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Boron-lead multiple bonds in the PbB2O– and PbB3O2– clusters

Boron-lead multiple bonds in the PbB2O– and PbB3O2– clusters ARTICLE https://doi.org/10.1038/s42004-022-00643-1 OPEN Boron-lead multiple bonds in the PbB O and PbB O clusters 3 2 1,3 1,3 2,3 2 2 2 Wei-Jia Chen , Teng-Teng Chen , Qiang Chen , Hai-Gang Lu , Xiao-Yun Zhao , Yuan-Yuan Ma , ✉ ✉ 2 2 2 1 Qiao-Qiao Yan , Rui-Nan Yuan , Si-Dian Li & Lai-Sheng Wang Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively – – rare. Here we report the observation of boron-lead multiple bonds in PbB O and PbB O , 2 3 2 which are produced and characterized in a cluster beam. PbB O is found to have an open- shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell 2– – [Pb≡B–B≡O] contains a B≡Pb triple bond. PbB O is shown to have a Y-shaped structure 3 2 with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between 2– – – + [Pb≡B–B≡O] /[Pb=B(B≡O) ] and the isoelectronic [Pb≡B–C≡O] /[Pb=B(C≡O) ] car- 2 2 bonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements. 1 2 Department of Chemistry, Brown University, Providence, RI 02912, USA. Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, 3 ✉ 030006 Taiyuan, China. These authors contributed equally: Wei-Jia Chen, Teng-Teng Chen, Qiang Chen. email: chenqiang@sxu.edu.cn; lisidian@sxu.edu.cn; lai-sheng_wang@brown.edu COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 1 1234567890():,; ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ue to its electron deficiency, boron tends to form multi- 1–5 center bonds in both its compounds and at nanoscales . DBoron is also capable of forming multiple chemical bonds with transition metals, such as in borylene (:BR) compounds, which usually involve a transition metal (M) with different ligands (L ), L MBR . The bonding between the metal and bor- n n ylene fragment is interpreted as B→M σ-donation and M(dπ)→B 7–10 back-donation . The similarities between borylenes and car- benes (:CR ) suggest the bonding between boron and metal should be a double bond. However, the B–M bond lengths vary in a wide range, depending on the ligands and the R group . Since 12,13 the syntheses of the first transition-metal borylene complexes , 14–24 considerable progresses have been achieved in this area . Boron-metal triple-bond characters were first suggested in [(OC) CrBSiH ] with the B–Cr bond length of 1.871 Å , slightly 5 3 shorter than the B=Cr double-bond length of 1.89 Å derived from Pyykkö’s covalent radii . Several transition-metal complexes with B–M bond lengths shorter than B=M double bonds have been characterized in both solid compounds and gaseous 15–19 molecules , among which some have B–M bond lengths comparable to those computed from Pyykkö’s triple-bond cova- lent radii. The first electron-precise transition-metal-boron triple- bond complex was identified recently by combined photoelectron spectroscopy (PES) and quantum chemistry calculations, in the – 27 linear ReB O species with a B≡Re triple bond . In fact, com- plexes with transition-metal-boron bond lengths shorter than 28,29 30,31 B≡M triple bonds and even B≣M quadruple bonds have been characterized by joint gas-phase experimental and ab initio theoretical studies. Compared with the transition-metal-boron multiple-bond complexes, compounds with multiple bonding between boron and main-group metal elements are rare , even though multiple bonds of boron with light main-group elements are common. This is understandable because main-group metal elements have valence ns and np orbitals with large differences in orbital radii, decreasing the hybridization of these orbitals and making it dif- ficult for heavy elements to form strong multiple bonds. The first molecules observed to contain main-group-metal-boron multiple – – 32 bonds are the linear Bi B and BiB O species , featuring two 2 2 – – B=Bi double bonds in Bi B and a B≡Bi triple bond in BiB O . 2 2 Besides these, boron has only been found to form double bonds 24,33–35 with heavy main-group elements . Here we report the observation of B–Pb multiple bonds in – – Fig. 1 Photoelectron spectra of PbB O .a At 355 nm. b At 266 nm. c At two molecular anions, PbB O and PbB O ,byajointPES and 2 3 2 193 nm. The vertical lines represent resolved vibrational progressions. theoretical study. Well-resolved photoelectron spectra were obtained for these two species in the gas phase and used to yielded the first vertical detachment energy (VDE) of 2.26 eV and an elucidate their structures and bonding. Theoretical calculations adiabatic detachment energy (ADE) of 2.19 eV evaluated from its and chemical bonding analyses showed that PbB O has an onset, which also represents the electron affinity (EA) of neutral open-shell [Pb☱B–B≡O] linear structure with a B–Pb bond 2– PbB O. Band A was observed at 2.40 eV with a short vibrational order of 2.5, whereas the closed-shell [Pb≡B–B≡O] contains a 2 −1 progression with the frequency of 890 cm . Band B consisting of a B≡Pb triple bond. The PbB O species has a Y-shaped struc- 3 2 single peak was observed at 2.71 eV. Two weak features labeled as C ture, [Pb=B(B≡O) ] , which consists of a B = Pb double bond were resolved near the detachment threshold at 355 nm and they coordinated by two boronyl ligands. Comparisons of the 2– – turned out to be part of a broad vibrational progression