Acceleration of CO2 insertion into metal hydrides: ligand, Lewis acid, and solvent effects on reaction kinetics

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Nitrous oxide activation by picoline-derived Ni-CNP hydrides

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Performance of homogeneous catalysts viewed in dynamics

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

Does the Spin State and Oriented External Electric Field Boost the Efficiency of Fe(II) Pincer Catalyst toward CO2 Hydrogenation Reaction?

In this study, we have explored the catalytic reactivities of four PNP-pincer supported Fe(II) complexes, namely, [(^iPrPN^MeP)FeH₂(CO)] (1), [(^iPrPN^MeP)FeH(CO)(BH₄)] (2), [(^iPrPN^HP)FeH₂(CO)] (3), and [(^iPrPN^MeP)FeH(BH₄)] (4) (^iPrPN^MeP = MeN{CH₂CH₂(PⁱPr₂)}₂ and ^iPrPN^HP = HN{CH₂CH₂(PⁱPr₂)}₂) toward reductive CO₂ hydrogenation for formate production. Our density functional theory and ab initio complete active space self-consistent field study have identified three fundamental steps in this catalytic transformation: (i) anchoring of the CO₂ molecule in the vicinity of the metal using noncovalent interactions, (ii) catalyst regeneration via H₂ cleavage, and (iii) formate rebound step leading to catalytic poisoning. The variations in the catalytic efficiency observed among these catalysts were attributed to either easing of steps (i) and (ii) or the hampering step (iii). This can be achieved in various chemical/non-chemical ways, for instance, (a) incorporation of strong-field ligands such as CO facilitating single-state reactivity and eliminating two-state reactivity that generally enhances the rate and (b) inclusion of Lewis acids such as LiOTf and strong bases found to either avoid catalytic poisoning or ease the H-H cleavages, to enhance the rate of reaction (c) evading mixing of excited open-shell singlet states to the ground closed-shell singlet state that hampers the catalytic regeneration. We have probed the role of oriented external electric fields (OEEFs) in the entire mechanistic profile for the best and worst catalyst, and our study suggests that imposing OEEFs opposite to the reaction axis (z-axis) fastens the catalytic regeneration step and, at the same time, hampers catalytic poisoning. The application of OEEFs is found to regulate the energetics of various spin states and can hamper two-state reactivity, therefore increasing the efficiency. Thus, this study provides insights into the CO₂ hydrogenation mechanism where the role of bases/Lewis acid, ligand design, spin states, and electric field in a particular direction has been established and is, therefore, likely to pave the way forward for a new generation of catalysts.

Large changes in hydricity as a function of charge and not metal in (PNP)M–H (de)hydrogenation catalysts that undergo metal–ligand cooperativity

Ligand pKa and metal hydricity scale with one another in (de)hydrogenation catalysts that undergo metal–ligand cooperativity, irrespective of metal or ligand identity. Anionic hydrides are significantly more hydridic than their neutral counterparts.

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re(^Rbpy)(CO)₃H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, ^tBu, Me, H, Br, COOMe, CF₃) to CO₂ and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(^Rbpy)(CO)₃H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Insights into the Capture of CO2 by Nickel Hydride Complexes

As a desired feedstock for sustainable energy source and for chemical synthesis, the capture and utilization of CO2 have attracted chemists’ continuous efforts. The homogeneous CO2 insertion into a nickel hydride complex to generate formate provides insight into the role of hydrogen as an active hydride form in the hydrogenation of CO2, which serves as a practicable approach for CO2 utilization. To parameterize the activities and to model the structure–activity relationship in the CO2 insertion into nickel hydride, the comprehensive mechanism of CO2 insertion into a series of square planar transition metal hydride (TM–H, TM = Ni, Pd, and Co) complexes was investigated using density functional theory (DFT) computations. The stepwise pathway with the TM-(H)-formate intermediate for the CO2 insertion into all seven square planar transition metal hydride (TM–H) complexes was observed. The overall rate-determining step (RDS) was the nucleophilic attraction of the terminal O atom on the Ni center in Ni-(H)-formate to form Ni-(O)-(exo)formate. The charge of the Ni atom in the axially vacant [Ni]+ complex was demonstrated as the dominant factor in CO2 insertion, which had an excellent linear correction (R2 = 0.967) with the Gibbs barrier (ΔG‡) of the RDS. The parameterized activities and modeled structure–activity relationship provided here light the way to the design of a more efficient Ni–H complex in the capture and utilization of CO2.

