Temperature 13C NMR Spectra Research Articles

Unveiling the mutual effects of hydroxyl and dibenzhydryl groups in α‐diimino‐nickel precatalysts for direct synthesis of polyethylene elastomers

The properties and practical applications of polyethylene are closely associated with the polymer molecular weight, polydispersity, degree of branching, and the type of branches and its arrangement within microstructure. In this study, a set of structurally diverse unsymmetrical 1‐(2,6‐dibenzhydryl‐4‐hydroxylphenylimino)‐2‐(2,6‐(R)‐4‐(R1)phenylimino)acenaphthene‐nickel dibromide precatalysts (where Ni2Me [R = Me, R1 = H], Ni2Et [R = Et, R1 = H], Ni2iPr [R = iPr, R1 = H], Ni3Me [R, R1 = Me], Ni2Et,Me [R = Et, R1 = Me]) has been prepared and studied for ethylene polymerization. Molecular structure analysis of Ni2Me and Ni2Et complexes revealed distorted tetrahedral geometries at nickel. The polymerization activities are extremely high for all precatalysts upon activation with either EASC or MMAO aluminum reagents, a trend notably pronounced in the case of the EASC‐activated systems, entering activity in the range of 10 million g mol−1 h−1 at 30°C. The Ni2Me demonstrated exceptional performance at higher temperatures, achieving an activity of 2.4 × 106 g mol−1 h−1 at 100°C and generated high molecular weight polyethylene (Mw = 105 g mol−1) with narrow polymer dispersity across all reaction temperatures (Ð ≈ 1.5). High temperature 13C NMR spectra identified 97–142/1000C branches in resulting polyethylene, a feature significantly dependent on the reaction temperature. In terms of mechanical properties, stress–strain analysis indicated that polyethylene synthesized at lower temperatures displayed superior tensile strength compared with that produced at higher reaction temperatures, while a reverse trend was observed in strain recovery analysis. The high strain recovery (up to 72%) of these polyethylene highlights the characteristic properties of thermoplastic elastomers.

Dynamic solution behaviour of metal complexes of the hexaamine cage ligand Me8tricosane

The solid state structures of the hexaamine cage complexes [M(Me8tricosane)]2+ (M = Zn, Cd, Hg) were previously shown to feature highly symmetric octahedral or trigonal prismatic coordination geometries. Here we show that the solution NMR spectra of each of these three complexes are far more complex than what would be expected if the solid state structures were retained after dissolution. The large numbers of resonances observed in the solution NMR spectra reflects establishment of equilibria involving multiple diastereoisomers with different nitrogen atom configurations. Support for this explanation was provided by the 15N HSQC spectrum of the Zn complex, and by variable temperature 1H and 13C NMR spectra of each of the three metal complexes. In addition, the results of calculations performed using DFT methods showed that the differences in energy between diastereoisomers were sometimes very small, and therefore unlikely to present a significant barrier to isomerisation processes.

Solvent Effects on the UV Absorption Spectrum of Carmofur

In this article, we reported that carmofur could be induced by some solvent to produce conformational alteration. Ultraviolet (UV) spectra were used to study the conformation alteration of carmofur. Upon the addition of acid in the some solvent, UV spectroscopy of carmofur could change gradually. When base was added to this system, UV spectroscopy of carmofur could return to the original state, and the change process was reversible. The variable temperature 1H and 13C-NMR spectrum were used to testify that temperature did not have any effect on the conformation alteration of carmofur in Acetonitrile: Trifluoroacetic-acid (9:1). These two conformers of carmofur were structurally stable in Acetonitrile: Trifluoroacetic-acid (9:1).

