Carbones and Carbon Atom as Ligands in Transition Metal Complexes

This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL2 as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and π) donation to the metal atom [M]⇇CL2. The second part of this review reports complexes which have a neutral carbon atom C as ligand. Carbido complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR3 → [M]-CR2 → [M]-CR → [M]-C. This review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands.


Introduction
Transition metal compounds with metal-carbon bonds are the backbone of organometallic chemistry. Molecules with M-C single bonds are already known since 1849 when Frankland reported the accidental synthesis of diethyl zinc while attempting to prepare free ethyl radicals [1,2]. Molecules with a [M]=CR 2 double bond (carbene complexes) or a [M]≡CR triple bond (carbyne complexes) were synthesized much later [3][4][5][6]. Two types of compounds with metal-carbon double or triple bonds having different types of bonds are generally distinguished, which are named after the people who isolated them first. Fischer-type carbene and carbyne complexes are best described in terms of dative bonds following the Dewar-Chatt-Duncan (DCD) model [7,8] [M] CR 2 and [M (-) ]

Introduction
Transition metal compounds with metal-carbon bonds are the backbone of organo chemistry. Molecules with M-C single bonds are already known since 1849 when Fra reported the accidental synthesis of diethyl zinc while attempting to prepare free ethyl [1,2]. Molecules with a [M]=CR2 double bond (carbene complexes) or a [M]≡CR trip (carbyne complexes) were synthesized much later [3][4][5][6]. Two types of compound metal-carbon double or triple bonds having different types of bonds are ge distinguished, which are named after the people who isolated them first. Fischer-type and carbyne complexes are best described in terms of dative bonds following the Chatt-Duncan (DCD) model [7,8] [M]⇄CR2 and [M (─) ] CR (+) , whereas Schro alkylidenes and alkylidynes are assumed to have electron-sharing double and triple [M]=CR2 and [M]≡CR [9][10][11].
This review deals with transition metal complexes with metal-carbon bonds to tw of ligands, which have only recently been isolated and theoretically studied. One type o are carbones CL2 [12], which are carbon(0) compounds with two dative bonds to a carbo in the excited 1 D state L→C ←L where the carbon atom retains its four valence electrons lone pairs that can serve as four-electron donors [13,14]. Thus, carbones CL2 are four-e donor ligands whereas carbenes CR2 are two-electron donors. Carbenes have a forma to one or two monodentate Lewis acids A or protons or to a single bidentate Lewis acid as shown in Figure 1. The large second proton affinity is a characteristic feature of carbones, which distinguishes them from carbenes [16]. Examples of all cases are known and are described below.
vacant p(π) orbital that can accept electrons in donor-acceptor complexes M⇄CR2 whereas carbones are double (σ and π) donors in complexes [M]⇇CL2. A good Lewis acid acceptor fragment A for a carbene complex has a vacant σ orbital and an occupied π orbital whereas a suitable acceptor for a carbone is a double Lewis acid with vacant σ and π orbitals as shown in Figure 1a,b. If the Lewis acid A has an occupied π orbital, it would lead to π repulsion with the π lone pair of the carbone CL2, whereby the repulsive interaction is reduced if L is a good π acceptor (Figure 1c). The two electron lone pairs of a carbone may bind to one or two monodentate Lewis acids A or protons or to a single bidentate Lewis acid as shown in Figure 1. The large second proton affinity is a characteristic feature of carbones, which distinguishes them from carbenes [16]. Examples of all cases are known and are described below. It is important to realize that the two electron lone-pairs of a carbone CL2 may additionally engage in π-backdonation to the ligands L whose strength depends on the availability of vacant π orbitals of the ligands L. Stronger π acceptor ligands L enhance the π-backdonation L←C →L which leads to wider bending angles at the carbon atom ( Figure 2). The significant bending of free C(CO)2 [17,18] can straightforwardly be explained in terms of dative bonding in carbon suboxide C3O2 [19,20]. The π-acceptor strength of ligands L thus modulates the donor interaction of the carbone CL2.  It is important to realize that the two electron lone-pairs of a carbone CL 2 may additionally engage in π-backdonation to the ligands L whose strength depends on the availability of vacant π orbitals of the ligands L. Stronger π acceptor ligands L enhance the π-backdonation L←C →L which leads to wider bending angles at the carbon atom ( Figure 2). The significant bending of free C(CO) 2 [17,18] can straightforwardly be explained in terms of dative bonding in carbon suboxide C 3 O 2 [19,20]. The π-acceptor strength of ligands L thus modulates the donor interaction of the carbone CL 2 .
fragment A for a carbene complex has a vacant σ orbital and an occupied π orbital whereas a suitable acceptor for a carbone is a double Lewis acid with vacant σ and π orbitals as shown in Figure 1a,b. If the Lewis acid A has an occupied π orbital, it would lead to π repulsion with the π lone pair of the carbone CL2, whereby the repulsive interaction is reduced if L is a good π acceptor (Figure 1c). The two electron lone pairs of a carbone may bind to one or two monodentate Lewis acids A or protons or to a single bidentate Lewis acid as shown in Figure 1. The large second proton affinity is a characteristic feature of carbones, which distinguishes them from carbenes [16]. Examples of all cases are known and are described below. It is important to realize that the two electron lone-pairs of a carbone CL2 may additionally engage in π-backdonation to the ligands L whose strength depends on the availability of vacant π orbitals of the ligands L. Stronger π acceptor ligands L enhance the π-backdonation L←C →L which leads to wider bending angles at the carbon atom ( Figure 2). The significant bending of free C(CO)2 [17,18] can straightforwardly be explained in terms of dative bonding in carbon suboxide C3O2 [19,20]. The π-acceptor strength of ligands L thus modulates the donor interaction of the carbone CL2.  The following list gives some essential features of carbones and their differences to carbenes. At the same time we want to stress that the distinction between carbenes and carbones are just a useful classification of compounds, which are a helpful model to explain the structures and reactivity of molecules. Nature does not exhibit a strict distinction line and there are complexes with electronic structures that have intermediate features between both classes of compounds. Carbenes and carbones are two ordering principles like ionic and covalent bonding. Intermediate cases are common and yet, the two concepts are essential ingredients of chemistry. The first part of this review summarizes experimental and theoretical work about transition metal complexes with carbone ligands [M]-CL 2 .

