Fischer carbene complexes remain favourite targets , and vehicles for new discoveries

Exciting new variations in Fischer-type carbene complex composition and reactivity have been realised by following or modifying well-established synthetic approaches such as metal carbonyl functionalization and modification of existing carbene ligands. The formation of targeted complexes for organic synthesis, carbene-containing chelates, and polynuclear carbene complexes, by employing ‘click chemistry’, warrants discussion. Transmetallation and α,α-dehydrogenation of ethers and amines have come into their own as viable synthetic methods to access carbene complexes with unique properties and activities. Successful mediation of carbene complex formation with pincer ligands has proved its worth. Quantum chemistry has become essential for supporting or initiating mechanistic proposals, but heuristic approaches such as invoking the vinylology principle to describe substituted phenylcarbene complexes are still valuable in the interpretation of bonding properties and the classification of complex types. Electrochemical studies now also constitute a powerful part of the experimental characterization tool kit.


Introduction
Ever since the ground-breaking publication by Fischer and Maasböl fifty years ago, 1 introducing stable, isolable and fully characterized carbene complexes of group 6 metals, the versatility of the carbene ligand class has not failed to fascinate the chemical community. The complete antipoles of Fischer's electrophilic complexes, the nucleophilic, so-called alkylidenes, present in high oxidation state transition metal complexes, were announced 10 years later, in 1974, by Schrock,2 which opened up the research field even further. Many other types of carbene ligands have now been exposed in detailsee Table 1 for examples. 3 Most of them defy simple categorization as exclusively Fischer or Schrock types. 29 Others have such distinctive and useful coordination influences, making electrophilicity or nucleophilicity not their most significant features. Obviously, the metal and its oxidation state as well as the other ligands present play a role in the large variations in reactivity found, for example, in the simple coordinated secondary, nonheteroatom-stabilized, ligands :CHR (R = aryl). 12, 30 Fischer-type carbene complexes (FCCs) have shown exceptional merit as synthons in organic and organometallic synthesis, and numerous review articles have been devoted to describing and organising the results. The latest of such articles usually contain references to earlier literature. 5,[31][32][33] The important role played by the Spanish research groups, under the leadership of Barluenga, Sierra and others, is worth mentioning here. Often the applicable mechanisms in the syntheses have been explored by quantum chemical calculations. 34 In addition to yielding organic products, interaction with carbene ligands, using other coordination modes, has afforded numerous coordination complexes. 35 FCCs have found application in diverseand sometimes unexpectedresearch areas. The formation of mixed valence and mixed metal Lindqvist complexes, 36 as well as stable gold and palladium, 37,38 and monodisperse chromium nanoparticles 39 are cases in point.
In this Perspective, classical Fischer carbene complexes of the general form L n MvC(XR 1 )R 2 are considered. With the growing interest in other types of carbene complexes (Table 1),

H. G. Raubenheimer
Helgard Raubenheimer has been Professor of Inorganic Chemistry and Head of Department at the Universities of Johannesburg and Stellenbosch. He is now Professor Extraordinary at the latter institution. His main interests are in the fields of carbene complex chemistry and the chemistry of gold. He regularly collaborates with Hubert Schmidbaur (TU Munich).
it is remarkable that research into such complexes is still flourishing. Sustained activity and application is stimulated, and guaranteed, by the preparation of new complex variations within the family as well as by a thorough understanding of their bonding (on both quantum chemical and empirical levels) and mechanism of formation. These aspects receive attention here and the literature is reviewed for the past decade. The only other reactions dealt with in depth here are the conversions of one carbene complex to another. Several relevant review articles have appeared during the review period and they are referred to in the appropriate sections.

The Fischer route
The synthetic approach followed in the laboratory of E. O. Fischer during the 1970s and 1980s is often still usedsometimes with small modificationsto obtain new and useful compounds or to find new applications for known complex types when the carbene ligands are designed differently to the original ones.