Recent progress in homogeneous hydrogenation of carbon dioxide to methanol

The requirement of running a new generation of fuel production is inevitable due to the limitation of oil production from reservoirs. On the other hand, enhancing the CO2 concentration in the atmosphere brings global warming phenomenon and leads to catastrophic disasters such as drought and flooding. Conversion of carbon dioxide to methanol can compensate for the liquid fuel requirement and mitigate CO2 emissions to the atmosphere. In this review, we surveyed the recent works on homogeneous hydrogenation of CO2 to CH3OH and investigated the experimental results in detail. We categorized the CO2 hydrogenation works based on the environment of the reaction, including neutral, acidic, and basic conditions, and discussed the effects of solvents’ properties on the experimental results. This review provides a perspective on the previous studies in this field, which can assist the researchers in selecting the proper catalyst and solvent for homogenous hydrogenation of carbon dioxide to methanol.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The kinetics of carbon dioxide insertion into a pincer-supported palladium methyl complex are studied. The complex inserts carbon dioxide at room temperature, and we explore both solvent and Lewis acid effects on carbon dioxide insertion.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives

Urgent solutions are needed to efficiently convert the greenhouse gas CO₂ into higher-value products. In this work, fac-Mn(bpy)(CO)₃Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO₂ conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et₃N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)₃Br and [Mn(bpy)(CO)₃]^- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et₃N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et₃N is replaced with Et₂NH. The origin of this difference lies in the ability of Et₃N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO₂, while with Et₂NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO₂ in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO₂-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO₂ to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(μ-H)3]3+ Clusters

CO₂ insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO₂ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe₃H₃L², 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe₃H₃L¹ (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO₂ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD₃ requires Fe-O bond formation in the product to offset an endergonic CO₂ insertion. μ₃-hydride coordination is noted to lower the activation barrier for the first CO₂ insertion event in DFT calculations.

Efficient carbon dioxide hydrogenation to formic acid with buffering ionic liquids

The efficient transformation of CO₂ into chemicals and fuels is a key challenge for the decarbonisation of the synthetic production chain. Formic acid (FA) represents the first product of CO₂ hydrogenation and can be a precursor of higher added value products or employed as a hydrogen storage vector. Bases are typically required to overcome thermodynamic barriers in the synthesis of FA, generating waste and requiring post-processing of the formate salts. The employment of buffers can overcome these limitations, but their catalytic performance has so far been modest. Here, we present a methodology utilising IL as buffers to catalytically transform CO₂ into FA with very high efficiency and comparable performance to the base-assisted systems. The combination of multifunctional basic ionic liquids and catalyst design enables the synthesis of FA with very high catalytic efficiency in TONs of >8*10⁵ and TOFs > 2.1*10⁴ h⁻¹.

Basic ionic liquids provide a buffering effect that enables the efficient synthesis of free formic acid from CO₂ hydrogenation. Here, a highly efficient catalytic system that transforms CO₂ to formic acid without the need of strong bases is demonstrated, avoiding the formation of formate salts.

Cobalt-catalyzed carboxylation of aryl and vinyl chlorides with CO2

The transition-metal-catalyzed carboxylation of aryl and vinyl chlorides with CO2 is rarely studied, and has been achieved only with a Ni catalyst or combination of palladium and photoredox. In this work, the cobalt-catalyzed carboxylation of aryl and vinyl chlorides and bromides with CO2 has been developed. These transformations proceed under mild conditions and exhibit a broad substrate scope, affording the corresponding carboxylic acids in good to high yields.

Visible-Light Photocatalytic Conversion of Carbon Dioxide by Ni(II) Complexes with N4S2 Coordination: Highly Efficient and Selective Production of Formate

The efficient and selective light-driven conversion of carbon dioxide to formate is a scientific challenge for green chemistry and energy science, especially utilizing visible-light energy and earth-abundant catalytic materials. In this report, two mononuclear Ni(II) complexes of pyridylbenzimidazole (pbi) and pyridylbenzothiazole (pbt), such as Ni(pbt)(pyS)₂ (1) and Ni(pbi)(pyS)₂ (2) (pyS = pyridine-2-thiolate), were prepared and their reactivities studied. The two Ni complexes were examined for CO₂ conversion using eosin Y as a photosensitizer upon visible-light irradiation in a H₂O/ethanol solvent. The photoreaction of CO₂ catalyzed by complexes 1 and 2 selectively affords formate with a high efficiency (14 000 turnover number) and a high catalytic selectivity of ∼99%. Undesirable proton reduction pathways were completely suppressed in the photocatalytic reactions with these sulfur-rich Ni catalysts under CO₂. Hydrogen photoproduction was also studied under argon. Their kinetic isotope effects and influence of solution pH for formate and H₂ production in the photocatalytic reactions are described in relation to the reaction mechanisms. These bioinspired Ni(II) catalysts with N/S ligation in relation to [NiFe]-hydrogenases are the first examples of early transition metal complexes affording such high selectivity and efficiencies, providing a future path to design solar-to-fuel processes for artificial photosynthesis.

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

Thermodynamic and kinetic hydricity of transition metal hydrides

This review of thermodynamic and kinetic hydricity provides conceptual overviews, tutorials on how to determine hydricity both experimentally and computationally, and salient case studies.