1.
Carbones are neutral carbon(0) compounds of the general formula CL 2 , which possess two electron lone pairs of electrons of σ and π symmetry, respectively. 2.
Carbones CL 2 have dative σ bonds L→ C ←L and weaker π backdonation L← C →L which resemble donor-acceptor bonds in transition metal complexes.
The carbon atom of carbones has very large electron densities and thus, unusually large negative partial charges. 4.
In contrast to carbenes, carbones exhibit high first and second proton affinities (PAs) in the region of about 290 and 150-190 kcal/mol, respectively. The second PA is a sensitive probe for the divalent C(0) character of a CL 2 molecule. Carbones can take up one and two protons with formation of [HCL 2 ] + cations or [H 2 CL 2 ] 2+ dications, respectively. 5.
Carbones have a bent equilibrium geometry where the bending angle becomes wider when the ligand L is a better π acceptor. 6.
Carbones can take up one or two monodentate Lewis acids A building the complexes A←C(L 2 ) and A←C(L 2 )→A or one bidentate Lewis acid A⇔C(L 2 ).
To the thematic of carbones several review articles were reported previously; A general overview on species that bear two lone pairs of electrons at the same C-center are summarized in [21], transition metal adducts of carbones are described in [22], and those of main group fragments in [23]. Two contributions, [24] and [25], in the series Structure and Bonding (Springer Edition) also deal with carbone transition metal addition compounds.
The  [26]. The first neutral carbon complex [M]-C, which was prepared and structurally characterized was reported in 2002 by by Heppert and co-workers [27]. They isolated the diamagnetic 16 valence electron ruthenium complexes [(PCy 3 )LCl 2 Ru(C)] (L = PCy and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene; Cy = Cyclohexyl) by a metathesis facilitated reaction. Quantum chemical calculations of model compounds suggested that the Ru-C bond in the complexes is best described by an electron-sharing double bond like in Schrock carbenes, which is reinforced by a donor bond [Ru] → =C| [28]. The field of neutral carbon complexes was systematically explored in recent years by Bendix [29]. This review summarizes in its second part the research in transition metal complexes with a naked carbon atom as ligand [M]-C that has been accomplished since 2002. The review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands.
From other linear or bent carbones with this skeleton, no transition metal complexes are described so far. In 1961, 1a was detected by Ramirez [30], and 1b-1d stem from the laboratory of Schmidbaurs group [31]. Later on, a series of related carbones were synthesized, but for which transition metal complexes are unknown so far. Quite recently the new amino substituted carbone 1e was published together with Zn and Rh addition compounds (See Scheme 1) [32]. In the 31P NMR spectra singlets at about −4.50 (1a), −6.70 (1b), −29.6 (1c), −22.45 (1d), and 12.5 In 1961, 1a was detected by Ramirez [30], and 1b-1d stem from the laboratory of Schmidbaurs group [31]. Later on, a series of related carbones were synthesized, but for which transition metal complexes are unknown so far. Quite recently the new amino substituted carbone 1e was published together with Zn and Rh addition compounds (See Scheme 1) [32]. In the 31P NMR spectra singlets at about −4.50 (1a), −6.70 (1b), −29.6 (1c), −22.45 (1d), and 12.5 ppm (1e) confirm the symmetric array of the compounds. All carbones have a bent structure but a linear form of 1a is realized if crystallized from benzene [33,34]. 1a has a short P-C distance of 1.633(4) Å and the P-C-P angle amounts to 130.1(6) • [35]. The carbone 1b exhibits a slightly longer P-C distance of 1.648(4) Å and the introduction of two less bulky methyl groups allows a more acute P-C-P angle of 121.8(3) • [36]. 1d has similar P-C bond distances of 1.645(12) Å 1.653 (14) Å and the acutest P-C-P angle in this series of 116.7 (7) • [37,38]. For 1c, gas phase electron diffraction studies result in a P-C distance of 1.594(3) Å and a P-C-P angle of 147.6(5) • assuming an apparent non-linearity but linearity in the average structure [37]. All structural parameters of 1e are close to those of 1a (P-C = 1.632(2) Å, P-C-P angle = 136.5(3) • [32]. In 1961, 1a was detected by Ramirez [30], and 1b-1d stem from the laboratory of Schmidbaurs group [31]. Later on, a series of related carbones were synthesized, but for which transition metal complexes are unknown so far. Quite recently the new amino substituted carbone 1e was published together with Zn and Rh addition compounds (See Scheme 1) [32]. In the 31P NMR spectra singlets at about −4.50 (1a), −6.70 (1b), −29.6 (1c), −22.45 (1d), and 12.5 ppm (1e) confirm the symmetric array of the compounds. All carbones have a bent structure but a linear form of 1a is realized if crystallized from benzene [33,34]. 1a has a short P-C distance of 1.633(4) Å and the P-C-P angle amounts to 130.1(6)° [35]. The carbone 1b exhibits a slightly longer P-C distance of 1.648(4) Å and the introduction of two less bulky methyl groups allows a more acute P-C-P angle of 121.8(3)° [36]. 1d has similar P-C bond distances of 1.645(12) Å 1.653 (14) Å and the acutest P-C-P angle in this series of 116.7(7)° [37,38]. For 1c, gas phase electron diffraction studies result in a P-C distance of 1.594(3) Å and a P-C-P angle of 147.6(5)° assuming an apparent non-linearity but linearity in the average structure [37]. All structural parameters of 1e are close to those of 1a (P-C = 1.632(2) Å, P-C-P angle = 136.5(3)° [32]. In Table 1, transition metal addition compounds between carbones with the P-C-P core are collected. All compounds show longer P-C bonds than the basic carbones as consequence of the competition of the occupied p orbital at C(0) between the two P-σ* orbitals and those of A. Table 1. Transition metal complexes with the carbones 1a to 1e including C-M and P-C bond lengths and P-C-P angles and 31 PNMR shifts in ppm.  Occupied d orbitals of Ni in the 1a-Ni(CO) 3 complex elongate the C-Ni bond to a carbone (2.110 Å) [39] but this leads to a relative short bond length to a NHC (1.971 Å) moiety [57]. In contrast, UCl 4 leads to a short bond to a carbone (2.411 Å) [51] indicating an appreciable U-C double bond character and a long one to a NHC base (2.612 Å) [58,59].