Polymer-supported carbene complexes. In contrast to previous approaches to affix carbene complexes to polymer supports by using phosphine ligands or by making the polymer the organic substituent of the carbene complex, more recently, the pentacarbonyl metal fragment was attached via a carefully selected isocyanide as the sixth ligand, starting from a Merrifield resin. 40 The carbene complex preparation then occurs as a solid-phase process by consecutive treatment with RLi and an alkylating agent, methyltriflate (MeOTf ) (eqn (1)).
In other examples, the linker chain length was increased. Reactions such as aminolysis, cyclopropanation and C-H insertion occur in the solid state, although not always in satisfactory yields. On completion of such a reaction, the starting pentacarbonyl complex, still attached to the resin, can be regener- Azobenzene-containing carbene ligands. In a contribution that particularly focussed on experimental variations when using the Fischer synthetic protocol, Garlichs-Zschoche and Dötz 41 described the preparation of two unique amphiphilic azobenzene aminosugar carbene complexes of chromium (1). Reaction conditions, choice of solvent, the metal attached to the organic group for CO nucleophilic attack, the alkylating agent, and time allowed for the reactions are the most important synthetic variables that were optimised for ensuring acceptable yields (Scheme 1).
Fluorinated carbene complexes. Replacing the nucleophilic reagent RLi in the Fischer approach with Me 3 SiCF 3 in the presence of fluoride ions and using MeOTf as alkylating agent, the first example of a perfluoroalkyl carbene complex has been prepared (eqn (2)). 42 It should be possible to transfer such a ligand to later transition elements (see below).
ð2Þ Unusual CO functionalizations. The first example of a simple methoxycarbene complex of platinum was prepared by straightforward base attack on coordinated CO, followed by alkylation. The platinum(II) in the produced complex was coordinated by a mediating PCP-type pincer ligand (Scheme 2). 43 Using another approach, chelated biscarbene complexes of chromium and tungsten could be formed upon consecutive CO functionalizations. 44 3-Bromo-5-methyl-thiophene was deprotonated by LDA and reacted with M(CO) 6 to form a 2-carbeniate. Subsequent Li/Br exchange with n-BuLi effected a second nucleophilic attack on a cis-positioned CO ligand, and alkylation yielded the product (Scheme 3). The preparation and reactivity of di-and polymetallic carbene complexes has been thoroughly dealt with in an earlier review article by Bezuidenhout and Lotz. 45 Deprotonation of N-coordinated N-methylimidazole in a cationic rhenium complex leads to internal Fischer-like nucleophilic attack on a coordinated CO ligand and affords carbene complex formation upon alkylation. 46 The dinuclear product represents a rare (see below) four-membered carbene-imine chelate eqn (3), Ar′ = 3.5-bis(trifluoromethyl)phenyl.

ð3Þ
An as yet unprecedented (methoxy)phospholylcarbene complex prepared according to an established method under carefully controlled conditions (eqn (4)), coordinates to a second (CO) 5 W fragment. 47 The NMR resonance of the carbene carbon in the product at δ 316.9, which is much higher than in known alkoxy(heteroaryl) carbene complexes (see references later in the text), indicates the possibility of unique reactivity, which has indeed been found.

ð4Þ
Changing the carbene ligand Certainly, the most versatile method to provide access to Fischer carbene complexes involves modification of an existing coordinated carbene. Numerous such reactions have been described previously, many of them in the course of utilizing carbene complexes in organic synthesis. 4 Cycloadditions and condensations. Alkynyl esters in the presence of N-methylmorpholine-N-oxide (NMO) and Fischer carbene complexes such as (CO) 5 MvC(OEt)Ph or (CO) 5 MvC(OEt)CuCPh (M = Cr, W) undergo self-condensation and coordination to the (CO) 5 M unit to afford pyran-2-ylidene Fischer complexes in relatively low yields (eqn (5)). 48 The exact mechanism of the transformation is still uncertain, although it seems likely that the NMO activates the original carbene complexes by other means than simply by ligand oxidation. Various other methods of preparing pyran-2-ylidene complexes are known. 4 Aumann and co-workers found remarkable differences between (β-imino)ethoxy-and (β-imino)thioethoxycarbene complexes of chromium and tungsten in reactions with imidoyl chlorides (Scheme 4). 49 The preference for either a metallic (di-π-methane) rearrangement (MDPMR) to yield (alkoxy)amino-carbene complexes (2), or an α-cyclization to afford thermolabile, coordinated pyrroles, is determined by an intricate interplay of kinetic and thermodynamic factors. A key role is played by a common intermediate carbene complex.
