2.1 Synthesis and Characterization of NHC-Supported 2-Tetrelavinylidenes 1-Si, 1-Ge and 1-GeMind

Entry to this chemistry was provided by the 1,2-dibromoditetrelenes (E)-(R)BrE=EBr(R) (E=Si, Ge; R=Tbb (C₆H₂−2,6-(CH(SiMe₃)₂)₂−4-^tBu), Mind (1,1,3,3,5,5,7,7-octamethyl-s-hydrindacen-4-yl)) and the diazoolefin (IDipp)CN₂ (IDipp = C[N(Dipp)CH]₂).³⁶ Heating of a 2 : 1 molar mixture of (IDipp)CN₂ and 1,2-dibromodisilene (E)-(Tbb)BrSi=SiBr(Tbb) in toluene at 90 °C was accompanied by a gradual color change of the reaction solution from orange-yellow to intense orange (Scheme 1). Reaction monitoring by ¹H NMR spectroscopy revealed the formation of the desired NHC-supported 2-silavinylidene 1-Si as a major product. 1-Si was isolated after work-up as an extremely air-sensitive, bright yellow solid in 59 % yield. Reaction of (IDipp)CN₂ with the 1,2-dibromodigermenes (E)-(R)BrGe=GeBr(R) (R=Tbb, Mind) proceeded rapidly in benzene at room temperature to afford selectively the isovalent 2-germavinylidenes 1-Ge and 1-GeMind, which were isolated after crystallization in high yields as air-sensitive, orange-brown and mustard-yellow solids, respectively (Scheme 1).

Compounds 1-Si, 1-Ge and 1-GeMind display high thermal stability, decomposing unselectively upon melting at 220 °C (1-Si), 162 °C (1-Ge) and 151 °C (1-Ge-Mind). Upon strict exclusion of air their solutions in (D₆)benzene are stable at 80 °C for several hours, the ¹H NMR spectra displaying no sign of decomposition.

1-Si, 1-Ge and 1-GeMind are the first single donor-stabilized 2-sila- and 2-germavinylidenes to be reported and were characterized by single crystal X-ray diffraction (sc-XRD) analyses, multinuclear NMR spectroscopy and UV/Vis spectroscopy. The molecular structures display the following features (Figure 3, Table 1 and ESI, chapter 5):

a) the Si1−C1 and Ge−C1 bond lengths are remarkably short (d(E−C^VNL): 1.657(2) Å (1-Si), 1.746(6) Å (1-Ge), 1.724(2) Å (1-GeMind); Table 1). In fact, the Si−C^VNL distance of 1-Si is considerably shorter than the shortest Si=C double bond length reported so far for a silene (1.702(5) Å, Me₂Si=C(SiMe₃)(Si^tBu₂Me)).³⁷ It is even shorter than the Si=C double bond lengths of 1-silaallenes (1.693(3)–1.704(4) Å, Table 1),^38-40 the NHC-supported 1-silavinylidenes A (1.792(4) Å)²⁰ and B (1.784(2) Å)²¹ and the NHC-supported iminosilenylidene (NHC)Si=C=N(Ar^Mes) (1.784(1) Å).¹⁹ Notably, the Si−C^VNL bond length is similar to that of the phosphane-supported silynes (2-(P(N^tBu)₂SiMe₂)-3-((Dipp)N)-norbornene)SiCP(NⁱPr₂)₂ (1.667(3) Å)⁴¹ and [(2-(P(N^tBu)₂SiMe₂)-3-((Dipp)N)-norbornene)SiCPPh₂(NⁱPr₂)][B(Ar^F)₄] (1.631(2) Å, Ar^F=C₆H₃−3,5-(CF₃)₂).⁴² Remarkably, the Ge−C^VNL bond of 1-GeMind is the shortest crystallographically determined Ge−C bond reported so far. It is much shorter than the Ge=C double bond lengths of the phosphagermaallene (Tipp)(^tBu)Ge=C=P(Mes*) (1.761(2) Å),⁴³ the 1-germaallene (Tipp)₂Ge=C=C(^tBu)(Ph) (1.783(2) Å),⁴⁴ and the phosphane-supported germyne (2-(P(N^tBu)₂SiMe₂)-3-((Dipp)N)-norbornene)GeCP(NⁱPr₂)₂ (1.887(5) Å).⁴⁵

