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High-Frequency 1H NMR Chemical Shifts of SnII and PbII Hydrides Induced by Relativistic Effects: Quest for PbII Hydrides Jan Vícha,*,† Radek Marek,‡ and Michal Straka*,§ †

Center of Polymer Systems, University Institute, Tomas Bata University in Zlín, Třída T. Bati 5678, CZ-76001 Zlín, Czech Republic CEITEC-Central European Institute of Technology, Masaryk University, Kamenice 5/A4, CZ-62500 Brno, Czech Republic § Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nám. 2, CZ-16610 Prague, Czech Republic ‡

S Supporting Information *

ABSTRACT: The role of relativistic effects on 1H NMR chemical shifts of SnII and PbII hydrides is investigated by using fully relativistic DFT calculations. The stability of possible PbII hydride isomers is studied together with their 1H NMR chemical shifts, which are predicted in the high-frequency region, up to 90 ppm. These 1H signals are dictated by sizable relativistic contributions due to spin−orbit coupling at the heavy atom and can be as large as 80 ppm for a hydrogen atom bound to PbII. Such high-frequency 1H NMR chemical shifts of PbII hydride resonances cannot be detected in the 1H NMR spectra with standard experimental setup. Extended 1 H NMR spectral ranges are thus suggested for studies of PbII compounds. Modulation of spin−orbit relativistic contribution to 1 H NMR chemical shift is found to be important also in the experimentally known SnII hydrides. Because the 1H NMR chemical shifts were found to be rather sensitive to the changes in the coordination sphere of the central metal in both SnII and PbII hydrides, their application for structural investigation is suggested.

1. INTRODUCTION Stable hydrides of heavier group 14 elements were considered to be restricted to tetravalent species until 2000,1 when the first hydrogen-bridged SnII hydride dimer2 [TrpSn(μ-H)]2, Trp = C6H3-2,6-(C6H2-2,4,6-iPr3)2, was prepared using large protecting terphenyl ligands.3 Further development in group 14 hydride chemistry was accelerated when the ability of subvalent GeII and SnII compounds to activate molecular hydrogen at mild conditions upon formation of corresponding low-valent hydrides was discovered.4−6 Subvalent group 14 hydrides have also drawn a lot of attention for their capability to react directly with unsaturated molecules, such as ketones, alkenes, and alkynes.7−9 Since then several synthetic procedures for preparation of SnII and GeII subvalent hydrides have been reported, including preparation of monomeric terminal Sn hydrides supported by bidentate10 or tridentate11 ligands and MH2 (M = Sn, Ge) hydrides stabilized in the coordination sphere of a transition metal.12 Despite considerable efforts13,14 no PbII hydrides have been characterized to date. Instead of the expected PbII hydride, synthetic procedures used for preparation of SnII and GeII hydrides yielded a lead analog of alkyne (plumbyne) TrpPb− PbTrp.14 However, it appears likely that TrpPb−PbTrp is generated in situ from unstable PbII hydride (TrpPbH)2 by rapid elimination of H2 molecule,13,14 a relatively common reaction among the p-block elements.15,16 The possibility of © XXXX American Chemical Society

(TrpPbH)2 stabilization by judicious choice of ligands was expressed13,14 yet has not been successful so far. Incidentally, the same approach led to successful preparation of a stable subvalent hydride of bismuth.17 We have recently predicted that light atom (LA) nuclei in subvalent TlI and PbII compounds directly bound to the heavy metal atom (HA) resonate at very high frequencies, with NMR chemical shifts up to 400 ppm for 13C and ∼1000 ppm for 29 18 Si. The reason behind these high-frequency signals are sizable (>200 ppm)18 relativistic effects induced at the light atom by the heavy atom, known as the Heavy Atom on the Light Atom (HALA) effects.19,20 In the SO HALA effect (SO = spin−orbit), which is typically dominant in covalently bound compounds, the SO coupling at HA mixes triplet states in the ground-state wave function of the molecule, thus introducing an additional shielding or deshielding at the LA via the Fermicontact mechanism.21−23 Due to the SO HALA effect, NMR resonances of LA nuclei in certain TlI and PbII compounds resonate at much higher frequencies than expected, thus being “pushed” outside the standard NMR chemical shift ranges. This provided an alternative explanation to why certain signals of light atoms are missing from experimental NMR spectra of TlI and PbII species.18 Received: July 1, 2016