fully bonding in [Pb≡B–B≡O] and [Pb=B(B≡O) ] with that in – + observed at 266 nm (Fig. 1b). The ADE and VDE for band C were [Pb≡B−C≡O] and [Pb=B(C≡O) ] also provide evidence for measured to be 3.20 eV and 3.34 eV, respectively, and the vibrational transition-metal-like properties for the central B atom. −1 progression yielded a frequency of 970 cm . The 266 nm spectrum also displayed a sharp peak D at 4.04 eV, closely followed by two weak peaks E (at 4.13 eV) and F (at 4.18 eV). At 193 nm (Fig. 1c), a Results and discussion – – new peak F was observed at 4.95 eV, beyond which the signal-to- The PES of PbB O . The photoelectron spectra of PbB O at three 2 2 noise ratios were poor and no additional PES bands could be defi- different photon energies are shown in Fig. 1. The spectrum at nitively identified. The VDEs of all the observed PES bands are given 355 nm revealed four detachment bands labeled as X, A, B, and C in Table 1, where they are compared with theoretical results. (Fig. 1a). The lowest binding energy band X corresponds to the detachment transition from the ground state of PbB O to that of – – neutral PbB O, whereas the higher binding energy bands represent The PES of PbB O . The PES spectra of PbB O (Fig. 2) dis- 2 3 2 3 2 detachment transitions to excited states of neutral PbB O. Band X played a much simpler pattern compared to that of PbB O . The 2 2 2 COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ARTICLE Table 1 The experimental data of PbB O and their assignments. VDE (expt.) Configurations Terms VDE (MRCI) Levels VDE (SO) Composition of SO coupled states 2 2 2 2 4 2 23 – a3 – a 3 – 1 + 3 X 2.26 1σ 2σ 3σ 4σ 1π 5σ 2π Σ 2.22 Σ 2.22 86.9% Σ + 11.8% Σ + 1.3% Π 3 – 3 – b 3 – 3 1 A 2.41 Σ , Σ 2.34 96.9% Σ + 2.2% Π + 0.9% Π 1 –1 1 c1 b 1 3 B 2.71 Δ 2.60 Δ 2.73 92.3% Δ + 7.7% Π 1 + c1 + b 1 + 3 – 3 C 3.34 Σ 2.90 Σ 3.17 82.7% Σ + 8.9% Σ + 8.4% Π 2 2 2 2 4 1 33 c3 b 3 1 D 4.04 1σ 2σ 3σ 4σ 1π 5σ 2π Π 3.82 Π 4.01 92.3% Π + 7.7% Δ 3 b 3 1 3 – E 4.13 Π 4.13 94.6% Π + 2.8% Π + 2.7% Σ 3 b 3 F 4.18 Π 4.33 100% Π 2 2 2 2 4 1 31 c1 b 1 3 3 – G 4.95 1σ 2σ 3σ 4σ 1π 5σ 2π Π 4.61 Π 5.07 96.3% Π + 3.2% Π + 0.5% Σ The observed features and their vertical detachment energies (VDEs) from the photoelectron spectra of PbB2O in comparison with theoretical values. All energies are given in eV. The first VDE was calculated at the CCSD(T) level. The higher VDEs were calculated with the SO coupling effect considered. The higher VDEs were calculated using the MRCI approach. (r = 2.122 Å) with a BO ligand coordinated to the central B B–Pb atom (Fig. 3a), similar to the previously reported linear BiB O – 27,32 41 and ReB O systems . The spin-orbit (SO) coupling effect was evaluated for PbB O using multi-reference configuration 42 2 interaction (MRCI) calculations . The SO coupling splits the Π 2 2 2 2 4 2 3 state (electron configuration: 1σ 2σ 3σ 4σ 1π 5σ 2π ) into two 2 2 2 sub-levels Π and Π , with Π being lower in energy by 3/2 1/2 3/2 0.27 eV. The strong SO coupling quenches the Renner–Teller effect in the linear monoanion. Adding one electron to PbB O 2– 1 + results in the closed-shell PbB O (C , Σ ), which has an even 2 ∞v shorter B–Pb bond length (r = 2.107 Å) (Fig. 3b). Removing B–Pb an electron from PbB O leads to the triplet ground state of 3 – neutral PbB O(C , Σ ), as shown Fig. 3c. 2 ∞v PbB O was found to have a closed-shell Y-shaped GM (C , 3 2 2v A ) featuring a terminal Pb with two BO units coordinated to the central B atom (Fig. 3d). It is 0.53 eV more stable than the second lowest-lying triplet isomer (C , A ) at the PBE0/AVTZ level 2v 2 (Supplementary Fig. 3a). The GM of neutral PbB O also 3 2 possesses a similar Y-shaped structure (C , B ) with the second 2v 1 isomer lying 0.61 eV higher in energy at the PBE0 level/AVTZ (Fig. 3e). The valence molecular orbitals (MOs) of PbB O (C , 2 ∞v 2 – 1 Π) and PbB O (C , A ) are shown in Supplementary Fig. 4. 3 2 2v 1 Comparison between the experimental and computational results. Detaching one electron from the 2π SOMO of PbB O Fig. 2 Photoelectron spectra of PbB O .a At 355 nm. b At 266 nm. The 3 2 3 – (Supplementary Fig. 4a) results in three final states in PbB O, Σ , vertical lines represent resolved vibrational progressions. 1 1 + 3 – Δ, and Σ , among which the triplet Σ state with a config- 2 2 2 2 4 2 2 uration of 1σ 2σ 3σ 4σ 1π 5σ 2π is the ground state. The 355 nm spectrum revealed one PES band X with partially resolved computed VDE/ADE of 2.22/2.19 eV at CCSD(T)/AVTZ are in vibrational structures (Fig. 2a). Band X gave rise to an ADE of excellent agreement with the experimental values at 2.26/2.19 eV. 2.96 eV and a VDE of 3.00 eV. Two vibrational progressions were To interpret the higher energy VDEs, we computed the term −1 discernible for band X with a high-frequency mode of 2060 cm values of the neutral excited states relative to the triplet ground −1 and a low-frequency mode of ~350 cm . Following a large state using the SA-CASSCF method with the MRCI interaction energy gap, a slightly broader band A at 4.07 eV was observed at and SO coupling effects considered simultaneously. In con- 266 nm (Fig. 2b). No higher binding energy detachment transi- 3 – sequence, the triplet Σ ground state splits into three closely tions were observed in the 193 nm spectrum (Supplementary 3 – 3 – 3 – spaced states Σ , Σ and Σ with the calculated VDEs of 0 –1 1 Fig. 1). The