Harmonizing the Electronic Structures of the Adsorbate and Catalysts for Efficient CO2 Reduction

In CO₂ electroreduction, the critical bottleneck lies in the CO₂ activation which requires high overpotentials. CO₂ activation is related to both the electronic structures of catalysts and those of adsorbates, thus an ideal catalyst should match its electronic structures with those of the adsorbate. Here, we harmonized the electronic structures of the adsorbate and Mn-doped In₂S₃ nanosheets for efficient CO₂ reduction. The introduction of Mn dopants into In₂S₃ nanosheets enhanced both the Faradaic efficiency (FE) for carbonaceous products and current density (j). At -0.9 V vs RHE, Mn-doped In₂S₃ nanosheets exhibited a remarkable FE of 92% for carbonaceous product at a high j of 20.1 mA cm^-2. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-doped In₂S₃ nanosheets during the activation of CO₂. Due to the unique electronic structures of the coadsorbed configurations, Mn-doped In₂S₃ nanosheets exhibited an energy barrier for CO₂ activation into HCOO* lower than that over pristine In₂S₃ nanosheets.

Synthesis of porous coordination polymers using carbon dioxide as a direct source

Porous coordination polymers (PCPs) were synthesized by using CO₂ and metal borohydrides as precursors. Borohydrides converted CO₂ into bridging ligands such as formate (HCO₂^-) or formylhydroborate ([BH(OCHO)₃]^-) which are available to construct porous architectures; one of them shows 380 m² g^-1 surface area.

Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies

The reduction of CO2 to formic acid by transition metal hydrides is a potential pathway to access reactive C1 compounds. To date, no kinetic study has been reported for insertion of a bridging hydride in a weak-field ligated complex into CO2; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(μ-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane (1) with CO2 monitored by 1H-NMR and temperature-controlled UV-vis spectroscopy. We found that 1 reacts with CO2 to traverse the reported monoformate (1-CO 2 ) and a diformate complex (1-2CO 2 ) at 298 K in toluene, and ultimately yields the triformate species (1-3CO 2 ) at elevated temperature. The second order rate constant, H/D kinetic isotope effect, ∆H ‡,and ∆S ‡for formation of 1-CO 2 were determined as 8.4(3)×10-4 M-1·s-1, 1.08(9), 11(1) kcal·mol-1, and -3(1)×10 cal·mol-1·K-1, respectively at 298 K. These parameters suggest that CO2 coordination to the iron centers does not coordinate prior to the rate controlling step whereas Fe-H bond cleavage does.

Development of an Improved System for the Carboxylation of Aryl Halides through Mechanistic Studies

The nickel-catalyzed carboxylation of organic halides or pseudohalides using carbon dioxide is an emerging method to prepare synthetically valuable carboxylic acids. Here, we report a detailed mechanistic investigation of these reactions using the carboxylation of aryl halides with (PPh₃)₂Ni^IICl₂ as a model reaction. Our studies allow us to understand several general features of nickel-catalyzed carboxylation reactions. For example, we demonstrate that both a Lewis acid and halide source are beneficial for catalysis. To this end, we establish that heterogeneous Mn(0) and Zn(0) reductants are multifaceted reagents that generate noninnocent Mn(II) or Zn(II) Lewis acids upon oxidation. In a key result, a rare example of a well-defined nickel(I) aryl complex is isolated, and it is demonstrated that its reaction with carbon dioxide results in the formation of a carboxylic acid in high yield (after workup). The carbon dioxide insertion product undergoes rapid decomposition, which ca These three oxidation states correspond to the onbe circumvented by a ligand metathesis reaction with a halide source. Our studies have led to both a revised mechanism and the development of a broadly applicable strategy to improve reductive carboxylation reactions. A critical component of this strategy is that we have replaced the heterogeneous Mn(0) reductant typically used in catalysis with a well-defined homogeneous organic reductant. Through its use, we have increased the range of ancillary ligands, additives, and substrates that are compatible with the reaction. This has enabled us to perform reductive carboxylations at low catalyst loadings. Additionally, we demonstrate that reductive carboxylations of organic (pseudo)halides can be achieved in high yields in more practically useful, non-amide solvents. Our results describe a mechanistically guided strategy to improve reductive carboxylations through the use of a homogeneous organic reductant, which may be broadly translatable to a wide range of cross-electrophile coupling reactions.

Isolation of the Metalated Ylides [Ph3 P-C-CN]M (M=Li, Na, K): Influence of the Metal Ion on the Structure and Bonding Situation

The isolation and structural characterization of the cyanido-substituted metalated ylides [Ph3 P-C-CN]M (1-M; M=Li, Na, K) are reported with lithium, sodium, and potassium as metal cations. In the solid-state, most different aggregates could be determined depending on the metal and additional Lewis bases. The crown-ether complexes of sodium (1-Na) and potassium (1-K) exhibited different structures, with sodium preferring coordination to the nitrogen end, whereas potassium binds in an unusual η2 -coordination mode to the two central carbon atoms. The formation of the yldiide was accompanied by structural changes leading to shorter C-C and longer C-N bonds. This could be attributed to the delocalization of the free electron pairs at the carbon atom into the antibonding orbitals of the CN moiety, which was confirmed by IR spectroscopy and computational studies. Detailed density functional theory calculations show that the changes in the structure and the bonding situation were most pronounced in the lithium compounds due to the higher covalency.