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DOI: 10.1021/acs.inorgchem.6b01575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Possible structures of group 14 hydride dimers, M = Si, Ge, Sn, Pb; R = H or carbon-based ligand. reducing the time required for the SCF procedure by approximately one-third in the case of parallel runs (>48 cores). The 1H NMR chemical shifts are referenced relative to TMS. Performance of the 4c-PBE Level in Calculations of 1H NMR Chemical Shifts in Heavy-Metal Hydrides. The performance of the four-component Dirac−Kohn−Sham (DKS) approach for calculations of δ(1H) in the heavy transition metal hydrides has been verified in the past with good results.27,30 The performance of the 4c-PBE level for experimentally known hydrides of main-group elements InIII, SnII, SnIV, and PbIV is tested in this work, see Table S1. Selected transition-metal complexes used in the previous studies are included for comparison.30 The overall performance of the 4c-PBE level for calculations of δ(1H) in the selected model set is rather good. The mean absolute deviation (MAD) of the 4c-PBE level from experiment is 0.9 ppm for the entire set and 0.7 ppm for the maingroup-element compounds. For details, see Table S1. Calculations of Energetics. Electronic energies of PbII−hydride isomers were calculated using the hybrid PBE0 functional with D3 dispersion correction51 at the two-component (2c) spin−orbit zerothorder regular approximation (SO-ZORA) level as implemented in the ADF package52−54 using the standard TZP basis set from the ADF2014 library for all atoms55,56 (noted as 2c-PBE0 level). This level was adopted given the known importance of dispersion effects in lead dimeric compounds57 and because dispersion correction is not available on the fully relativistic 4c-PBE level. Analysis of the NMR Chemical Shifts and Electronic Structure. MO analysis of NMR chemical shifts is currently unavailable at the 4c-PBE level; hence, MO analysis of δSO was done at the SO-ZORA level using the PBE40 functional, i.e., standard PBE0 functional with exact-exchange admixture set to 40%.58 This level provided excellent results in our previous studies of transition-metal complexes.38−40,59 It is referred to as 2c-PBE40. The orbital analysis of the NMR chemical shifts uses NMR shielding decomposition into canonical molecular orbitals as implemented in ADF2014 package, based on the NBO 6.0 module (Gennbo)60 interfaced to ADF code.61,62 Because only the sum (δs+p) of paramagnetic (δpara) and spin−orbit (δSO) NMR chemical shift contribution of individual MOs is obtained, analogous analysis was done at the scalar-relativistic ZORA 1c-PBE40 level (without spin− orbit contribution), too. δSO was then calculated as δSO = δs+p − δpara.27 For the sake of clarity, NMR chemical-shift contributions of degenerated spin−orbitals (spinors), arising from scalar-relativistic molecular orbitals due to the spin−orbit splitting at the 2c-PBE40 level, were summed up and are reported as contributions of the parental scalar-relativistic MOs. The contributions from the individual spinors thus correspond to one-half of the reported value. The atomic-orbital composition of M−H bonds was determined from the natural localized molecular orbitals (NLMO) obtained from natural bond orbital (NBO) analysis63 as implemented in the NBO 6.0 module (Gennbo)60 interfaced to ADF code.

The SO HALA relativistic contribution to the LA chemical shift, δSO(LA), is intimately related with the local electronic structure around the LA. The size and sign of the δSO(LA) reflects the composition and covalence of the HA−LA bond,24−27 the size of the HOMO−LUMO gap,28,29 and the composition of frontier MOs.18,26 Particularly, it depends on the LA s character of the HA−LA bond.24 Hence, the 1sbonded 1H nuclei are highly sensitive to the HALA effects.30 Indeed, reported experimental 1H NMR chemical shifts of existing SnII hydrides, δ(1H), which are up to 10 ppm larger than in SnIV compounds,10,31 suggest the presence of sizable deshielding SO HALA effects. Consequently, much larger SO HALA effects can be expected in the PbII compounds.18 This however brings us the question how large SO HALA effects can be expected in the heavier PbII hydrides? Are their 1H signals also shifted outside the standard NMR regions as was the case of 13C and 29Si NMR resonances?18 With 1H NMR spectroscopy being undeniably the best and sometimes the only method for detection and characterization of metal hydrides, prediction of correct spectral ranges is highly important for identification of new compounds. In this work, we evaluate the stability of various PbII hydrides and their isomers and predict their 1H NMR signals using fully relativistic density functional theory (DFT). High-frequency 1H NMR chemical shifts of PbII hydrides (up to 90 ppm) are found. Smaller but still significant δSO(1H) are found in SnII hydrides. 1 H NMR signals in both families of SnII and PbII hydrides sensitively reflect the changes in the coordination sphere of the central metal atom, which enhances the potential of 1H NMR spectroscopy in investigations of molecular topology in this family of compounds.