observed spectroscopic data for PbB O are given in 3 2 2.22, 2.34, and 2.34 eV, respectively, in excellent agreement with Table 2, where they are compared with the corresponding theo- the observed X and A bands (Table 1). The weak vibrational peak retical results. –1 with a spacing of 890 cm observed for band A is due to the B–B –1 stretching mode (ν ) with a computed frequency of 1019 cm (Supplementary Fig. 5a). Franck-Condon simulations suggest that Theoretical results. Figure 3 depicts the global minimum (GM) – 2– – there is unresolved low-frequency Pb–B stretching vibration in structures of PbB O , PbB O , PbB O, PbB O , and PbB O at 2 2 2 3 2 3 2 36–39 band X (Supplementary Fig. 6). The next two singlet final states the CCSD/AVTZ level of theory , with alternative low-lying 1 1 + Δ and Σ with calculated VDEs of 2.73 and 3.17 eV agree well isomers within 2.5 eV at the PBE0/AVTZ level collectively 2 0 shown in Supplementary Figs. 2, 3. The GM of PbB O possesses with bands B and C at 2.71 eV and 3.34 eV, respectively. The 2 −1 a highly stable linear structure (C , Π), which lies 0.88 eV lower observed frequency of 970 cm for the vibrational progression ∞v – 4 in energy than the second lowest-lying isomer PbB O (C , A”) resolved for band C is likely due to the B–B stretching mode (ν ), 2 s 4 3 – at the PBE0/AVTZ level. It consists of a short B–Pb bond which should have a similar frequency as the Σ ground state COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 3 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 Table 2 The experimental data of PbB O and their assignments. 3 2 a b Feature VDE/ADE (expt.) Configurations Terms VDE/ADE (theo.) TD-PBE0 CCSD(T) 2 2 2 2 212 c d X 3.00/2.96 ……1b 5a 1a 4b 6a 2b B 2.93/2.85 3.06/3.01 1 1 2 2 1 1 1 2 2 2 2 122 c d A 4.07 ……1b 5a 1a 4b 6a 2b A 3.84 3.97 1 1 2 2 1 1 1 The observed features and their vertical detachment energies (VDEs), as well as the first adiabatic detachment energy (ADE), from the photoelectron spectra of PbB O in comparison with theoretical 3 2 values. All energies are given in eV. The experimentally observed VDEs and ADE (shown in italic). The theoretically predicted VDEs and ADE. The VDEs and ADE calculated using the TD-PBE0 method. The first VDE and ADE were calculated using CCSD(T), while the second VDE was calculated using the TD-PBE0 method. – 2– – Fig. 3 Global minimum structures of PbB O and PbB O at different charge states. a PbB O . b PbB O . c PbB O. d PbB O . e PbB O . Bond lengths 2 3 2 2 2 2 3 2 3 2 and bond angles are given in Å and degrees, respectively, at the CCSD/AVTZ level of theory. Cartesian coordinates of these structures are given in Supplementary Table 2. (Supplementary Fig. 5). Detachment of one β-electron from the broad band A, which should contain an unresolved B–Pb 5σ orbital (HOMO-1, Supplementary Fig. 4a) results in the Π stretching vibrational progression. There is a large energy gap 3 3 neutral state, which gives rise to three SO states, Π , Π , and between HOMO-2 and HOMO-1 (3.04 eV at PBE0/AVTZ level), 2 1 agreeing well with the 193 nm spectrum which does not reveal Π , with calculated VDEs of 4.01, 4.13, and 4.33 eV, respectively, any new spectral features between 4.5 and 6.4 eV (Supplementary in agreement with the observed bands D at 4.04 eV, E at 4.13 eV, Fig. 1). The excellent agreement between the theoretical and and F at 4.18 eV (Table 1). Removing one α-electron from the 5σ experimental results confirms firmly the Y-shaped GM for orbital gives rise to the Π final state with a calculated VDE of PbB O . 5.07 eV, which accounts for band G at 4.95 eV. Overall, the the- 3 2 oretical results are in excellent agreement with the experiment, –/2– – confirming the linear GM for PbB O . Multiple B–Pb bonding in PbB O and PbB O . Adaptive 2 3 2 For PbB O , electron detachment from the 2b HOMO 3 2 1 natural density partitioning (AdNDP) analyses were performed – 2– – (Supplementary Fig. 4b) yields the B ground state of neutral on PbB O ,PbB O ,and PbB O to understand the nature of 2 2 3 2 PbB O . The calculated VDE/ADE of 3.06/3.01 eV at the 3 2 their chemical bonding. AdNDP basically transforms the MOs into CCSD(T)/AVTZ level are in good accord with the experimental more familiar bonding elements, such as lone pairs, two-center two- VDE/ADE of 3.00/2.96 eV. The low-frequency vibrational electron (2c-2e) bonds or multicenter delocalized bonds. As shown –1 progression (350 cm ) corresponds to the B–Pb stretching mode in Fig. 4a, the linear PbB O possesses one Pb 6 s and one O lone –1 with a calculated frequency of 318 cm (a mode) for neutral 1 pairs and a B≡O triple bond in the first row. The second row 2 −1 PbB O (C , B ), while the high-frequency mode (2060 cm ) 3 2 2v 1 depicts one 2c-2e B–B σ bond, one 2c-2e B–Pb σ bond, one 2c-2e originates from the B–O stretching mode with a calculated B–Pb π bond, and one 2c-1e B–Pb π bond, giving rise to a B–Pb –1 frequency of 2026 cm (a mode) (Supplementary Fig. 5b), as 1 bond order of 2.5. Thus, there is no sp hybridization in Pb, which confirmed by the Franck-Condon simulations (Supplementary uses its three valence 6p orbitals to form a triple bond with the Fig. 7 and Supplementary Table 1). It should be noted that the central B atom. Both B atoms undergo sp hybridization and form –1 observed frequency of 2060 cm in PbB O is similar to the triple bonds. The B–Pb