In a reaction that once again illustrates the versatility of alkynyl FCCs, 4 Sierra and co-workers 50 have shown that such compounds react with tropothione by formal regioselective [8 + 2] cycloaddition (Scheme 5) to give cycloheptathiophenecarbene complexes (4). DFT quantum chemical calculations indicate the intermediate formation of a nonaromatic, zwitterionic allene adduct (I).
Subsequent to the appearance of the Bezuidenhout review mentioned above, Baeza, Casarrubios and Sierra 51 published an important paper. They succeeded in showing that Fischer aminocarbene complexes of chromium and tungsten that contain both internal and terminal alkyne units in their organic side chains can be effectively converted into dinuclear, trinuclear, and tetranuclear carbene complexes by using 'click chemistry' with organic azides. The 1,3-dipolar addition reac-tions catalyzed by Cu(I) occurred readily and in good yields, although a few exceptions were noted. A highly diverse range of structures were constructed, but only a few representative examples (3)(4)(5) are shown in Scheme 6. Pauson-Khand cyclization of the (allyl)amino (R 1 = allyl) and alkyne substituents at the carbene carbon of selected compounds leads to a large number of C-C bond formations and CO insertions during bicyclic ring formation. (Amino)vinyl Fischer carbenes still exist at the periphery of the newly constructed molecules of increased complexity. In a simplified graphic version (Scheme 7), the cyclization in only one carbene unit is shown.
α-Deprotonation of Fischer carbene complexes. The acidity of α-hydrogens attached to the carbene carbon atom in Fischer carbene complexes is well known and has often been utilized in the past for their modification of the complexes by deproto-nation and electrophilic addition. Recent contributions from Raubenheimer's laboratory have been reviewed in this journal. 52 These include the formation of elusive four-membered carbene-heteroatom chelates as well as the preparation of remote NHC carbene complexes via the stereoselective generation of α,β-unsaturated Fischer carbenes.
The groups of Ortega-Alfaro and Alvarez-Toledano 53 combined efforts in preparing more examples of carbene-S and carbene-N five-membered chelates by reaction of α-deprotonated aminocarbene complexes with PhNCS and employing two different alkylating agents, and ultrasound activation in one instance, to afford 6 and 7 (Scheme 8).
In a very interesting application, Sarkar and co-workers 54 deprotonated standard (CO) 5 WvC(OMe)Me to allow the introduction of two polyethylene glycol (PEG) tethers by allyl attachment to the carbanionic centre (8, Scheme 9). The resulting product, when n = 7, is sufficiently soluble in water (55 mg l −1 ) to be biochemically useful. Bovine serum albumin reacts, by aminolysis, with this 'PEGylated' carbene complex to produce a bioconjugate with marker (heavy metal; ν CO ) characteristics (Scheme 10). In a recent contribution, first protein and then rabbit IgG were immobilized on gold and glass surfaces by reaction with a Fischer carbene function. 55 Anionic metaloxycarbene complexes of the Fischer-type were found to react with a variety of metal complexes, by different pathways, to form unusual products, depending on the R-group in [(CO) 5 MC(O)R] − and the second metal employed. Particularly interesting are examples of isomorphous, mer-configurated, tris 'complexes of complexes' of Cr(III), Fe(III) and Co(III), that form when R = 4-methyl-2thiazolyl. 52 Alkene-carbene and alkyne-carbene chelates are well known. López-Cortés and co-workers 56 reported the first examples of allene-aminocarbene complexes prepared by a rare functionalization of primary (amino)(ferrocenyl)carbene complexes of Cr, Mo and W (eqn (6)).