b) the C^VNL−C^NHC bond lengths (d(C1−C2): 1.358(3) Å (1-Si), 1.376(9) Å (1-Ge), 1.377(3) (1-GeMind)) are significantly longer than the C^cent=C^ter double bond lengths of 1-tetrelaallenes 1.314(2) −1.325(4) Å, Table 1).^{38-40, 44} Moreover, the C^VNL−C^NHC bond lengths lie outside the longer end of the range of the C^cent−C^NHC bond lengths reported for carbodicarbenes containing at least one NHC moiety (1.318 −1.346 Å).^46-48

c) the dicoordinated vinylidene center C^VNL displays a bent geometry (∡(Si1−C^VNL−C^NHC)=142.9(2)° (1-Si), ∡(Ge−C^VNL−C^NHC)=131.3(5)° (1-Ge), 135.0(2)° (1-GeMind)). The angle at C^VNL is smaller than those of 1-silaallenes (∡(Si−C^cent−C^ter)=172.0° −174.2°, Table 1)^38-40 and the 1-germaallene (^tBu)(Ph)C=C=GeTipp₂ (∡(Ge−C^cent−C^ter)=159.2°),⁴⁴ but considerably larger compared to the ∠(C^NHC−Si−Si) angle of the NHC-supported disilavinylidene (Z)-(SIDipp)Si=SiBr(Tbb) (97.6(1)°, SIDipp=C[N(Dipp)CH₂]₂).¹⁷ Of note, the angles at C^VNL lie in the range of the ∠(C^NHC−C^cent−C^NHC) angles reported for carbodicarbenes containing at least one NHC moiety (134.8° −146.2°).^46-48

d) the silicon center is trigonal planar coordinated (Σ∡(Si1)=359.7°) and the 2-silavinylidene mean-plane defined by the atoms Si1, C^VNL, Br and C^Tbb (C29) is orthogonal to the NHC mean-plane passing through the atoms of the N-heterocyclic ring as evidenced by the silavinylidene-NHC interplanar angle β_Si=C/NHC of 89.9(1)° or the twist angle θ_Si=C/NHC of 89.7(2)° between the NHC mean-plane and the silylene plane defined by the atoms Si1, Br and C^Tbb. This arrangement leads to a nearly C_s-symmetric structure with the plane of symmetry passing through the 2-silavinylidene core and bisecting the NHC five-membered ring. The IDipp and Tbb groups of 1-Si adopt an anti-periplanar conformation with a torsion angle ∠(C2−C1−Si1−C29) of 172.9(3)°. These conformational features are also found in the disilicon congener (Z)-(SIDipp)Si=SiBr(Tbb): β=93.5(1)°, θ=87.1(1)°, torsion angle (∡(C^NHC−Si−Si−C^Tbb)=177.3(2)°).¹⁷ The corresponding structural parameters of 1-Ge and 1-GeMind are very similar to those of 1-Si (Table 1). Of note, the Σ∡(Ge) in 1-Ge and 1-GeMind is different from that of the 1-germaallene (^tBu)(Ph)C=C=Ge(Tipp)₂, which contains a slightly trigonal pyramidal bonded Ge atom (Σ∡(Ge)=348.4°).⁴⁴

Based on the comparative assessment of the structural parameters, the E−C^VNL (E=Si, Ge) bond of 1-E (E=Si, Ge) and 1-GeMind can be described as E=C^VNL double bonds with a considerable E≡C^VNL triple bond character, and the C^VNL−C^NHC bonds have a partial double bond character in line with the results of the Natural Resonance Theory (NRT) in NBO basis, according to which the resonance hybrid of the gas phase optimized structures (1-Si)_calc and (1-Ge)_calc is mainly composed of three Natural Lewis Structures (NLSs), a bent 1-tetrelaallene type (1-E-a), a cationic tetrelyne type (1-E-b) and an ylide type structure (1-E-c) (Figure 4).