1-M 31 P NMR C-M P-C P-C-P Ref
The cation [1a-ReO 3 ] + holds the longest one with 1.771(8) Å indicating an appreciable C=Re double bond character. This feature applies also in part to 1a-UCl 4 and 1c-W(CO) 2 N 3 with elongated P-C bonds(See Scheme 2); a partial C-U double bond is confirmed by theoretical calculations. Similar long P-C bonds are found in the trication [1aH-Ag-1aH] 3+ , in 1a-(AuCl) 2 (See Scheme 3), and in 1b-Ni 2 (CO) 5 (See Scheme 4), where the carbone provides each two electrons to two accepting Lewis acids as depicted in Figure 1d. Occupied d orbitals of Ni in the 1a-Ni(CO)3 complex elongate the C-Ni bond to a carbone (2.110 Å) [39] but this leads to a relative short bond length to a NHC (1.971 Å) moiety [57]. In contrast, UCl4 leads to a short bond to a carbone (2.411 Å) [51] indicating an appreciable U-C double bond character and a long one to a NHC base (2.612 Å) [58,59].
The cation [1a-ReO3] + holds the longest one with 1.771(8) Å indicating an appreciable C=Re double bond character. This feature applies also in part to 1a-UCl4 and 1c-W(CO)2N3 with elongated P-C bonds(See Scheme 2); a partial C-U double bond is confirmed by theoretical calculations. Similar long P-C bonds are found in the trication [1aH-Ag-1aH] 3+ , in 1a-(AuCl)2(See Scheme 3), and in 1b-Ni2(CO)5(See Scheme 4), where the carbone provides each two electrons to two accepting Lewis acids as depicted in Figure 1d. The P-C-P angles are in the range between 115° and 132° reflecting the required space of the appropriate Lewis acid. The 31 P NMR shift of the carbone 1a amounts to about −5 ppm and those of the related addition compounds are shifted to lower fields and range between 4 ppm and 30 ppm. All iron(II) complexes of 1a and 1b are paramagnetic and 31 P NMR spectra could not be obtained.
For the 31 P NMR spectrum of the carbone 1b, a shift of −6.70 ppm was recorded [31]. With exception of 1b-Ni(CO)3 which resonate at 2.6 ppm, low field shifts between 12 and 22 ppm were found when 1b act as a four electron donor [40].
Further, 1e-ZnCl2 (See Scheme 5) [32] and 1a-ZnI2 [53] have closely related structural parameters but exhibit shorter C-Zn bond lengths than to related NHC-addition compounds (Δ = 0.051 Å) [60]. In both compounds a nearly perpendicular array of the ZnX2 and the PCP plane are found. No tendency for an additional N-coordination to the amino ligand of 1e is recorded for the ZnCl2 addition compound. In contrast the Rh-C distances in 1e-Rh(CO)2(acac) are longer (Δ = 0.117 Å) than in the corresponding NHC compound [61] and a partial π interaction was found by DFT calculation. Rh also shows no tendency for coordination of the adjacent amino groups [32]. The P-C-P angles are in the range between 115 • and 132 • reflecting the required space of the appropriate Lewis acid. The 31 P NMR shift of the carbone 1a amounts to about −5 ppm and those of the related addition compounds are shifted to lower fields and range between 4 ppm and 30 ppm. All iron(II) complexes of 1a and 1b are paramagnetic and 31 P NMR spectra could not be obtained.
For the 31 P NMR spectrum of the carbone 1b, a shift of −6.70 ppm was recorded [31]. With exception of 1b-Ni(CO) 3 which resonate at 2.6 ppm, low field shifts between 12 and 22 ppm were found when 1b act as a four electron donor [40].
Further, 1e-ZnCl 2 (See Scheme 5) [32] and 1a-ZnI 2 [53] have closely related structural parameters but exhibit shorter C-Zn bond lengths than to related NHC-addition compounds (∆ = 0.051 Å) [60]. In both compounds a nearly perpendicular array of the ZnX 2 and the PCP plane are found. No tendency for an additional N-coordination to the amino ligand of 1e is recorded for the ZnCl 2 addition compound. In contrast the Rh-C distances in 1e-Rh(CO) 2 (acac) are longer (∆ = 0.117 Å) than in the corresponding NHC compound [61] and a partial π interaction was found by DFT calculation. Rh also shows no tendency for coordination of the adjacent amino groups [32].
were found when 1b act as a four electron donor [40].
Further, 1e-ZnCl2 (See Scheme 5) [32] and 1a-ZnI2 [53] have closely related structural parameters but exhibit shorter C-Zn bond lengths than to related NHC-addition compounds (Δ = 0.051 Å) [60]. In both compounds a nearly perpendicular array of the ZnX2 and the PCP plane are found. No tendency for an additional N-coordination to the amino ligand of 1e is recorded for the ZnCl2 addition compound. In contrast the Rh-C distances in 1e-Rh(CO)2(acac) are longer (Δ = 0.117 Å) than in the corresponding NHC compound [61] and a partial π interaction was found by DFT calculation. Rh also shows no tendency for coordination of the adjacent amino groups [32]. Scheme 5. Selected transition metal complex with the carbone 1e as two electron donor ligand. Scheme 5. Selected transition metal complex with the carbone 1e as two electron donor ligand.
Various cationic complexes where reported with the pincer ligand 2a (See Figure 4) and group 10 metal halides and one dication with the group 11 metal Au. The 31 P NMR shifts range between 32 and 41 ppm(See Table 2). As with 1a the carbone carbon atom of 2a is basic enough to accept a proton to generate complexes of the type 2aH-MCl dications with all group 10 elements (See Scheme 6).
Various cationic complexes where reported with the pincer ligand 2a (See Figure 4) and group 10 metal halides and one dication with the group 11 metal Au. The 31 P NMR shifts range between 32 and 41 ppm(See Table 2). As with 1a the carbone carbon atom of 2a is basic enough to accept a proton to generate complexes of the type 2aH-MCl dications with all group 10 elements (See Scheme 6).  Table 2. Transition metal complexes with the phosphine based pincer ligands 2a and the pyridyl based pincer ligand 2b; C-M and P-C distances are included and 31 P NMR shifts in ppm.