ð6Þ Transmetallation Important contributions towards the preparation of new carbene complexes by transmetallation (or metal exchange) using, particularly, Rh, Pd and (later) Au as metals to replace Cr or W, have been made by the Spanish schools. Developments that have taken place after the appearance of the last review article by Gómez-Gallego et al. 57 are discussed below.
Sierra and co-workers 58 recently showed, both experimentally and theoretically, that carbene transmetallation from Cr to Rh or Pd is a key step in the catalyzed formation of quinolone alkaloids by cyclization of group 6 (2-aminoethyl)carbene complexes. Barluenga and co-workers 59 succeeded in preparing the first examples of mixed complexes that contain both Fischer carbene and NHC ligands coordinated to Rh(I) (eqn (7)), whereas Fañanás-Mastral and Aznar 60 made gold complexes bearing NHC and alkenylcarbene ligands by reacting (CO) 5 CrvC(OMe)CHvCHPh with (NHC)AuCl in the presence of AgSbF 6 . Various remote NHCs, prepared from Fischer carbenes, were transferred to Rh and Au to prepare the first such compounds of the latter element. 61 Strasser et al. 62 utilized (tht)AuCl to transfer a FCC that carries a 2-phenylthiazol-5-yl group from (CO) 5 W to isolobal AuCl (eqn (8)). The single proton on the thiazole ring exhibits an unusual shape and positional variation with temperature in its NMR spectra, probably because of Au⋯H hydrogen bonding interaction. 63 It is interesting that tungsten carbeniates are also readily transferable from W to Au when suitable complex fragments of the two metals are used.

ð8Þ
Although diheterocarbene complexes are not discussed in detail in this paper (see, for example, ref. 10 and 11 for recent articles on them), one very interesting example is mentioned below. Ruiz and co-workers 64 prepared such a complex positioned within an N-heterometallocyclic carbene ligand that is coordinated to gold (10), by double deprotonation, metallation with (Ph 3 P)AuCl, and final alkylation of a diaminocarbene complex of manganese(I) carbonyl. A reasonable schematic rationalization of the process, which includes metal exchange and internal nucleophilic attack on coordinated CO, is given in Scheme 11. Upon heating, the kinetic zwitterionic product (9) also reacts by a formal exchange of Me and Ph groups. Again, all metal-carbene interactions are represented by double bonds to allow the arrow formalism to be applied.
Various other intermolecular metal exchange reactions in bi-and polymetallic systems have been dealt with in the review article by Bezuidenhout and Lotz, 45 and are not repeated here. A very recent example, however, deserves attention.