The NHC-supported 2-tetrelavinylidenes 1-Si and 1-Ge were characterized in solution by multinuclear NMR spectroscopy. In the ²⁹Si NMR spectrum 1-Si displays a characteristic singlet signal for the unsaturated Si atom at δ_Si=−37.0 ppm. This appears at much higher field than those of 1-silaallenes (13.1–61.7 ppm, Table 2),^{38-40, 49} the NHC-supported disilavinylidene (Z)-(SIDipp)Si=SiBr(Tbb) (δ_SiBr(Tbb)=86.0 ppm),¹⁷ the bromosilene Br(R_s)Si=CH(C₆H₃-3,5-^tBu₂) (85.2 ppm, R_s=C(SiMe₃)₂(CH₂^tBu)),⁵⁰ or the 1,2-dibromodisilene (E)-(Tbb)BrSi=SiBr(Tbb) (84.1 ppm),⁵¹ and is also noticeably high-field shifted in comparison to those of naturally polarized silenes (δ_Si=17.5–144.2 ppm).⁵² This suggests that the Si=C double bond of 1-Si is electronically different from the Si=C double bond of naturally polarized silenes and may be attributed to the cationic silyne character of 1-Si (Figure 4). In fact, the ²⁹Si chemical shift of 1-Si compares very well with that of the phosphane-supported cationic silyne [(2-(P(N^tBu)₂SiMe₂)-3-((Dipp)N)-norbornene)SiCPPh₂(NⁱPr₂)][B(Ar^F)₄] (δ_Si=−32.5 ppm, Ar^F=C₆H₃−3,5-(CF₃)₂).⁴² Notably, the ¹J(Si,C^VNL) coupling constant of 1-Si of 147 Hz is considerably larger than those of silenes (¹J(Si,C^sp2)≈84 Hz) and even slightly larger than the largest ¹J(Si,C^cent) reported so far for 1-silaallenes (142.4 Hz).^{39, 40}

In the ¹³C NMR spectrum the signal of the vinylidene carbon atom C^VNL appears at a much higher field (δ_C^VNL/ppm=142.1 (1-Si), 167.7 (1-Ge), 161.5 (1-GeMind)) compared to that of the central carbon atom of 1-tetrelaallenes (δ_C^cent=223.6–282.3 ppm, Table 2), but lies closer to that of the central carbon atom of carbodicarbenes containing at least one NHC group (δ_C^cent=110.2, 140.5 ppm, Table 2). Thus, the C^VNL chemical shifts of 1-E and 1-GeMind indicate an ylidic character of the C^VNL−C^NHC bond (C^VNL(δ−)−C^NHC(δ+)).

2.2 Studies of the Stereodynamics of 1-Si and 1-Ge by Dynamic NMR Spectroscopy

Compounds 1-Si and 1-Ge show dynamics in solution on the NMR time scale. Given the nearly C_s-symmetric molecular structure of 1-E (Figure 3) the C²/C⁶ bonded isopropyl groups of the enantiotopic Dipp substituents are diastereotopic and hence should give rise to two septet signals for the four methine protons in the intensity ratio of 2 : 2. Similarly, two ¹H NMR singlet signals in the intensity ratio of 18 : 18 would be expected for the diastereotopic SiMe₃ groups of the enantiotopic C^2,6-bonded disyl groups of the Tbb substituent. However, the ¹H NMR spectra of 1-E recorded at 273 K in (D₈)THF display only one methine septet signal (Figure 5 (left)) and in the case of 1-Ge one broad SiMe₃ signal (ESI, chapter 3.3).