2. METHODS Molecular Structures. Molecular geometries are based on published X-ray data, where applicable, or prepared in silico for model compounds. Structures were optimized using the PBE0 functional32,33 and def2-TZVPP basis set34 for all atoms with corresponding relativistic effective core potentials (ECPs)35 for the metal centers (ECP substituting 28 electrons for Sn and 60 electrons for Pb). Dispersion correction (D3) by Grimme was used.36 This level of theory has been justified by several previous studies of metal complexes.37−40 The implicit conductor-like screening solvent model (COSMO) as implemented in Turbomole 7.041 with solvent parameters corresponding to the experimental solvents was used in geometry optimizations of the experimentally known compounds, while structures of hypothetical model compounds were optimized in vacuo. Calculations of NMR Chemical Shifts. The 1H NMR chemical shifts were calculated using the four-component (4c) Dirac−Kohn− Sham (DKS) relativistic approach within the Dirac−Coulomb framework42,43 as implemented in the ReSpect 3.42 code44 with PBE functional45,46 and uncontracted Dyall’s valence triple-ζ basis set47−49 for NMR spectator atoms and all atoms within 3 bonds of the spectator LA. More distant atoms were treated using Dyall’s valence double-ζ basis set47−49 to reduce computational time. This level of theory is noted as 4c-PBE in this work. A novel, more efficient diagonalization-free algorithm was employed in the self-consistent field procedure within the four-component relativistic scheme,50 effectively

3. RESULTS AND DISCUSSION 3.1. Stability of Dimeric PbII Hydrides. The simplest subvalent molecular hydride PbH2 is highly unstable and in a free form has only been observed in a low-temperature neon matrix at temperatures below 10 K.64 On the other hand, a whole family of potentially stable group 14 dimeric (RMH)2 hydrides (M = Si, Ge, Sn, Pb; R = H or carbon-based ligand) was predicted by Trinquier at the beginning of 1990s.65,66 Their main structural motifs are shown in Figure 1. Isomer I was B


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Inorganic Chemistry

Figure 2. Relative energies of PbII dimeric hydrides (HPbR)2 with respect to the increasing size of ligand R, calculated at the 2c-PBE0 level with D3 dispersion correction. R = H, Me (methyl), Ph (phenyl), TrpMe (C6H3-2,6-(C6H2-2,4,6-(CH3)3)2).

Figure 3. Structures of possible PbII hydrides 1−8 and their 1H NMR chemical shifts δCALC(1H) and δSO(1H). Compounds 1−5 are derived from structures I−V using TrpMe as ligand R. Compounds 6−8 are PbII analogues of existing SnII species.10−12 iPr = isopropyl.

predicted to be by far the most stable for Si and Ge,65,66 while for Sn and Pb the most stable was the μ-H-bridged isomer III.65,66 Indeed, the first prepared germylene hydride dimer has a structural arrangement of I,67 whereas the first stannylene hydride dimer adopts hydrogen-bridged conformation III in the solid state.2 However, later calculations68 of SnII hydride dimers featuring various ligands R revealed that all four isomers are energetically rather close to each other (within 7 kcal/mol), and depending on the ligand, the bridged structure III may not always be the

most stable one. In fact, for the large terphenyl ligand R, isomers IV and I were considerably more stable than isomer III.68 This led to experimental preparation of the first hydride with an asymmetric structure motif IV.68 The relative stability of hypothetical PbII hydrides featuring various ligands R, ranging from proton through methyl and phenyl to the large terphenyl TrpMe ligand (TrpMe = C6H32,6-(C6H2-2,4,6-(CH3)3)2), was estimated on the 2c-PBE0 level. The results are summarized in Figure 2 and Table S2. C


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Figure 4. Structures of experimentally known SnII hydrides 9,12 10,11 11,10 and 12 and their 1H NMR chemical shifts, δEXP(1H), δCALC(1H), and δSO(1H) in ppm.