bond length of 2.122 Å lies between the 3 2 previously reported B≡O symmetric stretching frequencies of B=Pb double-bond length (2.13 Å) and the B≡Pb triple-bond –1 1950, 2040, 1980, and 1935 cm in B O ,B O ,B O , and the length (2.10 Å) predicted from Pyykkö’scovalentatomicradii , 3 2 4 2 4 3 43–45 bare BO, respectively . The observed active vibrational modes consistent with the 2.5 bond order for the B–Pb multiple bond. are consistent with the geometry changes from the ground state of Hence, the linear PbB O species can be described as – – the anion to that of the neutral (Fig. 3) and the nature of the 2b 1 [Pb☱B–B≡O] . Adding one electron to PbB O yields the closed- 2– HOMO that mainly involves B–Pb π bonding and weak B–O shell and electron-precise PbB O which contains an ideal Pb≡B antibonding interactions (Supplementary Fig. 4b). The second triple bond including one 2c-2e Pb–B σ bond and two degenerate 2– VDE derived from electron detachment from the 6a HOMO-1 1 2c-2e Pb–B π bonds, i.e.[Pb≡B–B≡O] , as detailed in the second 2– was calculated to be 3.84 and 3.97 eV at the PBE0 and CCSD(T) row of Fig. 4b. The B–Pb bond length in PbB O is reduced to levels, respectively, consistent with band A at 4.07 eV. The 6a 1 2.107 Å, in good accord with the B≡Pb triple-bond length (2.10 Å) HOMO-1 is a strong B–Pb σ bonding orbital, consistent with the derived from Pyykkö’s triple-bond covalent atomic radii . 4 COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ARTICLE – 2– – Fig. 4 Chemical bonding anaylses using AdNDP. a PbB O . b PbB O . c PbB O . The occupation numbers (ONs) are indicated. The corresponding Lewis 2 2 3 2 electronic structures are also depicted. – 2 The central B atom in PbB O (Fig. 3d) undergoes sp Recently, boron has been shown to exhibit “metallomimetic” 3 2 hybridization and the AdNDP bonding analyses of the electron- properties , such as the formation of stable borylene dicarbonyl precise monoanion are straightforward (Fig. 4c). It contains one complex, in which two CO ligands are coordinated to a monovalent Pb 6 s lone pair, two O lone pairs, and two B≡O boronyl ligands boron via donor-acceptor bonds , a prototypical transition-metal in the first row. The second row displays two 2c-2e B–B σ bonds behavior. Another unexpected metallomimetic property of boron is between the central B and two BO ligands, one 2c-2e B–Pb σ its capability to form a half-sandwich complex with the aromatic 3− 7 − 51 bond, and one 2c-2e B–Pb π bond, giving rise to a terminal Pb=B B ligand in the recently observed [(η -B )-B-BO] species . 7 7 double bond. The B–Pb bond length of 2.152 Å is comparable to Since BO is isoelectronic with CO, we optimized the geometric the B=Pb double-bond length of 2.13 Å from Pyykkö’s covalent and electronic structures of the linear [Pb≡B−C≡O] and Y-shaped 26 – – + radii . PbB O can thus be formulated as [Pb=B(B≡O) ] . [Pb=B(C≡O) ] and found that these carbonyl complexes exhibit 3 2 2 2 47,48 Natural resonance theory (NRT) analyses at the PBE0/AVTZ similar AdNDP bonding patterns as their boronyl counterparts – 2– – level give rise to a B–Pb bond order of 2.39 for PbB O , 2.84 for [Pb≡B–B≡O] and [Pb=B(B≡O) ] , respectively (see Fig. 4 and 2 2 2– – PbB O , and 1.84 for PbB O , consistent with the bond order Supplementary Fig. 11), with the two 2c-2e B–Pb π bonds in 2 3 2 2– designations of 2.5, 3, and 2 for these species on the basis of [Pb≡B–B≡O] (Fig. 4b) changed to two 3c-2e Pb-B-C π bonds in electron counts, respectively. The covalency of the Pb–B multiple [Pb≡B−C≡O] (Supplementary Fig. 11a) and the 2c-2e B−Pb π bonds can be understood from the similar electronegativities of bond in [Pb=B(B≡O) ] (Fig. 4c) extended to a 4c-2e Pb-B-C2 π – 2– – + Pb (2.33) and B (2.04). The PbB O , PbB O and PbB O bond in [Pb=B(C≡O) ] (Supplementary Fig. 11b). These carbonyl 2 2 3 2 2 species are the first experimentally confirmed molecules with complexes may be more viable for chemical syntheses if a suitable –/2– B–Pb multiple bonds. We have also calculated MB O and ligand can be found to coordinate to Pb. MB O for M=Ge and Sn and found that both the Ge and Sn In conclusion, we have characterized the first B–Pb multiple 3 2 –/2– – species have similar GM structures and bonding patterns as their bonds in the PbB O and PbB O molecular anions, using 2 3 2 Pb counterparts (see Supplementary Figs. 8–10). photoelectron spectroscopy and ab initio calculations. Excellent agreement between the theoretical and experimental data confirms that the global minimum of PbB O has an open-shell Transition-metal-like behaviors of the central boron in [Pb≡B linear structure with a B–Pb bond order of 2.5, whereas the − + 2– – −CO] and [Pb=B(CO) ] . The electron deficiency of boron has closed-shell PbB O contains a B≡Pb triple bond. The PbB O 2 2 3 2 led to novel structures and bonding in various boron compounds. species has been shown to have a Y-shaped structure with a COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 5 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 terminal B=Pb double bond and two BO ligands coordinated to References 1. Lipscomb, W. N. The boranes and their relatives. Science 196, 1047–1055 the central B atom. Theoretical calculations indicate that –/2– – (1977). MB O and MB O with M=Ge and Sn display similar 2 3 2 2. Alexandrova, A. 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Boron-lead multiple bonds in the PbB2O– and PbB3O2– clusters