Two dynamic processes were resolved by variable temperature (VT) ¹H NMR experiments in the temperature range of 183–333 K (ESI, chapter 3). The first process involves a NHC rotation about the C^VNL−C^NHC bond, as evidenced, for example, by the broadening-coalescencing-sharpening of the two methine septet signals (C^2,6−CHMe_AMe_B) of the Dipp groups in 1-Si and 1-Ge upon increasing the temperature from the slow exchange limit (Figure 5 (left); ESI, chapter 3.2.1). The second process involves a Tbb rotation about the E−C^Tbb bond (E=Si, Ge) leading to a site exchange of the SiMe₃ groups (ESI, chapter 3.2.2). Standard activation parameters of the NHC and the Tbb rotation were determined by Eyring analysis of the temperature dependent exchange rate constants (k/Hz), which were obtained by gNMR full lineshape analysis of the variable temperature (VT) ¹H NMR spectra (Figure 5 (right), Table 3). In addition, an analysis of the effect of the systematic errors in natural linewidth (Δν°_1/2) on the activation parameters was carried out, which was shown to give a minimal enthalpy-entropy compensation in Eyring analysis for the NHC rotation allowing reliable estimates of the standard activation enthalpy (Δ^≠H°) and standard activation entropy (Δ^≠S°) (ESI, chapter 3.3).⁵³

The standard activation enthalpy (Δ^≠H°) for the NHC rotation in 1-Si (34.4(1.9) kJ ⋅ mol⁻¹) and 1-Ge (30.5(1.9) kJ ⋅ mol⁻¹) is lower than that for the Tbb rotation (1-Si: Δ^≠H°=54.4(6.2) kJ ⋅ mol⁻¹; 1-Ge: Δ^≠H°=48.5(4.9) kJ ⋅ mol⁻¹). It is also lower than the standard activation enthalpy for the C=C bond rotation in a series of 1,1-diamino alkenes containing one or two acceptor groups (Δ^≠H°=71.0–98.8 kJ ⋅ mol⁻¹; ESI, chapter 3.4),⁵⁴ the latter being in general much lower than those of normal olefins (~270 kJ ⋅ mol⁻¹). Moreover, the standard free activation energy (Δ^≠G°) for the NHC rotation in 1-Si (45.2(3.0) kJ ⋅ mol⁻¹) and 1-Ge (43.1(3.1) kJ ⋅ mol⁻¹) is comparable to the free activation energy of C=C double bond rotation in a 1,1-diborylallene ((Δ^≠G(183 K)=36.1 kJ ⋅ mol⁻¹),⁵⁵ for which a strongly polarized C=C bond has been suggested to account for the facile planarization of the allene core.

Overall, these comparisons suggest a considerable push-pull character of the C^VNL−C^NHC partial double bond of 1-Si and 1-Ge in agreement with the results of −i) the NBO analysis showing 0.5 non-Lewis electrons in the π*(C^VNL−C^NHC) NBO (see below; ESI, chapters 6.5 and 6.6), ii) the NRT analysis revealing that natural Lewis structures with a C^VNL−C^NHC single bond contribute in total ~40 % to the resonance hybrid (Figure 4) and iii) the potential energy surface scans (PES) of the torsion angle ∠(Si−C^VNL−C^NHC−N1) of (1-Si)_calc showing that the coplanar rotamer (1-Si−C)_calc (∠(Si−C^VNL−C^NHC)=143.0°, θ_Si=C/NHC=7.0°) lies +45.0 kJ ⋅ mol⁻¹ higher in energy relative to (1-Si)_calc (∠(Si−C^VNL−C^NHC)=143.8°, θ_Si=C/NHC=87.6°; ESI, chapter 6.8).

Notably, the standard activation entropy for the NHC rotation in 1-Si and 1-Ge is considerably negative (Δ^≠S°/J ⋅ (mol ⋅ K)⁻¹=−36.2(8.4) (1-Si), −42.3(9.1) (1-Ge)). Large negative activation entropies were obtained by dynamic NMR spectroscopy for the C=C bond rotations of push-pull olefins with a planar ground-state structure in polar solvents, and were attributed to the charge separated transition-state featuring orthogonal arrangements of the two CR₂ fragments (Δ^≠S°/ J ⋅ (mol ⋅ K)⁻¹=(−27.2)–(−51.7)). In comparison, large positive activation entropies were found for the C=C bond rotation of twisted olefins with a considerable charge separation in the ground-state, for which rotation proceeds via a planar transition-state.⁵⁴ Taking this into consideration, the activation entropy of 1-Si and 1-Ge suggests a higher degree of charge separation in the transition-state than the ground-state during NHC rotation.