δ(1H) trends; the following results presented for δSO(1H) are also valid for δ(1H). Notably, no straightforward correlations are observed between δSO(1H) and the hydrogen 1s character of the Pb− H bond in Table S3. Rather, the size δSO(1H) is dictated mainly by the admixture of the Pb 6p AO in the frontier orbitals, particularly the HOMO and LUMO. This is in accord with observations for 13C NMR in our previous report18 where we demonstrated that significant δSO(MO → MO*) are only obtained when both MO and MO* (vacant MO) contain a sufficiently large contribution of the heavy atom 6p AO character. Interestingly, the δSO(1H) of 1−4 dimeric species correlate with their thermodynamic stabilitythe δSO(1H) is the largest for thermodynamically the least stable structures. δSO(1H) of the most stable dimeric hydride isomer 3 has the lowest value (26 ppm), whereas signals of other hydride dimers are more deshielded, 32 ppm in 2 (average of Ha and Hb), 35 ppm in 4, and 48 ppm in 1 (the least stable isomer), see Figures 2 and 3. In other words, compounds to be the most likely prepared are the ones with the lowest δ(1H). Nevertheless, even the lowest 1 H NMR signal of 3 is predicted at a rather high frequency of 31 ppm, which is still outside the experimental 1H NMR chemical shift range of known sixth period hydrides (up to ∼25 ppm).70 Another structure-related observation should be noted for the series of monomeric species, 5−8, where δSO(1H) decreases with increasing coordination number of the Pb atom, see Figure 3. This empirical observation can be justified based on the availability of vacant 6p*-based Pb orbitals for magnetic couplings. Thus, the largest values of δSO are obtained for doubly coordinated PbII compounds, such as 5, where the 6p* Pb AO does not participate in bonding. The corresponding spin−orbit active MO* is then composed of 94% of Pb 6p* AO, which results in δSO(1H) ≈ 80 ppm, see Table S2 and Figure 3. When the coordination number of PbII increases, the 6p* AO orbital becomes involved in coordination bonding, and its availability for magnetic coupling is reduced. For instance, in the three-coordinate compound 8, the formal vacant 6p* Pb AO is partially used for ligand bonding. The spin−orbit active MOs* of 8 still have ∼30% of Pb 6p* character on average, which results in sizable δSO(1H) = 45 ppm. In the fourcoordinated hydride 7, the Pb 6p* AO is utilized in bonding with two nitrogen atoms, leaving only ∼8% Pb 6p* character in the spin−orbit active MOs* and δSO(1H) = 26 ppm. An even lower Pb 6p* AO character (2.5%) in 6 features a strongly electron-pair-donating carbene ligand, further enhanced by the presence of an electron acceptor W(CO)5 group. As a result, a small (in this context) δSO(1H) of 9 ppm is predicted for 6. Given the sensitivity of δSO(1H), hence also the total δ(1H), to the bonding situation around the heavy metal and given the