P
C-M P-C P-C-P Ref Figure 4. Tripodal basic pincer ligand 2a with its tautomers, the anionic pincer ligand 2cH − and the pyridyl pincer ligand 2b. Table 2. Transition metal complexes with the phosphine based pincer ligands 2a and the pyridyl based pincer ligand 2b; C-M and P-C distances are included and 31 P NMR shifts in ppm.  . This coordination mode is continued in a series of dicopper complexes presented by the same working group and prepared as depicted in Scheme 7. The addition of [Cu]PF6 to 2b followed by treatment with two eq. of PR3 generated the cationic complexes [2b-(CuPPh3)](PF6)2 and [2b-(CuP{C6H4OMe}3](PF6)2, respectively; 2b-(CuCarb)2 was obtained from 2b-(CuCl)2 and two eq. of CarbH/NaO t Bu (CarbH = carbazol) [63]. A series of complexes with the N,C,N pincer ligand sym-bis(2-pyridyl) tetraphenylcarbodiphosphorane (2b) were reported recently by the group of Sundermeyer. Remarkable is the molybdenum complex 2b-[Mo 2 (CO) 7 ] in which 2b provides four pairs of electrons for donation to a Mo 2 unit with an Mo-Mo separation of 3.0456(5) Å [64]. This coordination mode is continued in a series of dicopper complexes presented by the same working group and prepared as depicted in Scheme 7. The addition of [Cu]PF 6 to 2b followed by treatment with two eq. of PR 3 generated the cationic complexes [2b-(CuPPh 3 )](PF 6 ) 2 and [2b-(CuP{C 6 H 4 OMe} 3 ](PF 6 ) 2 , respectively; 2b-(CuCarb) 2 was obtained from 2b-(CuCl) 2 and two eq. of CarbH/NaO t Bu (CarbH = carbazol) [63]. Further, 2cH-CuPPh3 is an example of a complex with a deprotonated form of 2a and longer P-C distances are observed due to the protonation of the central carbon atom [63].
Further, 2cH-CuPPh 3 is an example of a complex with a deprotonated form of 2a and longer P-C distances are observed due to the protonation of the central carbon atom [63].
Molecules 2020, 25, x FOR PEER REVIEW 12 of 54 31 PNMR shifts are in the narrow range of 33 and 36 ppm (See Table 3) [70]. More complicated is the formation of 3a-Pt(CO), which stems from the hydrolysis of the related 3a-Pt(CCl2) complex (not isolated) [71].    Table 3. Transition metal complexes with ortho metallated tripodal pincer ligand 3a derived from 1a and the related pincer ligand 3b and 31 P NMR shifts. The carbone complex 3b-Pt(CO) was obtained from reacting the yldiide platinum complex (see Scheme 9) with 1 atm CO that inserts into the N-Si bond of the yldiide. The carbone complex 3b-Pt(CO) was obtained from reacting the yldiide platinum complex (see Scheme 9) with 1 atm CO that inserts into the N-Si bond of the yldiide. Scheme 9. Two mesomeric forms of 3b-Pt(CO); 3ba favors a tricarbene coordination at Pt(0) whereas 3bb is consistent Pt(II) forming two C-Pt s-bonds similar to 3a-Pt(CO). The short central C-Pt bond length of 2.002 Å indicates a partial doubly donation of the carbone C atom as shown in Figure 5. The planar environment at Pt is typical for Pt(II) and supports this view [72].