2.3 First Reactivity Study of 1-Si and 1-Ge: Synthesis and Characterization of NHC-Supported Bromogermynes 2-SiGe and 2-GeGe

Preliminary reactivity studies were carried out to probe the nature of the E=C^VNL bonds of 1-Si and 1-Ge. Both compounds undergo with GeBr₂(1,4-dioxane) at room temperature in benzene a 1,2-addition to the E=C^VNL bond to afford selectively the NHC-supported bromogermynes 2-SiGe and 2-GeGe, which after work-up were isolated as highly air-sensitive, yellow solids in high yields (Scheme 2, ESI, chapters 2.5 and 2.6). Compounds 2-SiGe and 2-GeGe are thermally stable solids, decomposing upon melting at 217 °C (2-SiGe) and 173 °C (2-GeGe).

The molecular structures of 2-SiGe and 2-GeGe were determined by sc-XRD analyses and feature a bent-dicoordinated Ge-center (∡(C^VNL−Ge−Br3)=101.97(4)° (2-SiGe), ∡(C^VNL-Ge2−Br3)=105.1(2)° (2-GeGe)) suggesting the presence of a stereochemically active lone pair at the Ge center (Figure 6). The bromogermyne cores of 2-SiGe and 2-GeGe display a distorted trans-bent geometry as evidenced by the torsion angle Br3−Ge/Ge2−C^VNL−E of 162.6(1)° and 149.7(1)°, respectively. The C^VNL center is trigonal planar coordinated (Σ∡(C^VNL)=359.0° (2-SiGe), 358.2° (2-GeGe)) and the NHC five-membered ring is considerably tilted out of the plane defined by the C^NHC, C^VNL and Ge atoms, as evidenced by the interplanar angle ϕ^NHC of 67.9(1)° (2-SiGe) and 88.8(3)° (2-GeGe). This conformation is different from that of the vinyl tetrylenes E{C(H)=NHC}₂ (E=Si, Ge; NHC=C[N(Dipp)CMe]₂),^{56, 57} which display a coplanar arrangement of the NHC five-membered ring (ϕ^NHC≈0°), but similar to that found for the NHC-supported ditetrelynes RE=ER(NHC) (ϕ^NHC=42.48(9)°–82.14(7)°; ESI, Table S24). Accordingly, the C^VNL−C^NHC bonds of 2-SiGe (1.444(2) Å) and 2-GeGe (1.447(7) Å) are significantly longer than those of the vinyl tetrylenes E{C(H)=NHC}₂ (1.375(2) Å (E=Si), 1.361(4) (E=Ge))^{56, 57} and also longer than those of 1-Si (1.358(3) Å) or 1-Ge (1.376(9) Å), and approach the values of C^sp2−C^sp2 single bonds (1.460 Å).⁵⁸ The Ge/Ge2−C^VNL bond lengths of 2-SiGe (1.923(1) Å) and 2-GeGe (1.921(5) Å) are 0.17 Å longer than the calculated Ge≡C triple bond length (1.754 Å) of the germyne (Tbt)Ge≡C(Tbt) (Tbt = C₆H₂−2,4,6-(CH(SiMe₃)₂)₃).⁵⁹ However, they are near the upper-end of the range of Ge=C^sp2 double bond lengths reported in the Cambridge Structural Database for germaethenes (1.770–1.894 Å), and are shorter than a typical Ge−C^sp2 single bond (2.01 Å). These comparisons suggest some π-bond character in the Ge/Ge2−C^VNL bonds of 2-SiGe and 2-GeGe, which is nicely reflected in the HOMO-1 of these compounds (ESI, chapter 6.11). A comparison of the d_E=E' bond lengths of the NHC-supported ditetrelynes RE=E'R(NHC) (E=Si, Ge; E’ = C, Si, Ge) with those of the ditetrelynes RE≡E'R shows that the magnitude of bond elongation Δd follows the trend Δd_GeC (0.17 Å)>Δd_SiSi (0.14 Å)>Δd_GeGe (0.06 Å) (ESI, Table S24) probably due to the increased E’−C^NHC interaction in the direction E'=C>Si>Ge.