Unlike SnII hydrides,68 the μ-H-bridged structure III is the most stable PbII hydride isomer in all cases. Although increasing the size of the ligands stabilizes other isomers, even for the largest tested ligand, isomers II and IV are predicted to be about +11 kcal/mol and isomer I about +14 kcal/mol less stable than isomer III. Hydrogen-bridged structure III is thus the most likely to be adopted by hypothetical PbII dimeric hydride. The calculated HOMO−LUMO gaps (ΔEHL) well correlate with relative energies of PbII dihydride isomers. The most stable isomer III also has the largest ΔE = 4.3 eV, closely followed by isomer II with 4.0 eV, while I and IV have ΔE of only 3.3 and 3.2 eV, respectively (for TrpMe ligand). Interestingly, ΔEHL for monomeric PbII hydride V is 4.1 eV, which is comparable with that of the most stable dimeric isomer III and corresponding Sn analogs calculated on the same level (∼4.0 eV). The ΔEHL is only an indicative factor of kinetic stability because it does not reflect the main problem in the preparation of PbII hydridesthe extreme steric dissatisfaction of large lead atom due to the proton’s small size. Whether this issue can be overcome, for instance, by sterically demanding multidentate ligands and/or formation of donor−acceptor adducts as was the case for Sn,10−12 remains to be seen. Nevertheless, besides “classical” hydride species I−V, we also included several PbII analogs of recently prepared SnII hydrides10−12 in the 1H NMR chemical shift calculations, as described in following section. 3.2. 1H NMR Chemical Shifts in Hypothetical PbII Hydrides. δ(1H) and δSO(1H) were calculated for isomeric dimer structures I−IV and monomer V featuring the TrpMe ligand (1−5 in Figure 3) as well as for PbII analogs of the experimentally known SnII compounds10−12 (6−8 in Figure 3). The 1H NMR resonances of hydride atoms in the studied PbII compounds are predicted to be found between 14 and 90 ppm, with a majority of signals located between 30 and 60 ppm, see Figure 3. Such a broad chemical shift range is due to the varying δSO(1H) ranging from +9 ppm to an extreme value of +80 ppm, see Figure 3. For comparison, 1H in PbIV compounds usually resonate within 5−10 ppm31 and SnII compounds within 6−14 ppm (see below).10−12 Values between −60 and +25 ppm have been reported for the heaviest of the transition metal hydrides.69,70 Predicted 1H NMR resonances of PbII hydrides are thus only comparable with those in the recently predicted and so-far hypothetical 5f element hydrides.71 To understand the nature of δSO(1H) variation, detailed MO analysis of δSO(1H) was performed, see Methods and Table S3 in the Supporting Information. Detailed explanation of the role of individual factors influencing δ SO in subvalent Pb II compounds is given elsewhere;18 here, we only discuss the main conclusions. Because δSO(1H) is a major part of the total predicted δ(1H) in PbII hydrides, and as such it dictates the D


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Inorganic Chemistry large range of δ(1H) in PbII hydrides, the δ(1H) has potential as an NMR “probe” for studies of electronic and topological structure in this family of compounds. Any utilization of the vacant 6p* Pb AO orbital, for instance, formation of dimeric species in the solution or solid state,68 should be reflected in considerable changes (units to tens of ppm) of δ(1H). Similar trends, although on a lower scale, should be observable for SnII hydrides. Because the SnII hydrides are known experimentally, we shall briefly discuss how relativity affects their 1H NMR properties. 3.3. Role of Relativity in 1H NMR Chemical Shifts of SnII Hydrides. δ(1H) and δSO(1H) were calculated at the 4cPBE level for SnII compounds 9, 10, and 11 (Figure 4) and compared with experimental data. Compound 12, which is a hypothetical monomeric unit of [TrpSn(μ-H)]2 and an analog of 5, is included as an example of a purely two-coordinated SnII compound. A good agreement between theory and experiment is observed, see Figure 4. Both calculated and experimental δ(1H) for 9−11 are found in the region of 6−14 ppm, with the δSO(1H) contribution being 1−7 ppm. Modulation of δ(1H) in SnII hydrides is thus predominantly caused by the δSO(1H) contribution, see Figure 4. The main difference in 1H NMR chemical shifts between SnII and SnIV compounds is hence due to the increased δSO(1H) as presumed (compare Table S1 and Figure 4). Although δSO(1H) in SnII compounds 9−12 is about seven times smaller than in the predicted PbII analogs 5−8, the modulation of δSO(1H) by changing the coordination number of the central metal is still well recognized. MO analysis reveals that the largest contribution to δSO(1H) arises from HOMO → LUMO magnetic coupling, see Table S4. The δSO(H → L) for 10, 11, and 12 as examples of four-, three-, and twocoordinated SnII compounds are illustrated in Figure 5 to demonstrate the modulation of δSO(1H) with respect to participation of 5p* Sn AO in bonding.

of 11 to 11% is reflected in larger δSO(H → L) = 1.8 ppm, while 94% of the Sn 5p* AO in the LUMO of the two-coordinated compound 12 results in a highly efficient HOMO−LUMO coupling,18 producing large δSO(H → L) = 8.3 ppm. Analogously to PbII compounds, when a vacant Sn 5p* AO becomes involved in the coordination bonding (three and four coordinated compounds), it is no longer available for efficient magnetic couplings responsible for large δSO, which leads to a significant reduction of the δSO(1H) size. However, due to the considerably smaller scale of δ(1H) changes in SnII hydrides as compared to PbII analogs, one should carefully consider the role of other effects (environment, dynamic processes) before deriving final conclusions about the coordination sphere of Sn based on hydride δ(1H).

