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B,C Covalent bond classification equivalency. Another important concept is the equivalent neutral class Figure 3C ; Parkin, the classification of the ligand changes with the presence of any charge or delocalization of charge. A donor ligand with x-electrons bound to a cationic metal center is equivalent to a donor ligand x-1 -electrons bound to a neutral metal center i. They also have an empty p orbital, which is available for backdonation Figure 4A.

Figure 4. For consistency in our discussion, we describe their bonding modes, differentiated by the extent of interaction between the metal orbitals and carbon's empty 2p orbitals. Again, the bonding mode depends on the electron accepting ability of the ligand framework.

Molecular orbitals

A more relevant bonding view of each example system will be further discussed in their corresponding sections. Carbene complexes are prevalent, well-discussed species in literature so we will not focus on presenting the extensive progress in the field of carbene chemistry. Interested readers are encouraged to review articles on this subject Ofele et al. Its protonated analog, cyclopropene, is a strained three-membered ring that is hugely thermodynamically unstable. The stability of the cation relative to the instability of the neutral species has elicited great interest in chemists and inspired synthetic and theoretical studies for decades.

Cyclopropenylium cations were first synthesized by Breslow when he synthesized triphenylcyclopropenylium cation Breslow, This was the first experimental verification of aromaticity in non-benzenoid molecules and it offered an important lesson: the energetic debt from ring strain can be compensated by aromatic stability Breslow and Chang, Although the cation has been widely investigated since its discovery, the number of metal-bound cyclopropenylium complexes is not as abundant.

A thorough review of this topic was presented by Komatsu and Kitagawa Stability from aromaticity was easily restored by abstraction of the R 2 group e. A variety of cyclopropenylidene complexes have since been reported. These carbene species have been extensively discussed and were thoroughly reviewed by Herrmann in Ofele et al. The bonding mode of cyclopropenylidene complexes is unambiguous and well-established by carbene chemistry and, therefore, will not be presented in this review. Figure 5. General synthesis for cyclopropenylium complexes: A Approach 1, B Approach 2.

C Influence of the ring substituent on the extent of back donation. They contain a gold atom that is bound to a formally divalent carbon atom and are applied in a variety of gold-catalyzed transformations.

Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital. Interactions. In. Chemistry Gr

Figure 6. A Bonding model for gold-carbene resonance structures. B Gem -diaurated carbocation species. Cationic gold I complexes have been extensively studied, so this review will focus only on what we consider to be one of the most intriguing species present during gold catalysis: the gem -diaurated carbocation species a carbocation that is stabilized by two gold atoms through Au-Au contacts Hashmi, Further, the substituents R 1 and R 2 influence this equilibrium. In order for efficient catalysis to take place, the equilibrium must be reversible, and the gem -diaurated species II is actually a less reactive off-cycle species than the corresponding vinyl gold I species I Brown et al.

This can be rationalized by the fact that the gem -diaurated species II is stabilized through Au-Au interactions, which makes it less reactive than the unstabilized vinyl gold I species I. The tolyl complex 7 represents the first 1,1-diaurated carbocation derived from benzene Nesmeyanov et al. According to its X-ray data, complex 8 shows an Au-C-Au bond angle of The two [AuPPh 3 ] units in the diaurated thienyl complex 9 have identical phosphorus environments based on the 31 P NMR, which shows only one peak.

The structure includes a short Au … Au distance of 2. The literature range of Au-Au distances for gem-diaurated compounds is 2. Since Hayter's report, other isolated cyclopropenylium-metal complexes were reported; Komatsu et al. The hapticity of the product depends on the ring's substituents, the metal, and the other ligands on the metal Figure 7.

Figure 7. The coordination mode and the ring-metal interaction are determined by the d electron count of the metal and by the ligand environment of the metal fragment as established by a molecular orbital approach developed by Hoffmann et al. Jemmis and Hoffmann, In short, the ML n group will adopt the position that maximizes stabilizing bonding interactions.