Synthesis, spectral (IR, UV–Vis and variable temperature NMR) characterization and crystal structure of (N-benzyl-N-furfuryldithicarbamato-S,S′)(thiocyanato-N)(triphenylphosphine)nickel(II)

Planar (N-benzyl-N-furfuryldithiocarbamato-S,S′)(thiocyanato-N)(triphenylphospine)nickel(II), [Ni(bfdtc)(NCS)(PPh3)], (1) was prepared from bis(N-benzyl-N-furfuryldithiocarbamato-S,S′)nickel(II), [Ni(bfdtc)2], (2) and characterized by elemental analysis, cyclic voltammetry, electronic, IR and variable temperature 1H and 13C NMR spectra. For complex 1, the thioureide vCN value is shifted to higher wavenumber compared to 2 and N13CS2 carbon signal observed for 1 is additionally shielded compared to the parent complex 2, suggesting increased strength of the thioureide bond due to the presence of the π-accepting phosphine. In the room temperature 13C NMR spectrum of 1, two pseudo doublets are observed in the aliphatic region. Variable temperature 13C NMR spectral studies suggest that the fast thiocyanate exchange appears to be responsible for the appearance of pseudo doublets. Single crystal X-ray structural analysis of 1 and 2 confirm the presence of four coordinated nickel in a distorted square planar arrangement with the NiS2PN and NiS4 chromophores, respectively. The NiS bonds are symmetric in 2 (2.1914(14) and 2.2073(13)Å). But significant asymmetry in NiS bond distances was observed in 1 (2.2202(8)Å and 2.1841Å). This observation clearly supports the less effective trans effect of SCN− over PPh3. Cyclic voltammetric studies revealed easier reduction of nickel(II) to nickel(I) in complex 1 compared to 2.

アコーディオン伸縮振動するπ共役系らせん高分子の合成とらせんピッチの制御

Poly(alkyl propiolate)s (PAP), e.g., Poly(n-heptyl propiolate) (PnHepP) and Poly((s)-2-octyl propiolate) (Ps2OctP) having an achiral or a chiral ester side chain were stereoregularly prepared with a Rh complex catalyst, [Rh(nbd)Cl]2 (nbd=norbornadiene), at 40 °C in methanol, respectively. The temperature dependence 1H and 13C NMR spectra of PAPs and their spectral simulations indicated that the restricted rotation around the ester O-C bond takes place through the so-called 3-site jump conformation. The spectra also exhibited that the helical conjugated main-chain of Ps2OctP was largely oscillating between the sequences of contracted helix and stretched helix in the time scale of the NMRs, although the degree of the oscilation of PnHepP was fairly small compared to that of Ps2OctP. The UV-vis and IR spectra of them supported that PAPs have at least two helical conformations, i.e., contracted and stretched helices. Therefore, these spectra supported the oscillation of the helix along the main-chain axis. The proposed model calculated by MMFF94 also indicated the accordion-like helix oscillation of contracted and stretched helices (HELIOS) of PAPs which was synchronized with restricted 3-site jump rotation around the ester O-C bond in the side-chain.

Studies on the s-cis–trans isomerism for some furan derivatives through IR and NMR spectroscopies and theoretical calculations

The s-cis–trans isomerism of two furan derivatives [2-acetyl- (AF) and 2-acetyl-5-methylfuran, (AMF)] was analyzed, using data from the deconvolution of their carbonyl absorption band in two solvents (CH2Cl2 and CH3CN). These infrared data showed that the O,O-trans conformers predominate in the less polar solvent (CH2Cl2), but these equilibria change in a more polar solvent (CH3CN) leading to a slight predominance of the O,O-cis conformers, in agreement with the theoretical calculations. The later results were obtained using B3LYP-IEFPCM/6-31++g(3df,3p) level of theory, which taking into account the solvent effects at IEFPCM (Integral Equation Formalism Polarizable Continuum Model). Low temperature 13CNMR spectra in CD2Cl2 (ca. −75°C) showed pairs of signals for each carbon, due to the known high energy barrier for the cis–trans interconversion leading to a large predominance of the trans isomers, which decreases in acetone-d6. This was confirmed by their 1HNMR spectra at the same temperatures. Moreover, despite the larger hyperconjugative interactions for the O,O-cis isomers, obtained from NBO data, these isomers are destabilized by the their Lewis energy.