Compounds 2-SiGe and 2-GeGe were also characterized by multi-nuclear NMR spectroscopy. The ¹H and ¹³C{¹H} NMR spectra in solution confirmed the solid-state structures of 2-SiGe and 2-GeGe and unraveled moreover as in case of 1-Si and 1-Ge hindered NHC and Tbb rotations about the respective C^VNL−C^NHC and E−C^Tbb bonds, which were studied by VT ¹H NMR experiments (ESI, chapter 3). Notably, the activation barriers Δ^≠G° for the NHC rotations in 2-SiGe and 2-GeGe are higher than in 1-Si and 1-Ge (Table 3), despite the longer C^VNL−C^NHC bonds in 2-SiGe and 2-GeGe compared to 1-Si and 1-Ge. This can probably be attributed to increased steric repulsion between the peripheral Dipp groups and the EBr₂Tbb group (E=Si, Ge) during NHC rotation in 2-SiGe and 2-GeGe leading to destabilization of the transition state. In the ¹³C{¹H} NMR spectra the chemical shift difference of the C^VNL and C^NHC atoms (2-SiGe: C^VNL=133.6 ppm, C^NHC=160.8 ppm; 2-GeGe: C^VNL=139.6 ppm, C^NHC=161.0 ppm) is indicative of a C^VNL(δ−)−C^NHC(δ+) bond polarization.

2.4 Quantum Chemical Calculations of (1-Si)_calc and (1-Ge)_calc

The electronic structure of 1-E (E=Si, Ge) was investigated by density functional theory (DFT) calculations at the B97-D3(BJ)−ATM/def2-TZVP level of theory (method-I). Gas phase structure optimizations afforded a minimum structure (1-E)_calc with bonding parameters in very good agreement with the experiment (ESI, chapter 6). In particular, the 2-tetrelavinylidene core of (1-E)_calc features a bent E−C^VNL−C^NHC skeleton and a trigonal-planar coordinated tetrel center E ((1-Si)_calc: ∠(Si−C^VNL−C^NHC)=143.8°, Σ∡(Si)=359.8°; (1-Ge)_calc: ∠(Ge−C^VNL−C^NHC)=132.5°, Σ∡(Ge)=358.5°) as found by the sc-XRD studies. Remarkably, unlike 1-Si a linear Si−C−C core was previously predicted for the NHC-supported 2-silavinylidenes (NHC)C=SiR₂ (NHC=C[N(^tBu)CH]₂; R=Me, ^tBu, Dipp, F, Cl)³³ on the B3LYP/def2-TZVPP level of theory and for the 1-silaallenes CH₂=C=SiR₂ (R=H, Me, SiH₃) on various levels of theory.⁶⁰ Scans of the potential energy hypersurface (PES) of (1-Si)_calc showed a continuous increase of the energy upon varying the Si−C^VNL−C^NHC angle from 140–180° leading to a linear isomer that is only 16 kJ ⋅ mol⁻¹ higher in energy than (1-Si)_calc (ESI, chapter 6.2). Notably, inspection of the single-point structures at each scan step revealed that linearization of the E−C^VNL−C^NHC skeleton is not accompanied by an NHC-rotation or a pyramidalization of the tetrel center. These results indicate, that the skeleton linearization of (1-Si)_calc is a low energy process, which however, cannot explain the stereodynamics in solution observed by VT NMR spectroscopy originating from NHC-rotation. In fact, PES scans of the rotation process provided a maximum barrier to the NHC-rotation (ESI, chapter 6.8), which compares well with the experimental barrier (Table 3). The C^VNL−C^NHC bond cleavage energy (BCE) of (1-E)_calc, that is required to give the singlet fragments ¹NHC and ¹C=EBr(Tbb) in the frozen geometry they adopt in (1-E)_calc, amounts 609 kJ ⋅ mol⁻¹ (E=Si) and 571 kJ ⋅ mol⁻¹ (E=Ge), respectively. The large BCE values suggest that 1-E can rather be described as imidazolium-2-tetrelavinylides. The 2-tetrelavinylidenes C=EBr(Tbb) have a triplet ground state with a small triplet-singlet gap ΔE_S→T (E_triplet–E_singlet) of −32 kJ ⋅ mol⁻¹ (E=Si) and −11 kJ ⋅ mol⁻¹ (E=Ge), respectively. Notably, a comparison of the BCE(C^VNL−C^NHC) with the sum of the triplet-singlet gaps (∑ΔE_S→T) of the fragments NHC (ΔE_S→T=330 kJ ⋅ mol⁻¹) and C=EBr(Tbb) shows that $$ BCE≈∑ΔE_S→T. This hints to a flat potential energy surface for the E−C^VNL−C^NHC bending in 1-E as predicted by the CGMT-model.^{61, 62}