For the following discussion, the molecules will be arbitrarily split into neutral fragments, C 3 R 3 and ML n. Figure 8. Descritpion of bonding mode between cyclopropenium and metal fragments. C Frontier orbitals interaction for M — R 3 C 3 : c. Cyclopropenium-metal complexes are prepared from the reaction between a cyclopropenium cation and a metal precursor, often a salt leading to the formation of a neutral species Figure 7A ; Donaldson and Hughes, When the ML 3 fragment is bound to the ring in this way Figure 8c.

In general, the bonding mode in cyclopropenium-transition metal complexes depends on the metal involved.

Walter Kohn: Nobel Laureate

The nature of the metal also affects the electronic configuration of the complex Table 1. This suggests that the backdonation into the C 3 ring is most significant in 13a. Table 1.

Kundrecensioner

Compiled M-C 3 ring distance data Churchill et al. Importantly, they concluded that the back-donation was likely directed toward the formally positively charged C 3 ring and not toward the cyclopentadiene Tuggle and Weaver, b. Prior studies by Hughes et al. In this case, only two frontier orbitals are suitable to interact with the ring Figure 8c. Higher in energy is b 1 , which is hybridized out away from the L groups and toward the cyclopropenium ring. Even higher in energy is 3a 1 , which is cylindrically symmetrical and is also hybridized away from L groups.

McClure and Weaver's platinum complex in was the first report of this unsymmetrical bonding McClure and Weaver, In , Mealli et al. They determined that the Ph 3 P 2 M unit progressively moved over the face of the cyclopropenium cation. They concluded that a smaller distance between the metal and one of the carbons in the ring resulted in increased tilting and twisting of the phenyl group directly connected to it. These geometric changes caused longer exocyclic C-C distances because of the decreased conjugation between the phenyl groups and the cyclopropenium ring Gompper and Bartmann, This type of interaction is consistent with the model C described in Figure 4B.

All of these complexes are consistent with the orbital models in Figure 9A. Figure 9. In addition, these metal-stabilized arylenium cations can be easily characterized by their 13 C NMR spectra. Consequently, some bending of the sp 2 -hybridized carbocationic center toward the metal atom has always been observed, which indicates the formation of double bond. Figure In 24 , the positive charge of the carbenium center is delocalized into the cyclopropenylium ring, which results in a bending angle of 6.

These geometric changes can be rationalized by the orbital model in Figure 10A. This data experimentally confirmed the predicted tendency toward strong bending of C6 Andrianov et al. The neutral complex 28b shows a bending angle of The bending of C6 causes a bonding interaction between C6 and the Cr-centered e s orbital Albright et al. B Geometric parameters of complex 29 by X-ray analysis. C Geometric parameters of complexes by DFT calculations. The bending angles for complexes 29a Kreindlin et al.

This is because a larger atomic radius leads to more overlap of filled metal d orbitals with the carbenium p orbital, resulting in strong carbenium backdonation and a large tilt angle Figure 10A. According to the orbital model, the electron-rich ligand can increase the electron density on the metal atom, resulting in carbenium backdonation and a large tilt angle.

Additionally, increasing the electron density of the metal center e. Cr is well-established in its ability to stabilize carbocations, including benzylic, phenonium, and benzonorbornenyl cations Tantillo et al. In , the groups of Houk Merlic et al. This is consistent with the substituent effects observed in Figure 11A. In the case of the cation, the low-lying LUMO interacts strongly with the symmetric occupied hybrid metal orbitals.

The overlap between the Cr and benzylic cation orbitals especially the d z 2 -like metal orbital is increased in two ways: 1 the distortion of the benzylic cation from planarity and, 2 shifting the chromium away from the center of the ring. A Chromium-stabilized carbocations. B Selected stabilizing orbital interactions between Cr CO 3 and benzylic cation. Cobalt, especially cationic dicobalt propargyl complexes, have played a significant role in organic synthesis since their discovery Nicholas and Pettit, ; Nicholas, ; McGlinchey et al.