As mentioned above, facile transmetallation of typical heteroatom-stabilized FCCs of Cr and W occurs with different gold(I) complexes. In the absence of a heteroelement in the diphenylcarbene complex of pentacarbonyltungsten, the reaction stops halfway and a cationic gold adduct (11) is formed (Scheme 12). 65 Such a compound with chromium as central metal is unstable, even at −50°C, and converts rapidly with loss of CO to form a bimetallic complex (12) in which the chromium atom is in contact with an ipso carbon of one of the phenyl groups, shielding it electronically. The contrasting smooth and complete transmetallation known for alkoxycarbene complexes can, however, be accomplished by placing two methoxy groups in the para positions of the phenyl rings of the starting chromium carbene complex. Cationic pentaand tetracarbonyl dinuclear products still form initially, but spontaneously convert to form a linear, diarylcarbene-gold complex (eqn (9)). 66 These results, as well as NMR measurements and crystal structure data, indicate that Fischer-like carbene complexes stabilized by remotely positioned heteroatoms can be generated not only by 1,2-insertion of a six-membered aromatic ring ('vinylology principle' 67see below), but by a 1,4-insertion of the C 6 H 4 ring ('extended vinylology') between the heteroatom fragment, e.g., OMe, and the carbene carbon. At any particular moment, only one of the two anisyl rings participates in π-electron delocalization towards the carbene carbon atom. (c) In contrast with the dehydrogenation of methyl ethers to give square planar Ir(I) carbene complexes, (PNP)Ir effects octahedral aminocarbene(dihydro)Ir(III) complexes by double C-H activation of methylamines (reaction 2). 70 The latter result is in agreement with an earlier report by Manzano and co-workers 73 who found that a PAPF bidentate PN ligand (PAPF = 1,2-[1-(dimethylamino)ethyl]-1-(diphenylphosphino)ferrocene) installed at Ru(II) effectively activates the attached aminomethyl unit to give a chelated aminocarbene ligand, (eqn (10)), whereas two related but less rigid and weaker bonded PN bidentate ligands, PPFA (2-[1-(dimethylamino)ethyl]-1-(diphenylphosphino)ferrocene) and PTFA ([η5-cyclopentadienyl][η5-4-(endo-dimethylamino)-3-(diphenylphosphino)-4,5,6,7-tetrahydro-1H-indenyl]) of iron(II)) are not prone to such conversion.
(d) The dihydride formed according to the procedure followed by Grubbs and co-workers when using THF as substrate (reaction 3, Scheme 13) is structurally related to the products mentioned in c. Heating leads to the expulsion of H 2 . 71 (e) Carbenes are not always formed during C-H activation with dehydrogenated (PNP)IrH 2 . Other outcomes include the formation of vinyl ethers (reaction 4) and activated (e.g., by oxidative addition, reaction 5) or decarbonylated (reaction 6) products. 71 (f ) Detailed quantum chemical calculations carried out by Yates and co-workers 74 support the mechanisms postulated by experimentation and give an excellent interpretation of the role played by norbornene by invoking a unique autocatalytic interplay between thermodynamics and kinetics during carbene complex formation.
In 2009, a short review article was published that deals mainly with reactions of the Fischer type iridium carbene complexes. 68 Certainly, one of the most unusual reactions is carbon dioxide reduction coupled with oxygen atom transfer to the Fischer carbene. [75][76][77] ð10Þ Schneider and co-workers 78 also used PNP pincer ligands for activating THF towards carbene formation along another synthetic route. Their chelating ligands of the types (HPNP) R (R = i Pr, t Bu) react with [IrCl(COE) 2 ] 2 (COE = cyclooctene) in the presence of NaX (X = BPh 4 ) or TlX (X = PF 6 ) to give the carbene complexes indicated in eqn (11). Obvious differences to the Grubbs' system are the neutral pincer ligand employed by the Schneider group compared to Grubbs' PNP amide, and the manner in which the active Ir(I) complex is generated. Important steps in the proposed, and experimentally sup-ported, mechanism include cyclooctenyl complex formation by oxidative substitution, Cl − abstraction, reductive elimination, and COE recoordination. Then, presumably, follows a slow α,α-dehydrogenation, leading to the formation of a carbene (dihydride) complex by COE substitution, as proposed by Yates and co-workers 74 for the Grubbs' reactions.