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ARTICLE https://doi.org/10.1038/s42004-022-00643-1 OPEN Boron-lead multiple bonds in the PbB O and PbB O clusters 3 2 1,3 1,3 2,3 2 2 2 Wei-Jia Chen , Teng-Teng Chen , Qiang Chen , Hai-Gang Lu , Xiao-Yun Zhao , Yuan-Yuan Ma , ✉ ✉ 2 2 2 1 Qiao-Qiao Yan , Rui-Nan Yuan , Si-Dian Li & Lai-Sheng Wang Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively – – rare. Here we report the observation of boron-lead multiple bonds in PbB O and PbB O , 2 3 2 which are produced and characterized in a cluster beam. PbB O is found to have an open- shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell 2– – [Pb≡B–B≡O] contains a B≡Pb triple bond. PbB O is shown to have a Y-shaped structure 3 2 with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between 2– – – + [Pb≡B–B≡O] /[Pb=B(B≡O) ] and the isoelectronic [Pb≡B–C≡O] /[Pb=B(C≡O) ] car- 2 2 bonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements. 1 2 Department of Chemistry, Brown University, Providence, RI 02912, USA. Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, 3 ✉ 030006 Taiyuan, China. These authors contributed equally: Wei-Jia Chen, Teng-Teng Chen, Qiang Chen. email: chenqiang@sxu.edu.cn; lisidian@sxu.edu.cn; lai-sheng_wang@brown.edu COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 1 1234567890():,; ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ue to its electron deficiency, boron tends to form multi- 1–5 center bonds in both its compounds and at nanoscales . DBoron is also capable of forming multiple chemical bonds with transition metals, such as in borylene (:BR) compounds, which usually involve a transition metal (M) with different ligands (L ), L MBR . The bonding between the metal and bor- n n ylene fragment is interpreted as B→M σ-donation and M(dπ)→B 7–10 back-donation . The similarities between borylenes and car- benes (:CR ) suggest the bonding between boron and metal should be a double bond. However, the B–M bond lengths vary in a wide range, depending on the ligands and the R group . Since 12,13 the syntheses of the first transition-metal borylene complexes , 14–24 considerable progresses have been achieved in this area . Boron-metal triple-bond characters were first suggested in [(OC) CrBSiH ] with the B–Cr bond length of 1.871 Å , slightly 5 3 shorter than the B=Cr double-bond length of 1.89 Å derived from Pyykkö’s covalent radii . Several transition-metal complexes with B–M bond lengths shorter than B=M double bonds have been characterized in both solid compounds and gaseous 15–19 molecules , among which some have B–M bond lengths comparable to those computed from Pyykkö’s triple-bond cova- lent radii. The first electron-precise transition-metal-boron triple- bond complex was identified recently by combined photoelectron spectroscopy (PES) and quantum chemistry calculations, in the – 27 linear ReB O species with a B≡Re triple bond . In fact, com- plexes with transition-metal-boron bond lengths shorter than 28,29 30,31 B≡M triple bonds and even B≣M quadruple bonds have been characterized by joint gas-phase experimental and ab initio theoretical studies. Compared with the transition-metal-boron multiple-bond complexes, compounds with multiple bonding between boron and main-group metal elements are rare , even though multiple bonds of boron with light main-group elements are common. This is understandable because main-group metal elements have valence ns and np orbitals with large differences in orbital radii, decreasing the hybridization of these orbitals and making it dif- ficult for heavy elements to form strong multiple bonds. The first molecules observed to contain main-group-metal-boron multiple – – 32 bonds are the linear Bi B and BiB O species , featuring two 2 2 – – B=Bi double bonds in Bi B and a B≡Bi triple bond in BiB O . 2 2 Besides these, boron has only been found to form double bonds 24,33–35 with heavy main-group elements . Here we report the observation of B–Pb multiple bonds in – – Fig. 1 Photoelectron spectra of PbB O .a At 355 nm. b At 266 nm. c At two molecular anions, PbB O and PbB O ,byajointPES and 2 3 2 193 nm. The vertical lines represent resolved vibrational progressions. theoretical study. Well-resolved photoelectron spectra were obtained for these two species in the gas phase and used to yielded the first vertical detachment energy (VDE) of 2.26 eV and an elucidate their structures and bonding. Theoretical calculations adiabatic detachment energy (ADE) of 2.19 eV evaluated from its and chemical bonding analyses showed that PbB O has an onset, which also represents the electron affinity (EA) of neutral open-shell [Pb☱B–B≡O] linear structure with a B–Pb bond 2– PbB O. Band A was observed at 2.40 eV with a short vibrational order of 2.5, whereas the closed-shell [Pb≡B–B≡O] contains a 2 −1 progression with the frequency of 890 cm . Band B consisting of a B≡Pb triple bond. The PbB O species has a Y-shaped struc- 3 2 single peak was observed at 2.71 eV. Two weak features labeled as C ture, [Pb=B(B≡O) ] , which consists of a B = Pb double bond were resolved near the detachment threshold at 355 nm and they coordinated by two boronyl ligands. Comparisons of the 2– – turned out to be part of a broad vibrational progression fully bonding in [Pb≡B–B≡O] and [Pb=B(B≡O) ] with that in – + observed at 266 nm (Fig. 1b). The ADE and VDE for band C were [Pb≡B−C≡O] and [Pb=B(C≡O) ] also provide evidence for measured to be 3.20 eV and 3.34 eV, respectively, and the vibrational transition-metal-like properties for the central B atom. −1 progression yielded a frequency of 970 cm . The 266 nm spectrum also displayed a sharp peak D at 4.04 eV, closely followed by two weak peaks E (at 4.13 eV) and F (at 4.18 eV). At 193 nm (Fig. 1c), a Results and discussion – – new peak F was observed at 4.95 eV, beyond which the signal-to- The PES of PbB O . The photoelectron spectra of PbB O at three 2 2 noise ratios were poor and no additional PES bands could be defi- different photon energies are shown in Fig. 1. The spectrum at nitively identified. The VDEs of all the observed PES bands are given 355 nm revealed four detachment bands labeled as X, A, B, and C in Table 1, where they are compared with theoretical results. (Fig. 1a). The lowest binding energy band X corresponds to the detachment transition from the ground state of PbB O to that of – – neutral PbB O, whereas the higher binding energy bands represent The PES of PbB O . The PES spectra of PbB O (Fig. 2) dis- 2 3 2 3 2 detachment transitions to excited states of neutral PbB O. Band X played a much simpler pattern compared to that of PbB O . The 2 2 2 COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ARTICLE Table 1 The experimental data of PbB O and their assignments. VDE (expt.) Configurations Terms VDE (MRCI) Levels VDE (SO) Composition of SO coupled states 2 2 2 2 4 2 23 – a3 – a 3 – 1 + 3 X 2.26 1σ 2σ 3σ 4σ 1π 5σ 2π Σ 2.22 Σ 2.22 86.9% Σ + 11.8% Σ + 1.3% Π 3 – 3 – b 3 – 3 1 A 2.41 Σ , Σ 2.34 96.9% Σ + 2.2% Π + 0.9% Π 1 –1 1 c1 b 1 3 B 2.71 Δ 2.60 Δ 2.73 92.3% Δ + 7.7% Π 1 + c1 + b 1 + 3 – 3 C 3.34 Σ 2.90 Σ 3.17 82.7% Σ + 8.9% Σ + 8.4% Π 2 2 2 2 4 1 33 c3 b 3 1 D 4.04 1σ 2σ 3σ 4σ 1π 5σ 2π Π 3.82 Π 4.01 92.3% Π + 7.7% Δ 3 b 3 1 3 – E 4.13 Π 4.13 94.6% Π + 2.8% Π + 2.7% Σ 3 b 3 F 4.18 Π 4.33 100% Π 2 2 2 2 4 1 31 c1 b 1 3 3 – G 4.95 1σ 2σ 3σ 4σ 1π 5σ 2π Π 4.61 Π 5.07 96.3% Π + 3.2% Π + 0.5% Σ The observed features and their vertical detachment energies (VDEs) from the photoelectron spectra of PbB2O in comparison with theoretical values. All energies are given in eV. The first VDE was calculated at the CCSD(T) level. The higher VDEs were calculated with the SO coupling effect considered. The higher VDEs were calculated using the MRCI approach. (r = 2.122 Å) with a BO ligand coordinated to the central B B–Pb atom (Fig. 3a), similar to the previously reported linear BiB O – 27,32 41 and ReB O systems . The spin-orbit (SO) coupling effect was evaluated for PbB O using multi-reference configuration 42 2 interaction (MRCI) calculations . The SO coupling splits the Π 2 2 2 2 4 2 3 state (electron configuration: 1σ 2σ 3σ 4σ 1π 5σ 2π ) into two 2 2 2 sub-levels Π and Π , with Π being lower in energy by 3/2 1/2 3/2 0.27 eV. The strong SO coupling quenches the Renner–Teller effect in the linear monoanion. Adding one electron to PbB O 2– 1 + results in the closed-shell PbB O (C , Σ ), which has an even 2 ∞v shorter B–Pb bond length (r = 2.107 Å) (Fig. 3b). Removing B–Pb an electron from PbB O leads to the triplet ground state of 3 – neutral PbB O(C , Σ ), as shown Fig. 3c. 2 ∞v PbB O was found to have a closed-shell Y-shaped GM (C , 3 2 2v A ) featuring a terminal Pb with two BO units coordinated to the central B atom (Fig. 3d). It is 0.53 eV more stable than the second lowest-lying triplet isomer (C , A ) at the PBE0/AVTZ level 2v 2 (Supplementary Fig. 3a). The GM of neutral PbB O also 3 2 possesses a similar Y-shaped structure (C , B ) with the second 2v 1 isomer lying 0.61 eV higher in energy at the PBE0 level/AVTZ (Fig. 3e). The valence molecular orbitals (MOs) of PbB O (C , 2 ∞v 2 – 1 Π) and PbB O (C , A ) are shown in Supplementary Fig. 4. 3 2 2v 1 Comparison between the experimental and computational results. Detaching one electron from the 2π SOMO of PbB O Fig. 2 Photoelectron spectra of PbB O .a At 355 nm. b At 266 nm. The 3 2 3 – (Supplementary Fig. 4a) results in three final states in PbB O, Σ , vertical lines represent resolved vibrational progressions. 1 1 + 3 – Δ, and Σ , among which the triplet Σ state with a config- 2 2 2 2 4 2 2 uration of 1σ 2σ 3σ 4σ 1π 5σ 2π is the ground state. The 355 nm spectrum revealed one PES band X with partially resolved computed VDE/ADE of 2.22/2.19 eV at CCSD(T)/AVTZ are in vibrational structures (Fig. 2a). Band X gave rise to an ADE of excellent agreement with the experimental values at 2.26/2.19 eV. 2.96 eV and a VDE of 3.00 eV. Two vibrational progressions were To interpret the higher energy VDEs, we computed the term −1 discernible for band X with a high-frequency mode of 2060 cm values of the neutral excited states relative to the triplet ground −1 and a low-frequency mode of ~350 cm . Following a large state using the SA-CASSCF method with the MRCI interaction energy gap, a slightly broader band A at 4.07 eV was observed at and SO coupling effects considered simultaneously. In con- 266 nm (Fig. 2b). No higher binding energy detachment transi- 3 – sequence, the triplet Σ ground state splits into three closely tions were observed in the 193 nm spectrum (Supplementary 3 – 3 – 3 – spaced states Σ , Σ and Σ with the calculated VDEs of 0 –1 1 Fig. 1). The observed spectroscopic data for PbB O are given in 3 2 2.22, 