Both of these resonance forms provide stability to the carbocation. B Selected representative cationic bimetallic propargyl complexes. A wide range of cationic dicobalt propargyl complexes or similar heterobimetallic complexes have been synthesized and characterized 37 - 44 , Figure 13B ; Gruselle et al.

The propargyl cation always preferentially coordinates to one of the metal atoms in each cluster due to accumulation of positive charge. In studies of these heterobimetallic complexes, the propargyl cation prefers to coordinate to Mo and Fe instead of Co 39, 41 , and 42 Gruselle et al. In 37 , the distances between the carbocationic center and the cobalt atom are 3. Cations 38 - 40 do not undergo Wagner-Meerwein rearrangement as a result of their stabilization, which otherwise occurs readily for uncomplexed 2-alkynylbornyl cations.

Analogously, stabilization of a propargyl cation by a molybdenum center results in a shielding of the molybdenum carbonyl signals from approximately — ppm in 40 Gruselle et al. For 41 and 42 , stabilization of a propargyl cation by an iron center shifts the carbonyl resonances to approximately ppm, while the cobalt carbonyl resonances are around — ppm Osella et al. The preferential coordination with a heavier metal within the heterobimetallic cations 39, 40 , and 43 is consistent with the reactivities of complex 20 and the conclusion in Figure 9B.

However, in complexes , the carbocation is bound to the lighter element, Fe. Most of the reported transition metal-carbocation complexes can fall into one of these two categories. In general, heavier metal atoms have larger radii, which can lead to stronger backdonation of filled d orbitals on the metal atom and greater stabilization of carbocations.

8.4 Molecular Orbital Theory

This results in weaker transition metal-carbocation interactions. On the other hand, an electron-rich ligand can increase the electron density on the metal atom, resulting in carbenium backdonation and greater stabilization. Transition metal-stabilized carbocations have been observed and characterized throughout the last century, but there is no comprehensive summary of the bonding modes of these transition metal-carbocation complexes.

To our surprise, most of this research was conducted and reported before and little attention has been given to the field during the last decades, even though much remains unknown about their properties, reactivities, and carbocation interactions with other transition metals e.

Because of their considerable synthetic value, it is of great importance to bring these metal-carbocation interactions back to the interest of the scientific community. All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Albright, T. Structure adn reactivity in organometallic chemistry.

An applied molecular orbital approach. Tetrahedron 38, — Cycloheptatriene and fulvene Cr CO 3 complexes. Amgoune, A. Amouri, H. Amyes, T. Experiments and calculations for determination of the stabilities of benzyl, benzhydryl, and fluorenyl carbocations: antiaromaticity revisited. Andrianov, V. Bandar, J. Aminocyclopropenium ions: synthesis, properties, and applications. Behrens, U. Ein Fulven-eisen-komplex? Benitez, D.

A bonding model for gold i carbene complexes. Bleiholder, C. Organometallics 28, — Bouhadir, G. Complexes of ambiphilic ligands: reactivity and catalytic applications. Braunschweig, H. Breslow, R. Triarylcyclopropenium ions. Synthesis and stability in the phenyl p-anisyl series. Brown, T.

Mechanistic analysis of gold I -catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis gold vinyl species and reversible C-O bond formation. Cais, M. Cheng, J. Hydride affinities of carbenium ions in acetonitrile and dimethyl sulfoxide solution. Chetcuti, M. Propargylic cations stabilized on nickel-molybdenum and nickel-tungsten bonds. Organometallics 21, — Chiang, T. Churchill, M. Ciabattoni, J. Di-t-butylcyclopropenone and substituted Di-t-butylcyclopropenyl Cations. Davis, R. Deno, N. Carbonium ions. Ditchfield, R. Organometallics 12, — Donaldson, W. Drew, M. Cyclopropenyl and oxocyclobutenyl complexes of molybdenum.