Transition Metal Complexes with P-C-P Five Membered Ring
The carbone 4 (see Figure 6) was obtained by deprotonation of the cation [4H] + . According to two P atoms in different chemical environments two doublets in the 31 P NMR spectrum were recorded at δ = 60.0 and 71.5 ppm; 2 JPP = 153 Hz. From X-ray determination stem the P-C(1) and P-C(2) distances of 1.644 (19) and 1.657(17) Å, respectively, and the P-C-P angle amounts to 104.82(10)° [73]. The bond lengths (see Table 4) are close to that reported for the carbone 1a. Scheme 9. Two mesomeric forms of 3b-Pt(CO); 3ba favors a tricarbene coordination at Pt(0) whereas 3bb is consistent Pt(II) forming two C-Pt s-bonds similar to 3a-Pt(CO). The short central C-Pt bond length of 2.002 Å indicates a partial doubly donation of the carbone C atom as shown in Figure 5. The planar environment at Pt is typical for Pt(II) and supports this view [72]. The carbone complex 3b-Pt(CO) was obtained from reacting the yldiide platinum complex (see Scheme 9) with 1 atm CO that inserts into the N-Si bond of the yldiide. Scheme 9. Two mesomeric forms of 3b-Pt(CO); 3ba favors a tricarbene coordination at Pt(0) whereas 3bb is consistent Pt(II) forming two C-Pt s-bonds similar to 3a-Pt(CO). The short central C-Pt bond length of 2.002 Å indicates a partial doubly donation of the carbone C atom as shown in Figure 5. The planar environment at Pt is typical for Pt(II) and supports this view [72].

Transition Metal Complexes with P-C-P Five Membered Ring
The carbone 4 (see Figure 6) was obtained by deprotonation of the cation [4H] + . According to two P atoms in different chemical environments two doublets in the 31 P NMR spectrum were recorded at δ = 60.0 and 71.5 ppm; 2 JPP = 153 Hz. From X-ray determination stem the P-C(1) and P-C(2) distances of 1.644(19) and 1.657(17) Å, respectively, and the P-C-P angle amounts to 104.82(10)° [73]. The bond lengths (see Table 4) are close to that reported for the carbone 1a.

Transition Metal Complexes with P-C-P Five Membered Ring
The carbone 4 (see Figure 6) was obtained by deprotonation of the cation [4H] + . According to two P atoms in different chemical environments two doublets in the 31 P NMR spectrum were recorded at δ = 60.0 and 71.5 ppm; 2 J PP = 153 Hz. From X-ray determination stem the P-C(1) and P-C(2) distances of 1.644(19) and 1.657(17) Å, respectively, and the P-C-P angle amounts to 104.82(10) • [73]. The bond lengths (see Table 4) are close to that reported for the carbone 1a. Molecules 2020, 25, x FOR PEER REVIEW 14 of 54  From the cyclic and asymmetric carbone 4 six transition metal complexes (see Scheme 10) are known in which the ligand acts as two electron donor via the C atom. As in the starting compound 4 the P 2 -C bond distances are slightly longer than P 1 -C bond. Addition of CuCl and AuCl(SMe2) to 4H + /tBuOK generates the compounds 4-CuOtBu and 4-AuOtBu, respectively. In CH3Cl2 or CHCl3 4-CuOtBu is converted into 4-CuCl [74]

Transition Metal Complexes with Asymmetric P-C-P Ligands
Several asymmetric carbones with orthometallation (5a-M, 5d-M), with an additional donor function (5c), or with a functionalized phenyl ring (5b) were reported that form TM complexes (see Figure 7).  From the cyclic and asymmetric carbone 4 six transition metal complexes (see Scheme 10) are known in which the ligand acts as two electron donor via the C atom. As in the starting compound 4 the P 2 -C bond distances are slightly longer than P 1 -C bond. Addition of CuCl and AuCl(SMe 2 ) to 4H + /tBuOK generates the compounds 4-CuOtBu and 4-AuOtBu, respectively. In CH 3 Cl 2 or CHCl 3 4-CuOtBu is converted into 4-CuCl [74]  From the cyclic and asymmetric carbone 4 six transition metal complexes (see Scheme 10) are known in which the ligand acts as two electron donor via the C atom. As in the starting compound 4 the P 2 -C bond distances are slightly longer than P 1 -C bond. Addition of CuCl and AuCl(SMe2) to 4H + /tBuOK generates the compounds 4-CuOtBu and 4-AuOtBu, respectively. In CH3Cl2 or CHCl3 4-CuOtBu is converted into 4-CuCl [74]

Transition Metal Complexes with Asymmetric P-C-P Ligands
Several asymmetric carbones with orthometallation (5a-M, 5d-M), with an additional donor function (5c), or with a functionalized phenyl ring (5b) were reported that form TM complexes (see Figure 7).

