a unique coordination environment to stabilize the whole system. Figure 3 | Pyramidalization angles for the fullerene carbon atoms in Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82. The carbon atoms closest to the Sc atoms are labeled with large circles. Download figure Download PowerPoint A comparison of Sc2@C3v(8)-C82 with other dimetallofullerenes was performed to reveal the nature of metal–metal bonding among existing [email protected],47 Experimental structural evidence has been found to support the direct metal–metal bonds in pristine M2@C2n (M = Y, Er, Lu).29–32 Similar to Sc2@C3v(8)-C82, a metal–metal bond is formed between two M2+ ions, resulting in a four-electron transfer between the metallic dimer and fullerene cage. Furthermore, recently reported [email protected]82 isomers revealed that the metal–metal bond can also be formed between two different lanthanide atoms.48 As shown in Table 1, the currently observed metal–metal bonds fall in the range of 3.27–3.75 Å, which is probably associated with the metal type, cage size, and metal–cage interaction. In addition, for dimetallofullerenes earlier reported, such as La2@Ih(7)-C8051 and Ce2@Ih(7)-C80,52 each lanthanide atom is separated and donates three electrons to the cage with formal charge distribution (M2)6+@(C2n)6−. However, when the fullerene cage is added by a benzyl group or a methyl group or when one cage carbon atom is substituted by nitrogen, a single-electron metal–metal bond will be formed between the two dissociative lanthanide atoms, resulting in different formal charge distribution, including (M2)5+@[Ih(7)-C80-R]5− (R = CH3 or CH2Ph, M = Y, La, Gd, Tb, Dy, Ho, and Er),26,27,49,53,54 and (M2)5+@(C79N)5− (M = Y, La, Gd, Tb, and Dy).25,50,55 It can be seen that the distance of such a one-electron metal–metal bond is generally longer than that of direct bonding occupied by two electrons, suggesting a weaker interaction between the lanthanide dimer connected by only one electron. Except for the lanthanide–lanthanide bonds, U2@Ih(7)-C80 is found to contain a weak actinide–actinide bond inside a C80 cage, in which the U–U is within the range of 3.46–3.79 Å, shorter than the predicted distance of an elongated weak U– U bond (3.9 Å).28 Table 1 | Metal–Metal Bond Distances of the Existing M2@C2n Characterized by Crystallography EMF Metal–Metal Distance (Å) References Sc2@C3v(8)-C82 3.36 This work Y2@Cs(6)-C82 3.64 30 Y2@C3v(8)-C82 3.50 30 Er2@Cs(6)-C82 3.62 31 Er2@C3v(8)-C82 3.41 31 Er2@C1(12)-C84 3.78 31 Er2@C2v(9)-C86 3.68 31 Lu2@Td(2)-C76 3.50 32 Lu2@D3h(5)-C78 3.27 32 Lu2@C2v(5)-C80 3.60 32 Lu2@Cs(6)-C82 3.60 29 Lu2@C3v(8)-C82 3.47 29 Lu2@C2v(7)-C84 3.56 32 Lu2@D2d(23)-C84 3.75 29 Lu2@Cs(8)-C86 3.49 32 Lu2@C2v(9)-C86 3.70 29 Lu2@Cs(15)-C86 3.34 32 Lu2@C1(26)-C88 3.57 32 [email protected]C3v(8)-C82 3.57 48 La2@Ih(7)-C80(CH2Ph) 3.71 49 Dy2@Ih(7)-C80(CH2Ph) 3.90 26 Tb2@C79N 3.90 25 Dy2@C79N 3.89 50 U2@Ih(7)-C80 3.46–3.79 28 To further understand the interactions between fullerenes and solvents or the NiII(OEP) molecules in both cocrystals, the interaction energies with the basis set superposition error (BSSE) correction were calculated by density functional theory (DFT). Results show that the interaction energies of Sc2@C3v(8)-C82 (ΔE1 = −4.93 kcal/mol, ΔE2 = −4.24 kcal/mol, and ΔE3 = −23.19 kcal/mol) are approximately equal to those of Sc2C2@C3v(8)-C82 (ΔE1 = −5.30 kcal/mol, ΔE2 = −4.19 kcal/mol, and ΔE3 = −23.47 kcal/mol). Herein, ΔE1, ΔE2, and ΔE3 represent the interaction