Vibrational assignments, normal coordinate analysis, B3LYP calculations and conformational analysis of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate

The Raman and infrared spectra of solid methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (MAMPC, C 7H 8N 4S 3) were measured in the spectral range of 3700–100 cm −1 and 4000–200 cm −1 with a resolution of 4 and 0.5 cm −1, respectively. Room temperature 13C NMR and 1H NMR spectra from room temperature down to −60 °C were also recorded. As a result of internal rotation around C–N and/or C–S bonds, eighteen rotational isomers are suggested for the MAMPC molecule (Cs symmetry). DFT/B3LYP and MP2 calculations were carried out up to 6-311++G(d,p) basis sets to include polarization and diffusion functions. The results favor conformer 1 in the solid (experimentally) and gaseous (theoretically) phases. For conformer 1, the two –CH 3 groups are directed towards the nitrogen atoms (pyrazole ring) and C S, while the –NH 2 group retains sp 2 hybridization and C–C N bond is quasi linear. To support NMR spectral assignments, chemical shifts ( δ) were predicted at the B3LYP/6-311+G(2d,p) level using the method of Gauge-Invariant Atomic Orbital (GIAO) method. Moreover, the solvent effect was included via the Polarizable Continuum Model (PCM). Additionally, both infrared and Raman spectra were predicted using B3LYP/6-31G(d) calculations. The recorded vibrational, 1H and 13C NMR spectral data favors conformer 1 in both the solid phase and in solution. Aided by normal coordinate analysis and potential energy distributions, confident vibrational assignments for observed bands have been proposed. Moreover, the CH 3 barriers to internal rotations were investigated. The results are discussed herein are compared with similar molecules whenever appropriate.

Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

The s- cis– trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl 4 and CHCl 3). These infrared data showed that the O,S- cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS 2/CD 2Cl 2 (−90 °C) and in acetone- d 6 (−80 °C) did not show pairs of signals for each carbon, due to the known low energy barrier (∼8 kcal mol −1) for the cis– trans interconversion. Data from NBO calculations show that the n O ( 2 ) → σ S-C 5 * and n O ( 2 ) → σ C 2 – C 3 * interactions occur only in the O,S -cis isomer and can explain its conformational preference.

Conformational analysis of N -phenyl- and N -trifyl-4,4-dimethyl-4-silathiane 1-sulfimides

N-Substituted 4,4-dimethyl-4-silathiane 1-sulfimides [R = Ph (1), CF3 (2)] were studied experimentally by variable temperature dynamic NMR spectroscopy. Low temperature 13C NMR spectra of the two compounds revealed the frozen ring inversion process and approximately equal content of the axial and equatorial conformers. Calculations of the 4-silathiane derivatives 1, 2 and the model compound [R = Me (3)] as well as their carbon analogs, the similarly N-substituted thiane 1-sulfimides [R = Ph (4), CF3 (5), Me (6)] at the DFT/B3LYP/6–311G(d,p) level in the gas phase and in chloroform solution using the PCM model at the same level of theory showed a strong dependence of the relative stability of the conformer on the solvent. The electronegative trifluoromethyl group increases the relative stability of the axial conformer. Copyright © 2010 John Wiley & Sons, Ltd.

Interpretation of vibrational and NMR spectra of allyl acrylate: An evidence for several conformers

The mid-IR, far-IR, and Raman spectra of allyl acrylate were measured and interpreted with support of the B3LYP/aug-cc-pVDZ calculated anharmonic vibrational spectra followed by the potential energy distribution analysis. The experimental 1H and 13C NMR spectra of allyl acrylate dissolved in CDCl 3 or C 6D 6 were interpreted by means of the B3LYP/aug-cc-pVDZ-su2 calculated NMR chemical shifts and J( 1H, 1H) and J( 1H, 13C) coupling constants. Exactly ten stable allyl acrylate conformers (five s- cis and five s- trans) were found after careful B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ scan of the conformational space. The experimental IR and Raman spectra are in good agreement with the theoretical spectra of the most stable conformers 1 with a presence of the second stable conformer 2, both exhibiting cis arrangement of the acrylic moiety. There are however two bands in the IR spectra, at ca. 1270 and 1260 cm −1, that definitely indicate the conformers with trans arrangement of the acrylic moiety to be present in liquid allyl acrylate. The bands at ca. 2990 and 1650 cm −1 are suggested to be due to Fermi resonances engaging CH and C C stretching vibrations, respectively. The careful inspection of the room temperature 1H and 13C NMR spectra of allyl acrylate suggest that a dominating form of the allyl acrylate molecule in an inert solvent exhibits the cis conformation of the acrylic moiety and an extended allyl group.