Echavarren, A. Non-stabilized transition metal carbenes as intermediates in intramolecular reactions of alkynes with alkenes. El Amouri, H. New bonding modes, fluxional behavior, and reactivity in dinuclear complexes bridged by four-electron donor unsaturated hydrocarbons. Ellis, J. Organometallics 22, — Espinosa Ferao, A. Fulvenization as characteristic geometric distortion in electron deficient ferrocenes. Tetrahedron 73, — Fischer, E. On the way to carbene and carbyne complexes. Fomin, V. Frenking, G. Gade, L. Cooperative reactivity of early-late heterodinuclear transition metal complexes with polar organic substrates.

Ghilardi, C. Crystal structure of the platinum derivative. Gleiter, R. Structural considerations. Organometallics 26, — Gompper, R. English 17, — Green, B. Chemistry of Bis cyclo-octa-1,5-idene platinum : reactions with electrophiles. Green, M. A new approach to the formal classification of covalent compounds of the elements. Gruselle, M. Harris, R. Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition. Hashmi, A. Dual gold catalysis. Gold catalysis in total synthesis.

Hayter, R. Hill, E. Carbonium ion stabilization by metallocene nuclei. Hopkinson, M. Merging visible light photoredox and gold catalysis.

Hughes, R. Competition between ligand substitution and cyclopropenyl migration to carbon monoxide followed by ring expansion to give oxocyclobutenyl ligands. Organometallics 5, — Oxidative addition of cyclopropenyl cations to zerovalent molybdenum and tungsten centers. Organometallics 4, — Jemmis, E. Cleaving CC bonds in cyclopropenium ions. Jia, M. Counterion effects in homogeneous gold catalysis. ACS Catal. Jiang, Y.

The evolution of cyclopropenium ions into functional polyelectrolytes. Jones, J.

Possible erratum in solutions to Orbital Interactions in Chemistry 2ed - Chemistry Stack Exchange

The bonding mode for most of the Z-type ligands described in the literature is unambiguous Amgoune and Bourissou, ; Owen, In order to discuss the bonding model of carbocations in depth, it is important to remind the reader about an important concept—donation and backdonation. This concept is represented by the Dewar-Chatt-Duncanson model Figure 3A with regard to metal-olefin interactions Nelson et al. Figure 3A shows how the complex can be described either as a metal-olefin adduct from modest backbonding resulting in an L-type ligand , or as a metallacyclopropane derivative due to extensive backbonding where the olefin serves as an LZ ligand, otherwise known as an X 2 ligand.

The equivalence between an LZ and X 2 system comes from the fact that both types require the involvement of two metal orbitals Figure 3B ; Parkin, The extent of backdonation strongly depends on the nature of the metal center. For example, a metal with a pair of electrons residing in a high energy orbital will favor strong backbonding interactions because of energy matching.

Examples of these types of ligands are C 2 H 4 and CO. Figure 3. A Chatt-Deward-Duncanson model. B,C Covalent bond classification equivalency. Another important concept is the equivalent neutral class Figure 3C ; Parkin, the classification of the ligand changes with the presence of any charge or delocalization of charge. A donor ligand with x-electrons bound to a cationic metal center is equivalent to a donor ligand x-1 -electrons bound to a neutral metal center i.

They also have an empty p orbital, which is available for backdonation Figure 4A. Figure 4. For consistency in our discussion, we describe their bonding modes, differentiated by the extent of interaction between the metal orbitals and carbon's empty 2p orbitals. Again, the bonding mode depends on the electron accepting ability of the ligand framework. A more relevant bonding view of each example system will be further discussed in their corresponding sections. Carbene complexes are prevalent, well-discussed species in literature so we will not focus on presenting the extensive progress in the field of carbene chemistry.