Figure 5. HOMO → LUMO magnetic couplings for 10, 11, and 12, their δSO(H → L) in ppm, ΔE in eV, and HOMO and LUMO composition with respect to Sn 5p and 5p* AO in %. All MOs are plotted with the cutoff of electron density at 0.05 au.

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4. CONCLUSIONS We studied the 1H NMR chemical shifts in SnII and PbII hydrides by the fully relativistic four-component density functional calculations using the PBE functional. The performance of the selected method was evaluated on the experimentally known data for a series of transition-metal and p-element hydrides and provided excellent results with mean absolute deviation from experiment below 0.9 ppm. Thermodynamic stability of various dimeric PbII hydride isomers of a general formula (RPbH)2 was evaluated using density functional theory as well. The hydrogen-bridged dimeric structure is estimated to be the most stable disregarding the size of R, with other isomers being about 11−14 kcal/mol higher in energy. The 1H NMR chemical shifts of the studied PbII hydrides are predicted to be found in a broad region between 14 (tetracoordinated PbII bis-adduct) and 90 ppm (monomeric two-coordinated PbII hydride), with the majority of signals located in the region between 30 and 60 ppm. The highfrequency δ(1H) signals in PbII hydrides originate from the large spin−orbit effects (up to 80 ppm) induced by the heavy Pb atom affecting the hydrogen atom. Resulting 1H resonances thus fall outside the range of known sixth-period hydride 1H NMR chemical shifts (from −60 to +25 ppm). This may explain the lack of hydride 1H NMR signals, although their presence in the reaction mixture was suspected. Hence, in searching for novel PbII hydride compounds, we suggest that high-frequency 1H NMR spectral regions up to 100 ppm are explored. The relativistic contribution to δ(1H) in PbII hydrides was shown to be sensitive to the coordination number of the central metal, reflecting the utilization of formal Pb 6p* AO by ligands in the coordination bonding. The δ(1H) thus has potential as an NMR “probe” for studies of electronic and topological structure in this family of compounds. Similar observations were made for 1H NMR chemical shift trends in experimentally known SnII hydrides which are also dictated by the HALA effects, although their values are considerably smaller than those in PbII hydrides. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01575. Evaluation of the 4c-PBE approach performance for calculations of 1H NMR chemical shifts; estimated stability of hypothetical PbII dimeric hydrides; analysis of bonding and δSO(1H) for 1−8; analysis of bonding and

Contrary to the HOMO composition and ΔE, the 5p* character of the LUMO is well correlated with δSO(H → L), see Figure 5. The negligible (4%) Sn 5p* character of the LUMO in compound 10 (below the threshold in Figure 5) leads to a small δSO(H → L) of ∼0.4 ppm at the spectator hydrogen atom. An increase of the Sn 5p AO contribution in the LUMO E


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δSO(1H) for 9−12; molecular coordinates for 1−12 (PDF)