Synthesis and Characterisation of the Cd(II) Complex of a Hexadentate(N4O2) Schiff Base Ligand; IR, NMR and Theoretical Studies

A new Cd(II) complex, Cd(L22pysa), prepared from the template condensation of a previously known ligand N(CH2CH2NH2)2(CH2C5H4N), (L22py), salicylaldehyde and [Cd(NO3)2].4H2O in equimolar ratio, has been characterised by IR and variable temperature 1H NMR and 13C NMR spectra. The gas-phase molecular structure of this complex was also studied by ab initio Hartree–Fock and DFT/B3LYP methods.

Ring Inversion Dynamics of Derivatives of Thianthrene Di- and Tetraoxide

The ring inversion barrier for thianthrene tetraoxide was determined by making use of the variable temperature 13C NMR spectra of the 2,7-diisopropyl derivative (DeltaG(double dagger) = 6.5 kcal mol(-1)). The barrier is lower than that measured for a trans thianthrene dioxide derivative (DeltaG(double dagger) = 9.35 kcal mol(-1)). These results agree well with ab initio theoretical predictions.

Generation of trans-spanning diphosphine ligands via alkene metathesis: Synthesis, structure, and dynamic behavior of a missing link in a series of square-planar platinum complexes

Reaction of KPPh 2 and Br(CH 2) 5CH CH 2 gives the phosphine PPh 2(CH 2) 5CH CH 2 (89%), which is treated with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C 6F 5)(S(CH 2CH 2–) 2)] 2 to yield trans-(Cl)(C 6F 5)Pt(PPh 2(CH 2) 5CH CH 2) 2 ( 4b, 80%). Ring-closing alkene metathesis (Grubbs’ catalyst) gives ▪ ( 5b, 84%), which features a trans-spanning diphosphine ligand. The Z/ E C C mixture is hydrogenated (1 atm, 10% Pd/C) to give ▪ ( 6b, 99%). The crystal structures of 4b and ( Z)- 5b are determined. In the former, both (CH 2) 5CH CH 2 moieties are directed on the same side of the platinum square plane. Low temperature 13C NMR spectra of 6b show two sets of signals for the diastereotopic P Ph 2 groups. These coalesce upon warming, which requires the passage of the chloride ligand through the macrocycle. Analysis by the complete bandwidth method gives Δ H ‡ and Δ S ‡ values of 6.0 ± 0.4 kcal mol −1 and −13.9 ± 2.6 eu. The 31P, 13C, and 2H CP/MAS NMR spectra of polycrystalline 6b and 6b- d 2 are studied, and indicate appreciable conformational mobility of the methylene chain in the solid state.

Reactions of 13CO with Ethoxycarbonylcarbene‐Bridged Dicobalt Carbonyl Complexes: [μ2‐{Ethoxycarbonyl(methylene)}‐μ2‐(carbonyl)bis(tricarbonylcobalt)(Co–Co)] and [Di‐μ2‐{ethoxycarbonyl(methylene)}bis(tricarbonylcobalt)(Co–Co)

AbstractIn a CH2Cl2 solution, under an atmospheric pressure of 13CO, the bridging and terminal CO ligands of the cobalt complex[μ2‐{ethoxycarbonyl(methylene)}‐μ2‐(carbonyl)bis(tricarbonylcobalt)(Co–Co)] (1) exchange with external 13CO at the same rate. The overall rate of CO exchange is kobs (10 °C) = 9.4 × 10–3 s–1 and kobs (25 °C) = (20.1 ± 1.2) × 10–3 s–1. In the presence of excess ethanol diethyl malonate (DEM), EtO213CCH2CO2Et is formed from 1 at a much smaller rate [kDEM (10 °C) = 0.40 × 10–5 s–1 and kDEM (25 °C) = (1.2 ± 0.1) × 10–5 s–1]. On the other hand, the complex [di‐μ2‐{ethoxycarbonyl(methylene)}bis(tricarbonylcobalt)(Co–Co)](2) does not exchange its CO ligands for 13CO at all at 10 or 25 °C. In the presence of excess ethanol, diethyl malonate with natural isotopic distribution, and 13CO, ligands containing complex 1 are formed simultaneously and at the same rate, which is kobs (10 °C) = (15.2 ± 1.1) × 10–5 s–1 and kobs (25 °C) = (33.5 ± 1.8) × 10–5 s–1. Variable temperature 13C NMR spectra of 1 and 2 reveal fluxional behavior for both complexes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Mechanism of the lithium–iodine exchange in an iodothiophene