Using advanced mass spectrometry and collision-induced dissociation (CID) supported by DFT calculations, Chen and co-workers 79 proposed the generation of gaseous, cationic, secondary FCCs (13,14) Insertions into Si-C and Si-H bonds CO insertion into Si-C bonds of actinide and early transition metal complexes is a well-established pathway to enolate complexes. Using an Fe(II) complex bearing the tridentate tetravalent silene ligand tris(3-trimethylsilyl-2-mercaptophenyl) methylsilane, Kawaguchi and co-workers 80 showed that treatment with CO results in an insertion of the molecule into one (substituted) phenyl-Si bond with concomitant C-coordination to form an iron-carbene bond. Two other CO molecules bind to iron to increase its coordination number from four to six (15 in eqn (12)). The close positioning of one of the thiolato sulphur atoms to the carbene carbon atom (1.872(4) Å) bears analogy to a thiametallacyclopropane complex, an intermediate during thiolysis of alkoxycarbene complexes that contain thiol ligands. 81

Scheme 14
ð12Þ Recently, in a highlight contribution, an unprecedented insertion of XylNC into a Si-H bond in a reaction between η 3 -H 2 SiRR′ σ-complexes of Ru(I) and the isocyanide was discovered (Scheme 15). 82 NMR studies at low temperatures indicate that an adduct is initially formed in which the isocyanide is associated with a six-coordinated Si centre. DFT computational modelling of the subsequent product formation shows 1,1hydrosilation (isocyanide insertion into the Si-H bond), iminoformyl migration to Ru, and N-bonding to Si to afford the aminocarbene product 16. The carbene ligand features one γ-agostic (3c2e) Si-H Ru bond and a weaker Ru-H Si association, in which the ruthenium provides the bridging hydride.

Oxidative substitution
The formation of (generally cyclic) carbene complexes by oxidative substitution was pioneered independently by the research groups of Lappert 83 and Stone 84 in the early 1970s. Later, it was applied to the preparation of normal and abnormal (some of them remote) pyridylidenes. 23 Fürstner and coworkers 85 used the approach to prepare prototypical Fischertype aminocarbene complexes of Pd(II) (eqn (13)). The method also lends itself to the formation of acyclic diaminocarbene complexes of Pd and Ni or diheterocarbene complexes of Pd; but these types, like NHCs and CAACs (see Table 1), have developed independently and are not discussed here.

ð13Þ
Experimental kinetic and thermodynamic studies The results discussed here are important in the context of converting one carbene complex to another. Since the 1980s, Bernasconi and co-workers have made important contributions towards understanding the thermodynamic and kinetic acidities of CH-active FCC complexes of group 6 metals. In one of the latest papers on this topic, Bernasconi and Ragains 86 report on deprotonation studies of [Cp*(CO) 2 FevC(OMe)Me] + , and use the opportunity to consider and briefly compare the different factors that determine proton transfer from both kinetic (using intrinsic rate constants, k o ) and thermodynamic (using pK a as parameter) viewpoints. Owing to the complexity of the many mutually supporting and opposing structural influences, rationalization of data is possible, but correct comparative predictions for given compounds in terms of rate of deprotonation or acid strength is unattainable.
The research group of Bernasconi has also published numerous articles on nucleophilic substitution at the carbene carbon atom. Their latest contribution 87 elaborates on two important results: (i) in the reaction of thiolate anions with Fischer type alkoxycarbene complexes, the leaving group expulsion is rate limiting with thiolates that are weak bases, whereas the nucleophilic attachment step is rate determining when such bases are strong; and (ii) weak base thiolates are inferior leaving groups compared to aryloxides of comparable basicities.
Rate constants for postulated tetrahedral adduct formation during hydrolysis of arylcarbene complexes, (CO) 5 CrvC(OR 1 ) Ar, with OH − , are adversely affected by an increase in the steric bulk of group R 1 . Such effects are similar to those found in the hydrolysis of esters, but they are more accentuated in carbene complexes. 88 Both the carbanionic and anionic amide nucleophiles malonitrile, CH(CN) 2 − , and cyanamide, NuCNH − , are more reactive nucleophiles for substituted alkoxy-or thioalkoxy carbene complexes than OH − . 89,90 Theoretical approaches A review article appeared in 2005 summarizing the status of quantum chemical calculations for Fischer-(and Schrock-) type carbene complexes. 91 Of importance is that it clearly shows that the interpretations of bonding in carbene complexes in terms of simple heuristic models based on exper-iment, measurement, and intuition have been substantiated by elaborate calculations. Interpretations are based on various procedures by making use of charge or energy partitioning. In doing this, new concepts have been defined that have not necessarily all been embraced yet by all experimental chemists. Dichotomies may arise between heuristic models and quantum mechanical interpretations, for example: stronger 'orbital interactions' do not necessarily translate into stronger chemical bonds; and calculated electron densities, in many examples, do not always concur with formal mesomeric structures.