Interested readers are encouraged to review articles on this subject Ofele et al. Its protonated analog, cyclopropene, is a strained three-membered ring that is hugely thermodynamically unstable. The stability of the cation relative to the instability of the neutral species has elicited great interest in chemists and inspired synthetic and theoretical studies for decades. Cyclopropenylium cations were first synthesized by Breslow when he synthesized triphenylcyclopropenylium cation Breslow, This was the first experimental verification of aromaticity in non-benzenoid molecules and it offered an important lesson: the energetic debt from ring strain can be compensated by aromatic stability Breslow and Chang, Although the cation has been widely investigated since its discovery, the number of metal-bound cyclopropenylium complexes is not as abundant.

A thorough review of this topic was presented by Komatsu and Kitagawa Stability from aromaticity was easily restored by abstraction of the R 2 group e. A variety of cyclopropenylidene complexes have since been reported. These carbene species have been extensively discussed and were thoroughly reviewed by Herrmann in Ofele et al. The bonding mode of cyclopropenylidene complexes is unambiguous and well-established by carbene chemistry and, therefore, will not be presented in this review.

Figure 5. General synthesis for cyclopropenylium complexes: A Approach 1, B Approach 2. C Influence of the ring substituent on the extent of back donation. They contain a gold atom that is bound to a formally divalent carbon atom and are applied in a variety of gold-catalyzed transformations. Figure 6. A Bonding model for gold-carbene resonance structures. B Gem -diaurated carbocation species. Cationic gold I complexes have been extensively studied, so this review will focus only on what we consider to be one of the most intriguing species present during gold catalysis: the gem -diaurated carbocation species a carbocation that is stabilized by two gold atoms through Au-Au contacts Hashmi, Further, the substituents R 1 and R 2 influence this equilibrium.

In order for efficient catalysis to take place, the equilibrium must be reversible, and the gem -diaurated species II is actually a less reactive off-cycle species than the corresponding vinyl gold I species I Brown et al. This can be rationalized by the fact that the gem -diaurated species II is stabilized through Au-Au interactions, which makes it less reactive than the unstabilized vinyl gold I species I.

The tolyl complex 7 represents the first 1,1-diaurated carbocation derived from benzene Nesmeyanov et al. According to its X-ray data, complex 8 shows an Au-C-Au bond angle of The two [AuPPh 3 ] units in the diaurated thienyl complex 9 have identical phosphorus environments based on the 31 P NMR, which shows only one peak.

The structure includes a short Au … Au distance of 2. The literature range of Au-Au distances for gem-diaurated compounds is 2. Since Hayter's report, other isolated cyclopropenylium-metal complexes were reported; Komatsu et al. The hapticity of the product depends on the ring's substituents, the metal, and the other ligands on the metal Figure 7.

Figure 7. The coordination mode and the ring-metal interaction are determined by the d electron count of the metal and by the ligand environment of the metal fragment as established by a molecular orbital approach developed by Hoffmann et al. Jemmis and Hoffmann, In short, the ML n group will adopt the position that maximizes stabilizing bonding interactions.

For the following discussion, the molecules will be arbitrarily split into neutral fragments, C 3 R 3 and ML n. Figure 8. Descritpion of bonding mode between cyclopropenium and metal fragments. C Frontier orbitals interaction for M — R 3 C 3 : c. Cyclopropenium-metal complexes are prepared from the reaction between a cyclopropenium cation and a metal precursor, often a salt leading to the formation of a neutral species Figure 7A ; Donaldson and Hughes, When the ML 3 fragment is bound to the ring in this way Figure 8c.

In general, the bonding mode in cyclopropenium-transition metal complexes depends on the metal involved. The nature of the metal also affects the electronic configuration of the complex Table 1. This suggests that the backdonation into the C 3 ring is most significant in 13a. Table 1. Compiled M-C 3 ring distance data Churchill et al.

Importantly, they concluded that the back-donation was likely directed toward the formally positively charged C 3 ring and not toward the cyclopentadiene Tuggle and Weaver, b. Prior studies by Hughes et al. In this case, only two frontier orbitals are suitable to interact with the ring Figure 8c. Higher in energy is b 1 , which is hybridized out away from the L groups and toward the cyclopropenium ring. Even higher in energy is 3a 1 , which is cylindrically symmetrical and is also hybridized away from L groups.