Solutions of 2-lithio-5-methylthiophene (4) were characterized using DNMR techniques and shown to be a mixture of monomer and dimer in THF–Et2O (3:2). The hypervalent iodine ate complex 5 (Ar2I–Li+), a presumed intermediate in the Li–I exchange with 2-iodo-5-methylthiophene, was observed by 13C and 7Li NMR spectroscopy at low temperatures (–130 °C). At higher temperatures, the ate complex coalesced with 2-lithio-5-methylthiophene. A kinetic scheme was developed, which accounts for the exchange of the monomer 4M, dimer 4D, and 2-iodo-5-methylthiophene (6) with the ate complex 5. The rates of the various exchanges were obtained through a DNMR analysis of the variable temperature 13C and 7Li NMR spectra, and the thermodynamic and activation parameters were calculated. The monomer 4M and the ate complex 5 have similar reactivity as aryl donors in the Li–I exchange reaction, but 4M is at least 1000 times as reactive as the dimer 4D towards the iodide.Key words: halogen–metal exchange, lithium iodinate, iodine ate complex, lithium reagent, aggregate reactivity.

The first 1,3,2-diazabora-[3]ferrocenophanes – molecular structures and dynamic behaviour in solution

Dilithiated 1,1 ′-bis(trimethylsilylamino)ferrocene ( 1) reacts with aminoboron dihalides ( 2) X 2B-N(R ′)R [X=Br, R ′=R=Et ( 2a); X=Cl, R ′=Me, R=CH 2Ph ( 2b), X=Cl, R ′=Et, R=Ph ( 2c)] to give 2-amino-1,3,2-diazabora-[3]ferrocenophanes ( 3a– c) for the first time. The steric constraints exerted by the [3]ferrocenophane unit and the presence of the N-SiMe 3 groups cause rather different B–N bonding situations in these tri(amino)boranes. The boron atom has the choice between three nitrogen atoms for BN(pp)π bonding: in the cases of 3a and 3b, it prefers the NEt 2 and the N(Me)CH 2Ph group, respectively, over the N-SiMe 3 groups, whereas in 3c the N(Et)Ph group appears to be the weaker π-donor. This can be concluded from the X-ray structural analyses carried out for 3a and 3c, and from the low temperature 1H, 13C, and 29Si NMR spectra of 3a– 3c.

Investigation on uranyl interaction with bioactive ligands. Synthesis and structural studies of the uranyl complexes with glycine and N-(2-mercaptopropionyl)glycine

Summary Three new uranium(VI) compounds have been prepared with glycine (Hgly), N-(2-mercaptopropionyl)glycine (Hmpg) and 2,2′-dithiobis(N-propionylglycine) (tpg). The crystal structure of [UO2(Hgly)2Cl2] has been solved, revealing the uranium atom to be seven-co-ordinated. The two zwitterionic glycines are coordinated to uranium atom by the carboxylate groups, one in a monodentate and the other in a bidentate mode. Solid state studies and low temperature 1H, 13C, 2D COSY and HMQC solution NMR spectra of the [UO2(mpg)(CH3COO)], show that mpg coordinates uranium atom by the carboxylate group.