2.34, and 2.34 eV, respectively, in excellent agreement with Table 2, where they are compared with the corresponding theo- the observed X and A bands (Table 1). The weak vibrational peak retical results. –1 with a spacing of 890 cm observed for band A is due to the B–B –1 stretching mode (ν ) with a computed frequency of 1019 cm (Supplementary Fig. 5a). Franck-Condon simulations suggest that Theoretical results. Figure 3 depicts the global minimum (GM) – 2– – there is unresolved low-frequency Pb–B stretching vibration in structures of PbB O , PbB O , PbB O, PbB O , and PbB O at 2 2 2 3 2 3 2 36–39 band X (Supplementary Fig. 6). The next two singlet final states the CCSD/AVTZ level of theory , with alternative low-lying 1 1 + Δ and Σ with calculated VDEs of 2.73 and 3.17 eV agree well isomers within 2.5 eV at the PBE0/AVTZ level collectively 2 0 shown in Supplementary Figs. 2, 3. The GM of PbB O possesses with bands B and C at 2.71 eV and 3.34 eV, respectively. The 2 −1 a highly stable linear structure (C , Π), which lies 0.88 eV lower observed frequency of 970 cm for the vibrational progression ∞v – 4 in energy than the second lowest-lying isomer PbB O (C , A”) resolved for band C is likely due to the B–B stretching mode (ν ), 2 s 4 3 – at the PBE0/AVTZ level. It consists of a short B–Pb bond which should have a similar frequency as the Σ ground state COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 3 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 Table 2 The experimental data of PbB O and their assignments. 3 2 a b Feature VDE/ADE (expt.) Configurations Terms VDE/ADE (theo.) TD-PBE0 CCSD(T) 2 2 2 2 212 c d X 3.00/2.96 ……1b 5a 1a 4b 6a 2b B 2.93/2.85 3.06/3.01 1 1 2 2 1 1 1 2 2 2 2 122 c d A 4.07 ……1b 5a 1a 4b 6a 2b A 3.84 3.97 1 1 2 2 1 1 1 The observed features and their vertical detachment energies (VDEs), as well as the first adiabatic detachment energy (ADE), from the photoelectron spectra of PbB O in comparison with theoretical 3 2 values. All energies are given in eV. The experimentally observed VDEs and ADE (shown in italic). The theoretically predicted VDEs and ADE. The VDEs and ADE calculated using the TD-PBE0 method. The first VDE and ADE were calculated using CCSD(T), while the second VDE was calculated using the TD-PBE0 method. – 2– – Fig. 3 Global minimum structures of PbB O and PbB O at different charge states. a PbB O . b PbB O . c PbB O. d PbB O . e PbB O . Bond lengths 2 3 2 2 2 2 3 2 3 2 and bond angles are given in Å and degrees, respectively, at the CCSD/AVTZ level of theory. Cartesian coordinates of these structures are given in Supplementary Table 2. (Supplementary Fig. 5). Detachment of one β-electron from the broad band A, which should contain an unresolved B–Pb 5σ orbital (HOMO-1, Supplementary Fig. 4a) results in the Π stretching vibrational progression. There is a large energy gap 3 3 neutral state, which gives rise to three SO states, Π , Π , and between HOMO-2 and HOMO-1 (3.04 eV at PBE0/AVTZ level), 2 1 agreeing well with the 193 nm spectrum which does not reveal Π , with calculated VDEs of 4.01, 4.13, and 4.33 eV, respectively, any new spectral features between 4.5 and 6.4 eV (Supplementary in agreement with the observed bands D at 4.04 eV, E at 4.13 eV, Fig. 1). The excellent agreement between the theoretical and and F at 4.18 eV (Table 1). Removing one α-electron from the 5σ experimental results confirms firmly the Y-shaped GM for orbital gives rise to the Π final state with a calculated VDE of PbB O . 5.07 eV, which accounts for band G at 4.95 eV. Overall, the the- 3 2 oretical results are in excellent agreement with the experiment, –/2– – confirming the linear GM for PbB O . Multiple B–Pb bonding in PbB O and PbB O . Adaptive 2 3 2 For PbB O , electron detachment from the 2b HOMO 3 2 1 natural density partitioning (AdNDP) analyses were performed – 2– – (Supplementary Fig. 4b) yields the B ground state of neutral on PbB O ,PbB O ,and PbB O to understand the nature of 2 2 3 2 PbB O . The calculated VDE/ADE of 3.06/3.01 eV at the 3 2 their chemical bonding. AdNDP basically transforms the MOs into CCSD(T)/AVTZ level are in good accord with the experimental more familiar bonding elements, such as lone pairs, two-center two- VDE/ADE of 3.00/2.96 eV. The low-frequency vibrational electron (2c-2e) bonds or multicenter delocalized bonds. As shown –1 progression (350 cm ) corresponds to the B–Pb stretching mode in Fig. 4a, the linear PbB O possesses one Pb 6 s and one O lone –1 with a calculated frequency of 318 cm (a mode) for neutral 1 pairs and a B≡O triple bond in the first row. The second row 2 −1 PbB O (C , B ), while the high-frequency mode (2060 cm ) 3 2 2v 1 depicts one 2c-2e B–B σ bond, one 2c-2e B–Pb σ bond, one 2c-2e originates from the B–O stretching mode with a calculated B–Pb π bond, and one 2c-1e B–Pb π bond, giving rise to a B–Pb –1 frequency of 2026 cm (a mode) (Supplementary Fig. 5b), as 1 bond order of 2.5. Thus, there is no sp hybridization in Pb, which confirmed by the Franck-Condon simulations (Supplementary uses its three valence 6p orbitals to form a triple bond with the Fig. 7 and Supplementary Table 1). It should be noted that the central B atom. Both B atoms undergo sp hybridization and form –1 observed frequency of 2060 cm in PbB O is similar to the triple bonds. The B–Pb bond length of 2.122 Å lies between the 3 2 previously reported