5-M 31 P NMR ( 2 J PP ) M-C P 1 -C P 2 -C P-C-P Ref.
Transition metal complexes of 5a-M 5a-Ptcod(C 8  19.3 nr nr nr [78] As depicted in Scheme 11, three neutral complexes of 1a are known in which one of its phenyl group is orthometallated to produce the 5a-M core. The 31 P NMR shift of the unchanged PPh 3 group range between about 6 and 13 ppm whereas for the orthometallated side shifts between 15 and 40 ppm where recorded. Both P-C distances do not differ markedly and amount to about 1.700 Å. As depicted in Scheme 11, three neutral complexes of 1a are known in which one of its phenyl group is orthometallated to produce the 5a-M core. The 31 P NMR shift of the unchanged PPh3 group range between about 6 and 13 ppm whereas for the orthometallated side shifts between 15 and 40 ppm where recorded. Both P-C distances do not differ markedly and amount to about 1.700 Å. phenyl group is orthometallated to produce the 5a-M core. The 31 P NMR shift of the unchanged PPh3 group range between about 6 and 13 ppm whereas for the orthometallated side shifts between 15 and 40 ppm where recorded. Both P-C distances do not differ markedly and amount to about 1.700 Å. The paramagnetic 5c-UCl 4 exhibits a short C-U distance indicative for a double dative bond of the carbone C atom as in 2b-UCl 4 and was obtained by reacting UCl 4 with the dication 5c-H 2 /NaHMDS. Upon further coordination of the pyridyl group (U-N = 2.537(4) Å) the U atom attains the coordination number 6 [41].
bond of the carbone C atom as in 2b-UCl4 and was obtained by reacting UCl4 with the dication 5c-H2/NaHMDS. Upon further coordination of the pyridyl group (U-N = 2.537(4) Å) the U atom attains the coordination number 6 [41].
[5c-AuPPh3] + was obtained from reacting the carbone 5c with [PPh3AuCl]/Na[SbCl6] (see Scheme 13). In the cationic complex [5c-(CuCl)((AuPPh3)]SbF6, the carbone 5c acts as a six-electron donor with a Cu-N distance of 2.267(6) Å and Cu-Au separation of 2.8483(10) Å. The Cu and Cl atoms are each disordered over two positions with occupancy of about 0.8 to 0.2. If CuCl is replaced by AuCl as in [5c-(AuCl)(AuPPh3)]SbF6 the C-AuPPh3 distance is slightly elongated and no coordination of the pyridyl N atom is observed. The Au-Au separation is with 3.1274(6) Å too long for a metallophilic interaction. In both compounds, the carbone C atom constitutes a chiral center according to four chemical different substituents and acts as a four-electron donor. The PPh3 group resonates between 15 and 27 ppm [76]. In the related symmetric pyridyl-free complex 1a-(AuCl)2, slightly shorter C-Au (2.076(3) Å) were recorded accompanied by longer P-C (1.776(3) Å) bond lengths [51]. Scheme 13. Selected structures of transition metal complexes with the mono pyridyl substituted carbone 5c.

Transition Metal Complexes of Carbones with Cyclobutadiene
The carbones 6a and 6b (see Figure 8) can also be seen as an all-carbon four-membered ring bent allene (CBA); 6a is stable for several hours at −20° but decomposes when warmed up to −5°. The optimized geometry reveals a very acute allene bond angle of 85.0° and coplanarity of the ring carbon atoms including the two nitrogen atoms. The C=C bonds of the allene fragment amount to 1.423 Å and are significantly longer than in typical linear allenes (1.31 Å). Short CN bonds of 1.36 Å indicate some double bond character. The CCC carbon atom resonates in the 13 C NMR spectrum at 151 ppm. The first and second proton affinities (PAs) are very high amounting to 307 and 152 kcal/mol [79].