energies between the EMFs and the toluene molecules over the top site of the fullerenes, the toluene over the side site of the fullerenes, and the NiII(OEP) molecules in Figure 1, respectively. The electrostatic potential (ESP) distribution of both endohedral fullerenes, as shown in Figures 4a and 4b, shows a high coincidence except for the shift of the Sc atom caused by the C2 endo-unit, which is consistent with the adapted orientation of the toluene molecule over the top site of the fullerenes. Consequently, similar orientations of the solvent molecules and the NiII(OEP) molecules in both crystals represent similar electronic configurations on the surfaces of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 cages. Figure 4 | ESP surface, HOMOs, and LUMOs of (a, c, and e) Sc2@C3v(8)-C82 and (b, d, and f) Sc2C2@C3v(8)-C82. Download figure Download PowerPoint DFT calculations at the M06-2X/6-31G(d) level were also performed using Gaussian 16 to clarify the essence of the Sc–Sc bond in the C3v(8)-C82 fullerene cage. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 are shown in Figures 4c–4f. The LUMOs of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 are both distributed over the endo-units and the cage surface, and the HOMO of Sc2C2@C3v(8)-C82 is also distributed over the encapsulated Sc2C2 unit and the cage surface. However, the HOMO of Sc2@C3v(8)-C82 is clearly centered between the two Sc atoms, suggesting bonding of the encapsulated Sc atoms, which is in agreement with previous calculations.24,56 In addition to Sc2@C3v(8)-C82, the theoretical calculations of Y2@C3v(8)-C82, Lu2@C3v(8)-C82, Er2@C3v(8)-C82, [email protected]C3v(8)-C82, and [email protected]C3v(8)-C82 reported by Popov et al.24 and Lu2@Td-C76 reported by Zhao et al.23 all share similar HOMO isosurfaces, which appear to be a distinct characteristics of endo-metal–metal bonds in dimetallofullerenes. In addition, natural bond orbital (NBO) analysis confirmed that the Sc–Sc bond with 1.97179 e occupancy is formed by contributions from the two Sc atoms (48.57% and 51.43%, respectively), powerfully demonstrating that a metal–metal covalent bond is formed by the contribution of one electron from each Sc atom. Conclusion We investigated the nature of Sc–Sc bond formation in Sc2@C3v(8)-C82 fullerene cage with a uniquely determined Sc2 moiety. A Sc–Sc single bond with a distance of 3.36 Å in Sc2@C3v(8)-C82 was unambiguously determined by verifying with another endohedral fullerene [Sc2C2@C3v(8)-C82] with a Sc2C2 moiety. This comparison showed a similar electronic configuration on the cage surface, resulting in similar interactions not only with the planar NiII(OEP) molecule but also with the solvent molecules. In addition, by comparing the pyramidalization angles of the cage carbon atoms, the extrusion effects were shown to depend on the endo-metals and environmental adaptability of the endo-units. Together with DFT calculations, this work provides significant insights into Sc–Sc bonding in fullerene cages and provides a precise understanding of the nature of metal–metal bond formation. Supporting Information Supporting Information is available and includes the experimental section; HPLC chromatograms and mass spectra of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 ( Figures S1 and S2); Vis-NIR spectra of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 in toluene ( Figure S3); Sc sites and the mirror-rated counterparts in Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82, and