On the other hand, the relative contributions by σand π-interactions in the metal-C(carbene) bond in terms of a Dewar-Chatt-Duncanson-type interpretation have now been placed on a firm footing, while an earlier proposal by Wang and co-workers, 92 and others, 91 to describe the π-interaction in the M-C(carbene)-X fragment of a FCC by a (3c4e) bond has not been refuted.

Structure and reactivity studies combined with quantum chemical calculations
Dedicated quantum mechanical calculations have also been used to interpret or supplement various experimental investigations. The calculations by Yates in the context of Grubbs' α,α-hydrogen activations, and the rationalization of carbene complex formation by mass spectroscopic activation, as discussed above, are such examples. These, and a few others, are highlighted below.
Conformation of Fischer type carbene complexes. Fischer carbene complexes, (CO) 5 MvC(OR 1 )R 2 , usually occur as anti isomers around the C(carbene)-O bond in the crystalline state. For the first time, by using DFT calculations and accepting correspondence between the vacuum and crystalline states, the observation has been explained in terms of σ CH and σ CH * (situated on the alkyl group, R 2 ) donor-acceptor interactions with symmetry-related π orbitals of neighbouring CO ligands. 93 The more obvious destabilizing steric repulsion between groups attached to the carbene carbon atom plays a decisive role in alkynylcarbene complexes, making the syn-conformation the preferred one. Complexes with syn-conformations are more polar than their anti analogues and are the preferred orientations in more polar solvents.
Carbene complex ionization. ESI-MS studies coupled with deuterium labelling, and DFT calculations to identify lowest energy intermediates, enabled Sierra and co-workers 94 Photochemistry. Quantum chemistry is essential for understanding the versatile and extremely useful photochemistry of Cr(0) and Mo(0) FCCs. After studying the electronic transitions, reactivities, ligand modifications, important solvent effects, operative mechanisms, and selectivity in such complexes, Sierra and co-workers summarized their most important results in a review article. 96

Scheme 16
Electrochemistry. Various groups have reported on the redox profiles of FCCs in the review period. In almost all cases, the interpretation of the results supported and extended by quantum chemical calculations, is in accordance with earlier work. 97,98 The current viewpoint is that, for example, oxidation occurs at the central metal and reduction at the carbene ligand, with most of the additional electronic charge centred at the carbene carbon atom and partly delocalized over attached heteroatoms and conjugated heteroaromatic rings. The delocalization is indicated by the shape of the LUMO orbitals of the neutral complexes or the HOMOs of the formed anionic radicals. [99][100][101][102] A number of further developments regarding FCCs have been reported, some of which are summarized below.
• Conjugation of the carbene complexes with an analogue of natural nucleic acids by aminolysis affects the electrochemical activity of the organometallic fragment modestly, and exploitation as electrochemical probes in bio-organometallic chemistry can be foreseen. 103 • A direct major influence on the oxidation potential of the carbene complexes is the π-acidity of the ligands attached to the metal. 99 • Increasing the effective electronegativity of X in complexes of the type (CO) 5 MvC(XR 1 )R 2 (X = O, NR 3 , S; R 1 = alkyl; R 2 = alkyl, aryl; R 3 = H, alkyl) and the electron-withdrawing influence of groups R 4 in (CO) 5 MvC(XR 1 )C 6 H 5 R 4 -p, the more readily the reduction occurs. 99,101 • The heteroatom in furyl and thienyl substituents plays a similar role as X and (amino)furylcarbenes are reduced more readily than their thienyl analogues. 99 It is unclear why the order is reversed in alkoxycarbenes. 101 • Carbene complexes of iron and chromium become stronger oxidants with the increased electron delocalization in the formed radical anion when R 2 = C(O)Me in aminocarbene complexes. The low-lying LUMO orbital of the neutral complex (or the HOMO of the radical anion) is delocalized over N, C(carbene), the aromatic ring, and attached carbonyl group in the para position. Such complexes are even much easier reduced than those with R 2 = CF 3 , where the LUMO is exclusively localized on the C(carbene)-N portion of the molecule. 104 This result is particularly interesting since conjugative stabilization of the carbocationic carbene carbon atom by phenyl or substituted phenyl groups is generally of little importance. 100,105,106 The electron charge of the added electron is much better delocalized in 2-heteroaryl substituents than in substituents linked in the 3-position. 103 Extending delocalization by the addition of a second conjugated thienyl group to an existing one facilitates reduction even further. 107 • Support for the atomic location of the single electron after one-electron oxidations and reductions in a variety of carbene complexes, including examples of Fe(0) and Mn(I), emanated from DFT calculations, ESR measurements, and mass spectrometry. 99,102 • Most authors report one-electron oxidation and reduction steps, but two-electron reductions have also been found for aminocarbene complexes, depending on the electrodes used. 99,101 Further oxidation of the central metal has also been observed.