Transition Metal Complexes of Carbones with Cyclobutadiene
The carbones 6a and 6b (see Figure 8) can also be seen as an all-carbon four-membered ring bent allene (CBA); 6a is stable for several hours at −20 • but decomposes when warmed up to −5 • . The optimized geometry reveals a very acute allene bond angle of 85.0 • and coplanarity of the ring carbon atoms including the two nitrogen atoms. The C=C bonds of the allene fragment amount to 1.423 Å and are significantly longer than in typical linear allenes (1.31 Å). Short CN bonds of 1.36 Å indicate some double bond character. The CCC carbon atom resonates in the 13 C NMR spectrum at 151 ppm. The first and second proton affinities (PAs) are very high amounting to 307 and 152 kcal/mol [79]. The molecular orbitals show that the HOMO and HOMO-1 have clearly the largest coefficients at the central carbon atom and exhibit the typical shape of lone-pair molecular orbitals with σ (HOMO) and π (HOMO-1) symmetry; however, with reversed order with respect to CDPs and CDCs. To emphasize the proximity of 6 to CDP carbones, we use the same symbolism mimicking a metal.
The free CBA 6b could not be obtained, but only the cationic 6bH + and 6bH2 2+ are known The molecular orbitals show that the HOMO and HOMO-1 have clearly the largest coefficients at the central carbon atom and exhibit the typical shape of lone-pair molecular orbitals with σ (HOMO) and π (HOMO-1) symmetry; however, with reversed order with respect to CDPs and CDCs. To emphasize the proximity of 6 to CDP carbones, we use the same symbolism mimicking a metal.
The free CBA 6b could not be obtained, but only the cationic 6bH + and 6bH 2 2+ are known and used as starting compounds for the syntheses of the related transition metal complexes [80]. The 13 C NMR shifts of the central carbon atom are shifted to higher fields relative to the starting free carbone ranging between 124 and 139 ppm (see Table 6). Table 6. Transition metal complexes with the all carbon ligand 6; 13 C NMR shifts (in ppm) of the donating carbon atom. Distances in Å, angles in deg.

Carbodicyclopropenylidene
Stephan described the first carbodicarbene stabilized by flanking cyclopropylidenes, named carbodicyclopropylidene 7 (see Figure 9) [81]. Neither the neutral singlet 1,2-diphenylcyclopropenylidene as carbene ligand L in 7 nor the carbone tetraphenylcarbodicyclopropenyliden (CDC) 7 itself are stable compounds at room temperature. The free carbene L has only been observed in an argon matrix isolated at 10 K and 7 could be characterized in solution by low temperature NMR spectroscopy; for the central carbon atom a 13 C NMR shift at δ = 133 ppm was recorded at −60 °C.
The first and second proton affinities of 7 were determined to be 283 and 153 kcal/mol, respectively. The molecular structure of 7 was determined by computational methods. Calculations reveal that the central carbon atom is in a linear environment the C-C distances were calculated at 1.308 Å and the C-C-C angle to 180°. The energy difference between the linear allenic structure and the bent arrangement is shallow amounting to 6.6 kcal/mol for a Neither the neutral singlet 1,2-diphenylcyclopropenylidene as carbene ligand L in 7 nor the carbone tetraphenylcarbodicyclopropenyliden (CDC) 7 itself are stable compounds at room temperature. The free carbene L has only been observed in an argon matrix isolated at 10 K and 7 could be characterized in solution by low temperature NMR spectroscopy; for the central carbon atom a 13 C NMR shift at δ = 133 ppm was recorded at −60 • C.
The first and second proton affinities of 7 were determined to be 283 and 153 kcal/mol, respectively. The molecular structure of 7 was determined by computational methods. Calculations reveal that the central carbon atom is in a linear environment the C-C distances were calculated at 1.308 Å and the C-C-C angle to 180 • . The energy difference between the linear allenic structure and the bent arrangement is shallow amounting to 6.6 kcal/mol for a bending angle of 140 • and 10 kcal/mol for 130 • . The highest occupied molecular orbital (HOMO) and HOMO-1 of 7 are degenerate and incorporate the p(π) orbitals of the C2-C1-C2a fragment.
The central C atom is more negatively charged (−0.19 a.u.) than the adjacent C atoms, suggesting nucleophilic character [81].

7-M 13 C NMR M-C C-C C-C-C Ref.
[ bending angle of 140° and 10 kcal/mol for 130°. The highest occupied molecular orbital (HOMO) and HOMO-1 of 7 are degenerate and incorporate the p(π) orbitals of the C2-C1-C2a fragment.
The central C atom is more negatively charged (−0.19 a.u.) than the adjacent C atoms, suggesting nucleophilic character [81].

Carbodicarbenes
Carbodicarbenes, CDCs, are neutral compounds where a bare carbon atom with its four electrons is stabilized by two NHC ligands which plays the role of a phosphine group as in carbodiphosphoranes, CDPs. Theoretical studies have demonstrated that this class of compounds could be stable and their existence was predicted by Frenking [82] and short times

Carbodicarbenes
Carbodicarbenes, CDCs, are neutral compounds where a bare carbon atom with its four electrons is stabilized by two NHC ligands which plays the role of a phosphine group as in carbodiphosphoranes, CDPs. Theoretical studies have demonstrated that this class of compounds could be stable and their existence was predicted by Frenking [82] and short times later realized by the group of Bertrand [83].