Interactions of Remote Alkyl Groups with Lanthanide Metal Centers: Synthesis, Characterization and Ligand Redistribution Reactions of Heterobimetallic Species Containing Trialkylaluminum Fragments

The reaction of [Ln(OAr)3]2 with four equivalents of trialkylaluminum leads to the formation of the bis-trialkylaluminum adducts (ArO)Ln[(μ-OAr)(μ-R)AlR2]2 [Ln = La, R = Me (1); Ln = La, R = Et (3); Ln = Sm, R = Et (4)]. The X-ray crystal structure of 1 reveals short La−C(bridging) distances of 2.800(5) and 2.759(5) Å. A reduced 1JC-H coupling constant of 110 Hz and a low energy ν(C−H) stretch in the solution and solid state IR spectra are consistent with a strong agostic La···H−C interaction in solution. Crystal data for 1: a = 11.708(2) Å; b = 18.416(4) Å; c = 20.949(4) Å; α = 90°; β = 90°; γ = 90°; V = 4516.8(15) Å3; Z = 4; R1 = 4.69%. The solid-state structure of the samarium ethyl derivative 4 reveals close contacts of 2.627(4) and 2.649(4) Å between the samarium center and the methylene carbons of the triethylaluminum groups. The room temperature 13C NMR spectrum of 4 exhibits a 1JC-H coupling constant of 102 Hz; additionally, the fluxional process that exchanges methyl groups in 1 and 2 is slow enough on the NMR time scale to allow distinct methylene groups in 4 to be observed. Crystal data for 4: a = 19.318(1) Å; b = 20.150(1) Å; c = 26.280(1) Å; α = 90°; β = 90°; γ = 90°; V = 10229.8(9) Å3; Z = 8; R1 = 3.53%. Thermolysis of 1−4 results in ligand redistribution to form [R2Al(OAr)]2 [R = Me, Et (5)] and other unidentified species. Crystal data for 5: a = 9.496(4) Å; b = 10.183(4) Å; c = 18.794(7) Å; α = 90.06(1)°; β = 92.394(7)°; γ = 114.976(6)°; V = 1645.6(11) Å3; Z = 2; R1 = 10.13%.

Bisquaric Acid: Unusual Solid State NMR, Electronic Structure, and a Predicted Order−Disorder Transition

The molecular structure of the recently synthesized compound bisquaric acid was investigated by high-resolution solid state 13C NMR spectroscopy and electronic structure calculations. Variable temperature 13C NMR spectra were found quite unusual compared with the solution phase spectra and not explainable on the basis of the literature data on squaric acid. In particular, the 13CsOH peaks were found to be coincident with the 13CdO peaks. An extensive electronic structure analysis using the density functional theory (B3LYP) was thus performed, including the calculation of the 13C shifts by the GIAO method. Indeed, the calculations indicate that the linker CsC bond has a substantial double-bond character, thereby leading to a strong deshielding of the CsOH carbons. In addition, the theoretical study shows significant differences between bisquaric acid and squaric acid in several molecular properties, such as aromaticity, hydrogen-bonding, dipole moment, and acid-base behavior. Unlike squaric acid, no phase transition was detected for bisquaric acid by 13C NMR. However, the theoretical results suggest the possibility of such a transition at temperatures lower than about 100 °C, but it should be probed by other methods such as specific heat measurements.

Crystalline (NN)C–M(NN) complexes: synthesis, structure, bonding and lability [M = Si, Ge, Sn or Pb; (NN) = 1,2-(ButCH2N)2C6H4

Each of the red-brown or red, isomorphous, crystalline adducts (NN)C–M(NN) [(NN) = 1,2-(ButCH2N)2C6H4 and M = Si, Ge, Sn or Pb] has been obtained from the carbene (NN)C and the appropriate silylene, germylene, stannylene or plumbylene M(NN) in benzene and crystallisation from a hydrocarbon. They are monomeric, with the three-co-ordinate Ccarb and M atoms in an almost planar (C) or pyramidal (M) environment. The C–M distances are more than ca. 10% longer than for a typical MII–C bond in an MR2 molecule. Variable temperature 13C and xM NMR spectra (xM = 29Si, 119Sn or 207Pb) in toluene-d8, as well as for the appropriate M(NN) and C(NN) precursors, have been recorded. The Ccarb and M chemical shift data show that the compounds readily dissociate in solution, their stability decreasing in the sequence Sn > Pb > Si > Ge. From the magnitude of the chemical shifts, their conformation and C–M distances of the adducts, it is concluded that the C–M bond in each adduct is best formulated as being electrostatic in nature, with the carbene moiety as electron donor and the M(NN) fragment as acceptor.