McClure and Weaver's platinum complex in was the first report of this unsymmetrical bonding McClure and Weaver, In , Mealli et al. They determined that the Ph 3 P 2 M unit progressively moved over the face of the cyclopropenium cation. They concluded that a smaller distance between the metal and one of the carbons in the ring resulted in increased tilting and twisting of the phenyl group directly connected to it.

These geometric changes caused longer exocyclic C-C distances because of the decreased conjugation between the phenyl groups and the cyclopropenium ring Gompper and Bartmann, This type of interaction is consistent with the model C described in Figure 4B. All of these complexes are consistent with the orbital models in Figure 9A. Figure 9. In addition, these metal-stabilized arylenium cations can be easily characterized by their 13 C NMR spectra.

Consequently, some bending of the sp 2 -hybridized carbocationic center toward the metal atom has always been observed, which indicates the formation of double bond. Figure In 24 , the positive charge of the carbenium center is delocalized into the cyclopropenylium ring, which results in a bending angle of 6. These geometric changes can be rationalized by the orbital model in Figure 10A.

This data experimentally confirmed the predicted tendency toward strong bending of C6 Andrianov et al. The neutral complex 28b shows a bending angle of The bending of C6 causes a bonding interaction between C6 and the Cr-centered e s orbital Albright et al. B Geometric parameters of complex 29 by X-ray analysis. C Geometric parameters of complexes by DFT calculations. The bending angles for complexes 29a Kreindlin et al.

This is because a larger atomic radius leads to more overlap of filled metal d orbitals with the carbenium p orbital, resulting in strong carbenium backdonation and a large tilt angle Figure 10A. According to the orbital model, the electron-rich ligand can increase the electron density on the metal atom, resulting in carbenium backdonation and a large tilt angle.

Additionally, increasing the electron density of the metal center e. Cr is well-established in its ability to stabilize carbocations, including benzylic, phenonium, and benzonorbornenyl cations Tantillo et al. In , the groups of Houk Merlic et al. This is consistent with the substituent effects observed in Figure 11A. In the case of the cation, the low-lying LUMO interacts strongly with the symmetric occupied hybrid metal orbitals. The overlap between the Cr and benzylic cation orbitals especially the d z 2 -like metal orbital is increased in two ways: 1 the distortion of the benzylic cation from planarity and, 2 shifting the chromium away from the center of the ring.

A Chromium-stabilized carbocations. B Selected stabilizing orbital interactions between Cr CO 3 and benzylic cation. Cobalt, especially cationic dicobalt propargyl complexes, have played a significant role in organic synthesis since their discovery Nicholas and Pettit, ; Nicholas, ; McGlinchey et al. Both of these resonance forms provide stability to the carbocation. B Selected representative cationic bimetallic propargyl complexes. A wide range of cationic dicobalt propargyl complexes or similar heterobimetallic complexes have been synthesized and characterized 37 - 44 , Figure 13B ; Gruselle et al.

The propargyl cation always preferentially coordinates to one of the metal atoms in each cluster due to accumulation of positive charge. In studies of these heterobimetallic complexes, the propargyl cation prefers to coordinate to Mo and Fe instead of Co 39, 41 , and 42 Gruselle et al.

In 37 , the distances between the carbocationic center and the cobalt atom are 3. Cations 38 - 40 do not undergo Wagner-Meerwein rearrangement as a result of their stabilization, which otherwise occurs readily for uncomplexed 2-alkynylbornyl cations. Analogously, stabilization of a propargyl cation by a molybdenum center results in a shielding of the molybdenum carbonyl signals from approximately — ppm in 40 Gruselle et al.

For 41 and 42 , stabilization of a propargyl cation by an iron center shifts the carbonyl resonances to approximately ppm, while the cobalt carbonyl resonances are around — ppm Osella et al. The preferential coordination with a heavier metal within the heterobimetallic cations 39, 40 , and 43 is consistent with the reactivities of complex 20 and the conclusion in Figure 9B.