their corresponding nearest cage carbons with distances and pyramidalization angles ( Figures S4 and S5); complete numberings of Sc2@C3v(8)-C82 and Sc2C2@C3v(8)-C82 ( Figure S6); and crystallographic information of Sc2@C3v(8)-C82·NiII(OEP)·2C7H8 and Sc2C2@C3v(8)-C82·NiII(OEP)·2C7H8 ( Table S1). Conflicts of Interest There is no conflict of interest to report. Funding Information Financial support for this research was provided by the National Natural Science Foundation of China (nos. 92061204, 21771152, and 21721001). References 1. Duncan Lyngdoh R. H.; Schaefer H. F.; King R. B.Metal–Metal (MM) Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc.Chem. Rev.2018, 118, 11626–11706. Google Scholar 2. Cao C.-S.; Shi Y.; Xu H.; Zhao B.Metal–Metal Bonded Compounds with Uncommon Low Oxidation State.Coord. Chem. Rev.2018, 365, 122–144. Google Scholar 3. Chem. Google Scholar Compounds with Google Scholar Google Scholar a of with a Google Scholar of a with between Google Scholar Bonds Chem. Google Scholar Y.; Bonded by Transition Google Scholar and Chem. Google Scholar in Chem. Google Scholar to Metal and of a of and Chem. Google Scholar R. Bonds and of a an to Transition Metal Chem. Google Scholar F.; and Chemistry of Google Scholar R. Bonds between Transition Chem. Google Scholar Metal–Metal Distances and Bond 1, Google Scholar R. a and of = The First Chem. Google Scholar in Chem. Google Scholar H.; of to and and in Compounds with Chem. Google Scholar a Bond between Chem. Google Scholar Xie Y.; R. Schaefer H. F.; of the Chem. Google Scholar Metal inside an and of Chem. Google Scholar Zhao a Bond in the Google Scholar H. F.; Popov the Metal within the Metal–Metal Bonds in and Google Scholar Xu H. (M = Y, and of inside Chem. Google Scholar F.; Popov with an between inside a Google Scholar H.; F.; Y.; and in a from Chem. Google Scholar Y.; Chen of a Chem. Google Scholar Y.; Zhao H.; Xie Y.; = of between inside Chem. Google Scholar H.; Xie Y.; of = and Google Scholar Xie Y.; and Theoretical of = of Metal–Metal Google Scholar Xie Y.; of = by Google Scholar Popov in as by of in Google Scholar H.; of Bonds within Google Scholar Zheng H.; H.; Zhao Y.; Zhao of of Metal the Metal–Metal in Google Scholar Bonds without Bonds to the Metal–Metal Bond A on in Google Scholar of Google Scholar H.; Y.; H.; of = of the in Google Scholar Chen H.; Y.; H.; of an Chem. Google Scholar Chen the an a Sc2C2 with in an Chem. Google Scholar Chen Sc2C2 inside a with A by C2 Chem. Google Scholar Chen of the in and of the and Chem. Google Scholar H.; Y.; An with within Chem. Google Scholar R. in Chem. Google Scholar H. of the with = and Google Scholar and Google Scholar F.; Popov Bonds inside Chem. Google Scholar Zhao H.; of with Google Scholar Chen for Metal–Metal in a Chem. Google Scholar Y.; F.; Shi A of with Google Scholar Y.; Y.; Y.; and of the Chem. Google Scholar Y.; of a by Chem. Google Scholar of and Chem. 118, Google Scholar F.; Chen F.; Popov with by a Metal–Metal Google Scholar H.; H. and of a with a State of = Chem. Google Scholar H.; Y.; of C2 on the of Google Scholar Information Chemical single-crystal

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Atomically Precise Insights into Metal–Metal Bonds Using Comparable Endo-Units of Sc ₂ and Sc ₂ C ₂

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Yang‐Rong Yao, Xiang‐Mei Shi, Shan-Yu Zheng, Zuo‐Chang Chen, Su‐Yuan Xie, Rong‐Bin Huang, Lan‐Sun Zheng

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