B≡O symmetric stretching frequencies of B=Pb double-bond length (2.13 Å) and the B≡Pb triple-bond –1 1950, 2040, 1980, and 1935 cm in B O ,B O ,B O , and the length (2.10 Å) predicted from Pyykkö’scovalentatomicradii , 3 2 4 2 4 3 43–45 bare BO, respectively . The observed active vibrational modes consistent with the 2.5 bond order for the B–Pb multiple bond. are consistent with the geometry changes from the ground state of Hence, the linear PbB O species can be described as – – the anion to that of the neutral (Fig. 3) and the nature of the 2b 1 [Pb☱B–B≡O] . Adding one electron to PbB O yields the closed- 2– HOMO that mainly involves B–Pb π bonding and weak B–O shell and electron-precise PbB O which contains an ideal Pb≡B antibonding interactions (Supplementary Fig. 4b). The second triple bond including one 2c-2e Pb–B σ bond and two degenerate 2– VDE derived from electron detachment from the 6a HOMO-1 1 2c-2e Pb–B π bonds, i.e.[Pb≡B–B≡O] , as detailed in the second 2– was calculated to be 3.84 and 3.97 eV at the PBE0 and CCSD(T) row of Fig. 4b. The B–Pb bond length in PbB O is reduced to levels, respectively, consistent with band A at 4.07 eV. The 6a 1 2.107 Å, in good accord with the B≡Pb triple-bond length (2.10 Å) HOMO-1 is a strong B–Pb σ bonding orbital, consistent with the derived from Pyykkö’s triple-bond covalent atomic radii . 4 COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 ARTICLE – 2– – Fig. 4 Chemical bonding anaylses using AdNDP. a PbB O . b PbB O . c PbB O . The occupation numbers (ONs) are indicated. The corresponding Lewis 2 2 3 2 electronic structures are also depicted. – 2 The central B atom in PbB O (Fig. 3d) undergoes sp Recently, boron has been shown to exhibit “metallomimetic” 3 2 hybridization and the AdNDP bonding analyses of the electron- properties , such as the formation of stable borylene dicarbonyl precise monoanion are straightforward (Fig. 4c). It contains one complex, in which two CO ligands are coordinated to a monovalent Pb 6 s lone pair, two O lone pairs, and two B≡O boronyl ligands boron via donor-acceptor bonds , a prototypical transition-metal in the first row. The second row displays two 2c-2e B–B σ bonds behavior. Another unexpected metallomimetic property of boron is between the central B and two BO ligands, one 2c-2e B–Pb σ its capability to form a half-sandwich complex with the aromatic 3− 7 − 51 bond, and one 2c-2e B–Pb π bond, giving rise to a terminal Pb=B B ligand in the recently observed [(η -B )-B-BO] species . 7 7 double bond. The B–Pb bond length of 2.152 Å is comparable to Since BO is isoelectronic with CO, we optimized the geometric the B=Pb double-bond length of 2.13 Å from Pyykkö’s covalent and electronic structures of the linear [Pb≡B−C≡O] and Y-shaped 26 – – + radii . PbB O can thus be formulated as [Pb=B(B≡O) ] . [Pb=B(C≡O) ] and found that these carbonyl complexes exhibit 3 2 2 2 47,48 Natural resonance theory (NRT) analyses at the PBE0/AVTZ similar AdNDP bonding patterns as their boronyl counterparts – 2– – level give rise to a B–Pb bond order of 2.39 for PbB O , 2.84 for [Pb≡B–B≡O] and [Pb=B(B≡O) ] , respectively (see Fig. 4 and 2 2 2– – PbB O , and 1.84 for PbB O , consistent with the bond order Supplementary Fig. 11), with the two 2c-2e B–Pb π bonds in 2 3 2 2– designations of 2.5, 3, and 2 for these species on the basis of [Pb≡B–B≡O] (Fig. 4b) changed to two 3c-2e Pb-B-C π bonds in electron counts, respectively. The covalency of the Pb–B multiple [Pb≡B−C≡O] (Supplementary Fig. 11a) and the 2c-2e B−Pb π bonds can be understood from the similar electronegativities of bond in [Pb=B(B≡O) ] (Fig. 4c) extended to a 4c-2e Pb-B-C2 π – 2– – + Pb (2.33) and B (2.04). The PbB O , PbB O and PbB O bond in [Pb=B(C≡O) ] (Supplementary Fig. 11b). These carbonyl 2 2 3 2 2 species are the first experimentally confirmed molecules with complexes may be more viable for chemical syntheses if a suitable –/2– B–Pb multiple bonds. We have also calculated MB O and ligand can be found to coordinate to Pb. MB O for M=Ge and Sn and found that both the Ge and Sn In conclusion, we have characterized the first B–Pb multiple 3 2 –/2– – species have similar GM structures and bonding patterns as their bonds in the PbB O and PbB O molecular anions, using 2 3 2 Pb counterparts (see Supplementary Figs. 8–10). photoelectron spectroscopy and ab initio calculations. Excellent agreement between the theoretical and experimental data confirms that the global minimum of PbB O has an open-shell Transition-metal-like behaviors of the central boron in [Pb≡B linear structure with a B–Pb bond order of 2.5, whereas the − + 2– – −CO] and [Pb=B(CO) ] . The electron deficiency of boron has closed-shell PbB O contains a B≡Pb triple bond. The PbB O 2 2 3 2 led to novel structures and bonding in various boron compounds. species has been shown to have a Y-shaped structure with a COMMUNICATIONS CHEMISTRY | (2022) 5:25 | https://doi.org/10.1038/s42004-022-00643-1 | www.nature.com/commschem 5 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00643-1 terminal B=Pb double bond and two BO ligands coordinated to References 1. Lipscomb, W. N. The boranes and their relatives. Science 196, 1047–1055 the central B atom. Theoretical calculations indicate that –/2– – (1977). MB O and MB O with M=Ge and Sn display similar 2 3 2 2. Alexandrova, A. 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