• Newly characterized and DFT-studied fac and mer isomers of the complexes (CO) 6 (dppe)CrvC(XR 1 )R 2 (X = O, NH; R 1 = Et, Cy; R 2 = 2-thienyl, 2-furyl) participate in various reactions during cyclic voltammetric study (Scheme 17). 108 The equilibrium between the neutral complexes in solution favours the lower energy mer isomer by way of a fast conversion. The fac + cation rapidly and quantitatively converts to the mer + isomer, which is the only one that exhibits a reduction peak in the cyclic voltammogram. Related redox relationships have been postulated earlier for disubstituted phosphine and phosphite derivatives of chromium hexacarbonyl. 109 Mechanistic considerations. Extending the comprehensive kinetic studies undertaken by Bernasconi and co-workers on the aminolysis of methoxy and thiomethoxy carbene complexes of Cr(0) and W(0), Solà and co-workers 110 undertook the first computational study of the mechanisms of such reactions; they concentrated on a chromium family. The detailed calculations take into account various reaction routes as well as solvent effects. The results indicate a stepwise process, including two transition states, that takes place by the initial formation of a tetrahedral zwitterionic intermediate, III. The ratedetermining step is the initial nucleophilic attack by NH 3 , which is inhibited by π-donation from the heteroatom X. The subsequent two steps in which MeXH is eliminated and the product formed, are in accordance with earlier proposals made by Bernasconi's group. 111

Other bonding considerations
Despite the fact that Hoveyda-Grubbs precatalysts and their derivatives (Scheme 18) contain no heteroatom attached to the carbene carbon atom, they can be classified as belonging to the Fischer carbene family on the basis of the vinylology principle 112 (referred to earlier) with the anchored, remote o-O i Pr group fulfilling a stabilizing role by π-conjugation with the electron-deficient carbene carbon atom. 67 The most important modifications that have been made to such catalysts to change their performance emerge from in steric or electronic effects. Substituents ortho to the O i Pr group cause a structural distortion and increased rate of metathesis reaction, 113 whereas nearly the same effect is achieved by substituting the aromatic ring in the position para to the O i Pr group by electron withdrawing NO 2 . 114 Stability of the Ru-O bond in the chelate is however increased (and activity decreased) by better π-delocalization (aromaticity) in the chelate ring, caused by the correct positioning of annelated aromatic rings (Scheme 18). 67 This active field of research with its own unique and complicated mechanistic uncertainties 115 has been reviewed. 116,117 Conclusions The wealth of new Fischer type carbene complexes prepared during the past 10 years holds challenges and can serve as a basis for investigation by organic chemists, theoreticians, kineticists and bio-inorganic chemists. The often unique processes utilized, including certain transmetallations, cycloadditions, α,α-dehydrogenations and even oxidative additions, should inspire synthetic organometallic chemists to find new novel methods of preparation and to further enlarge the existing library of FCCs.