(12) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; Rivard, E. Stabilization of the Heavy Methylene Analogues, GeH2 and SnH2, within the Coordination Sphere of a Transition Metal. J. Am. Chem. Soc. 2011, 133, 777−779. (13) Rivard, E.; Power, P. P. Recent Developments in the Chemistry of Low Valent Group 14 Hydrides. Dalton Trans. 2008, No. 33, 4336− 4343. (14) Pu, L.; Twamley, B.; Power, P. P. Synthesis and Characterization of 2,6-Trip2H3C6PbPbC6H3−2,6-Trip2 (Trip = C6H2−2,4,6-I-Pr3): A Stable Heavier Group 14 Element Analogue of an Alkyne. J. Am. Chem. Soc. 2000, 122, 3524−3525. (15) Gauvin, F.; Harrod, J. F.; Woo, H. G. Catalytic Dehydrocoupling: A General Strategy for the Formation of Element−Element Bonds. In Advances in Organometallic Chemistry; West, F. G. A. S. R., Ed.; Academic Press, 1998; Vol. 42, pp 363−405. (16) Clark, T. J.; Lee, K.; Manners, I. Transition-Metal-Catalyzed Dehydrocoupling: A Convenient Route to Bonds between MainGroup Elements. Chem. - Eur. J. 2006, 12 (34), 8634−8648. (17) Hardman, N. J.; Twamley, B.; Power, P. P. (2,6-Mes2H3C6)2BiH, a Stable, Molecular Hydride of a Main Group Element of the Sixth Period, and Its Conversion to the Dibismuthene (2,6Mes2H3C6)BiBi(2,6-Mes2C6H3). Angew. Chem., Int. Ed. 2000, 39, 2771−2773. (18) Vícha, J.; Marek, R.; Straka, M. High-Frequency 13C and 29Si NMR Chemical Shifts in Diamagnetic Low-Valence Compounds of TlI and PbII: Decisive Role of Relativistic Effects. Inorg. Chem. 2016, 55, 1770−1781. (19) Nakatsuji, H.; Takashima, H.; Hada, M. Spin-Orbit Effect on the Magnetic Shielding Constant Using the Ab Initio UHF Method. Chem. Phys. Lett. 1995, 233, 95−101. (20) Ballard, C. C.; Hada, M.; Kaneko, H.; Nakatsuji, H. Relativistic Study of Nuclear Magnetic Shielding Constants: Hydrogen Halides. Chem. Phys. Lett. 1996, 254, 170−178. (21) Manninen, P.; Lantto, P.; Vaara, J.; Ruud, K. Perturbational Ab Initio Calculations of Relativistic Contributions to Nuclear Magnetic Resonance Shielding Tensors. J. Chem. Phys. 2003, 119, 2623−2637. (22) Vaara, J.; Manninen, P.; Lantto, P. Perturbational and ECP Calculation of Relativistic Effects in NMR Shielding and Spin−Spin Coupling. In Calculation of NMR and EPR Parameters; Kaupp, M., Bühl, M., Malkin, V., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 2004; pp 209−226. (23) Maldonado, A. F.; Aucar, G. A. The UKB Prescription and the Heavy Atom Effects on the Nuclear Magnetic Shielding of Vicinal Heavy Atoms. Phys. Chem. Chem. Phys. 2009, 11, 5615−5627. (24) Kaupp, M.; Malkina, O. L.; Malkin, V. G.; Pyykkö, P. How Do Spin−Orbit-Induced Heavy-Atom Effects on NMR Chemical Shifts Function? Validation of a Simple Analogy to Spin−Spin Coupling by Density Functional Theory (DFT) Calculations on Some Iodo Compounds. Chem. - Eur. J. 1998, 4, 118−126. (25) Vícha, J.; Straka, M.; Munzarová, M. L.; Marek, R. Mechanism of Spin−Orbit Effects on the Ligand NMR Chemical Shift in Transition-Metal Complexes: Linking NMR to EPR. J. Chem. Theory Comput. 2014, 10, 1489−1499. (26) Vícha, J.; Foroutan-Nejad, C.; Pawlak, T.; Munzarová, M. L.; Straka, M.; Marek, R. Understanding the Electronic Factors Responsible for Ligand Spin−Orbit NMR Shielding in TransitionMetal Complexes. J. Chem. Theory Comput. 2015, 11, 1509−1517. (27) Greif, A. H.; Hrobárik, P.; Hrobáriková, V.; Arbuznikov, A. V.; Autschbach, J.; Kaupp, M. A Relativistic Quantum-Chemical Analysis of the Trans Influence on 1H NMR Hydride Shifts in Square-Planar Platinum(II) Complexes. Inorg. Chem. 2015, 54, 7199−7208. (28) Hegetschweiler, K.; Kuppert, D.; Huppert, J.; Straka, M.; Kaupp, M. Spin−Orbit-Induced Anomalous pH-Dependence in 1H NMR Spectra of CoIII Amine Complexes: A Diagnostic Tool for Structure Elucidation. J. Am. Chem. Soc. 2004, 126, 6728−6738. (29) Hyvärinen, M.; Vaara, J.; Goldammer, A.; Kutzky, B.; Hegetschweiler, K.; Kaupp, M.; Straka, M. Characteristic Spin−Orbit Induced 1H(CH2) Chemical Shifts upon Deprotonation of Group 9

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Stanislav Komorovsky and Michal Repisky (UiT−The Arctic University of Norway) for providing the latest version of the ReSpect code and technical support with the 4c-DKS calculations. This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic, Program NPU I (LO1504 to J.V.) and the Czech Science Foundation (grants 16-05961S to J.V. and 15-09381S to R.M.). Institutional support to MS was provided by the Czech Academy of Sciences, project RVO-61388963. Computational resources were provided by the CESNET, project LM2015042, and the CERIT Scientific Cloud, project LM2015085, provided under the programme “Projects of Large Research, Development, and Innovations Infrastructures”.

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