However, in complexes , the carbocation is bound to the lighter element, Fe. Most of the reported transition metal-carbocation complexes can fall into one of these two categories. In general, heavier metal atoms have larger radii, which can lead to stronger backdonation of filled d orbitals on the metal atom and greater stabilization of carbocations. This results in weaker transition metal-carbocation interactions. On the other hand, an electron-rich ligand can increase the electron density on the metal atom, resulting in carbenium backdonation and greater stabilization.

Transition metal-stabilized carbocations have been observed and characterized throughout the last century, but there is no comprehensive summary of the bonding modes of these transition metal-carbocation complexes. To our surprise, most of this research was conducted and reported before and little attention has been given to the field during the last decades, even though much remains unknown about their properties, reactivities, and carbocation interactions with other transition metals e. Because of their considerable synthetic value, it is of great importance to bring these metal-carbocation interactions back to the interest of the scientific community.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Albright, T. Structure adn reactivity in organometallic chemistry.

An applied molecular orbital approach. Tetrahedron 38, — Cycloheptatriene and fulvene Cr CO 3 complexes. Amgoune, A. Amouri, H. Amyes, T. Experiments and calculations for determination of the stabilities of benzyl, benzhydryl, and fluorenyl carbocations: antiaromaticity revisited.

Andrianov, V. Bandar, J. Aminocyclopropenium ions: synthesis, properties, and applications. Behrens, U. Ein Fulven-eisen-komplex? Benitez, D. A bonding model for gold i carbene complexes. Bleiholder, C. Organometallics 28, — Bouhadir, G. Complexes of ambiphilic ligands: reactivity and catalytic applications.

Braunschweig, H. Breslow, R. Triarylcyclopropenium ions. Synthesis and stability in the phenyl p-anisyl series. Brown, T. Mechanistic analysis of gold I -catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis gold vinyl species and reversible C-O bond formation. Cais, M.

Cheng, J. Hydride affinities of carbenium ions in acetonitrile and dimethyl sulfoxide solution. Chetcuti, M. Propargylic cations stabilized on nickel-molybdenum and nickel-tungsten bonds. Organometallics 21, — Chiang, T. Churchill, M. Ciabattoni, J. Di-t-butylcyclopropenone and substituted Di-t-butylcyclopropenyl Cations. Davis, R. Deno, N. Carbonium ions. Ditchfield, R. Organometallics 12, — Donaldson, W. Drew, M. Cyclopropenyl and oxocyclobutenyl complexes of molybdenum.

Echavarren, A. Non-stabilized transition metal carbenes as intermediates in intramolecular reactions of alkynes with alkenes. El Amouri, H. New bonding modes, fluxional behavior, and reactivity in dinuclear complexes bridged by four-electron donor unsaturated hydrocarbons. Ellis, J. Organometallics 22, — Espinosa Ferao, A. Fulvenization as characteristic geometric distortion in electron deficient ferrocenes. Tetrahedron 73, — Fischer, E. On the way to carbene and carbyne complexes. Fomin, V. Frenking, G. Gade, L. Cooperative reactivity of early-late heterodinuclear transition metal complexes with polar organic substrates.

Ghilardi, C. Crystal structure of the platinum derivative. Gleiter, R. Structural considerations. Organometallics 26, — Gompper, R. English 17, — Green, B. Chemistry of Bis cyclo-octa-1,5-idene platinum : reactions with electrophiles. Green, M. A new approach to the formal classification of covalent compounds of the elements.

Gruselle, M. Harris, R. Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition. Hashmi, A. Dual gold catalysis. Gold catalysis in total synthesis. Hayter, R.


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Hill, E. Carbonium ion stabilization by metallocene nuclei. Hopkinson, M. Merging visible light photoredox and gold catalysis. Hughes, R. Competition between ligand substitution and cyclopropenyl migration to carbon monoxide followed by ring expansion to give oxocyclobutenyl ligands. Organometallics 5, —