Analysis of the canonical molecular orbitals (MOs) of (1-Si)_calc shows that the HOMO-1 is the π(Si−C^VNL)_oop (oop: out of plane of the 2-silavinylidene core) with the LUMO being the π*(Si−C^VNL)_oop counterpart that is concentrated at Si. The HOMO is a multi-center π-orbital (Si−C^VNL/NHC^ring) with a large contribution of C^VNL, that is bonding between C^VNL and Si and non-bonding between C^VNL and C^NHC (Figure 7 and ESI, chapter 6.4).

NBO analysis of the wave function of (1-Si)_calc gave a leading natural Lewis structure with localized bond pair NBOs for the σ(Si−C^VNL), π(Si−C^VNL), σ(C^VNL−C^NHC), π(C^VNL−C^NHC), σ(Si−C^Tbb) and σ(Si−Br) bonds with a high Lewis electron occupancy of ≥ 1.9e (Table 4). Both the π(Si−C^VNL) and π(C^VNL−C^NHC) NBOs as well as the σ(Si−C^VNL) NBO are strongly polarized toward the C^VNL carbon atom (65–72 %) rationalizing the high charge accumulation at the C^VNL atom (Table 4). NBO analysis of (1-Ge)_calc gave very similar results (ESI, chapter 6.6).

Considerable delocalization is evidenced in (1-Si)_calc by the low Lewis occupancy of the lone pair NBOs LP(N1) and LP(N2) (1.61e each), the presence of non-Lewis electrons in the anti-bonding NBOs π*(C^VNL−C^NHC) (0.48e) and σ*(Si−Br) (0.20e) (Table 4), and a second order perturbation energy ∑(E⁽²⁾) of 346 kJ ⋅ mol⁻¹ for the two LP(N1/N2)→π*(C^VNL−C^NHC) π-type donor-acceptor interactions as well as an E⁽²⁾ energy of 46 kJ ⋅ mol⁻¹ for the π(C^VNL−C^NHC)→σ*(Si−Br) negative hyperconjugation, in line with the results of the NRT analysis (Figure 4; ESI, chapter 6.5).

Natural population analysis (NPA) reveals a considerable charge flow from the NHC to the 2-tetrelavinylidene fragment C=EBr(Tbb) of (1-E)_calc resulting in a charge accumulation in the C=EBr(Tbb) fragment (∑q(C=EBr(Tbb))=−0.36 (E=Si), −0.38 (E=Ge)) and a strongly negative NPA charge at the C^VNL center of −1.00 ((1-Si)_calc) and −0.89 ((1-Ge)_calc), which in combination with the strongly positive NPA charge at the tetrel center (q(Si)=+1.38, q(Ge)=+1.23) points to a very polar E^(δ+)−C^VNL(δ−) bond. This is in line with the results of NRT analysis revealing a considerable ionic contribution (40 % ((1-Si)_calc), 37 % ((1-Ge)_calc)) to the total NRT bond order of the E−C^VNL bond (NRT-BO: 2.0 ((1-Si)_calc), 1.9 ((1-Ge)_calc)) providing a rationale for the unusual shortening of the E=C^VNL bond. Notably, q(C^VNL) of (1-E)_calc lies between the NPA charge of the central carbon atom in the carbodicarbene C(NHC)₂ (NHC=C[N(Me)CH]₂) (q(C^cent)=−0.50) and the carbodiphoshorane C(PMe₃)₂ (q(C^cent)=−1.47).⁶³