C NMR
M-C C-C C-C-C Ref.
As depicted in Scheme 17, introduction of PdCl2P(OiPr)3 to 8e afforded the complex 8e-PdCl2P(OiPr)3; it features a square planar Pd center with a short interatomic distance of one phosphite oxygen atom and the carbon atom of the NHC molecule of 2.890 Å that is smaller than the sum of van der Waals radii. This indicates strong attractive interaction between the atoms [88]. The three Pd complexes 8e-PdCl2PPh3, 8e-PdCl2PTol3, and 8e-PdCl2PCy3 were The addition compounds 8c-RhCl(CO) 2 and 8d-RhCl(CO) 2 where obtained upon reacting the appropriate carbone 8c or 8d with [RhCl(CO) 2 ] 2 . Similarly, the addition of [Pd(allyl)Cl] 2 to 8c leads to the allyl complex 8c-PdCl(C 3 H 5 ) [85].
As depicted in Scheme 17, introduction of PdCl 2 P(OiPr) 3 to 8e afforded the complex 8e-PdCl 2 P(OiPr) 3 ; it features a square planar Pd center with a short interatomic distance of one phosphite oxygen atom and the carbon atom of the NHC molecule of 2.890 Å that is smaller than the sum of van der Waals radii. This indicates strong attractive interaction between the atoms [88]. The three Pd complexes 8e-PdCl 2 PPh 3 , 8e-PdCl 2 PTol 3 , and 8e-PdCl 2 PCy 3 were obtained by reacting the carbone 8e with the appropriate PdCl 2 PR 3 ; between the NHC and the aromatic phosphine substituents (Ph or Tol) an unexpected π-π interaction was detected. One Ph and one Tol group are nearly parallel to the imidazole rings with centroid-centroid distances of 3.25 Å (Ph) and 3.30 Å (Tol), respectively [89]. 8f-RhCl(CO)2 and 8g-RhCl(CO)2 stem from reacting the appropriate carbone with [RhCl(CO)2]2 [90]. The cod ligand of [Ir(cod)Cl]2 was replaced by bubbling CO through a mixture with 8h to generate the complex 8h-IrCl(CO)2 [91].
Some experimental findings indicate that carbodicarbenes also have catalytic properties for a wide range of transformations, which are currently being actively studied by several groups. Examples have been reported such as hydrogenation of inert olefins [92], C-C cross-coupling reactions [84], intermolecular hydroamination [93] and hydroheteroarylation [94]. It seems that this area is still in an infant stadium and it can be expected that CDCs may be found useful as catalyst for other reactions.

Tridentate Cyclic Diphosphino CDCs
The carbones 9a and 9b in Figure 12 are functionalized carbodicarbene in which the donating carbon atom is part of a seven membered ring. The neutral 9a and 9b could not be isolated, source for transition metal complexes are the related cations 9aH + and 9bH + (see Table 9) [93]. 8f-RhCl(CO) 2 and 8g-RhCl(CO) 2 stem from reacting the appropriate carbone with [RhCl(CO) 2 ] 2 [90]. The cod ligand of [Ir(cod)Cl] 2 was replaced by bubbling CO through a mixture with 8h to generate the complex 8h-IrCl(CO) 2 [91].
Some experimental findings indicate that carbodicarbenes also have catalytic properties for a wide range of transformations, which are currently being actively studied by several groups. Examples have been reported such as hydrogenation of inert olefins [92], C-C cross-coupling reactions [84], intermolecular hydroamination [93] and hydroheteroarylation [94]. It seems that this area is still in an infant stadium and it can be expected that CDCs may be found useful as catalyst for other reactions.
Some experimental findings indicate that carbodicarbenes also have catalytic properties for a wide range of transformations, which are currently being actively studied by several groups. Examples have been reported such as hydrogenation of inert olefins [92], C-C cross-coupling reactions [84], intermolecular hydroamination [93] and hydroheteroarylation [94]. It seems that this area is still in an infant stadium and it can be expected that CDCs may be found useful as catalyst for other reactions.

Tridentate Cyclic Diphosphino CDCs
The carbones 9a and 9b in Figure 12 are functionalized carbodicarbene in which the donating carbon atom is part of a seven membered ring. The neutral 9a and 9b could not be isolated, source for transition metal complexes are the related cations 9aH + and 9bH + (see Table 9) [93]. The neutral 9a and 9b could not be isolated, source for transition metal complexes are the related cations 9aH + and 9bH + (see Table 9) [93]. Table 9. Transition metal complexes with the carbones 9a and 9b; 13 C NMR signal of the central donating carbon atom.

9-M 13 C NMR M-C C-C C-C-C Ref.
Transition metal complexes with the carbone 9a 9a-RhCl 73

Tetraaminoallene (TAA) Transition Metal Complexes
The 13 C NMR shift of the central carbon atom amounts to 142.8 ppm. The first and second PAs of 10 are 282.5 and 151.6 kcal/mol, respectively [16,82].
The salt [10-AuPPh3]SbF6 in Scheme 19 is the only transition metal complex of TAA (see Figure 13), which has been reported so far. Both carbene moieties are planar, but are tilted relative to each other, to relieve allylic strain. The Au-C bond lengths amounts to 2.072(3) Å and the slightly different C-C dative bonds has interatomic distances of 1.406 (5)

Tetraaminoallene (TAA) Transition Metal Complexes
The 13 C NMR shift of the central carbon atom amounts to 142.8 ppm. The first and second PAs of 10 are 282.5 and 151.6 kcal/mol, respectively [16,82].
The salt [10-AuPPh 3 ]SbF 6 in Scheme 19 is the only transition metal complex of TAA (see Figure 13), which has been reported so far. Both carbene moieties are planar, but are tilted relative to each other, to relieve allylic strain. The Au-C bond lengths amounts to 2.072(3) Å and the slightly different C-C dative bonds has interatomic distances of 1.406 (5)

Transition Metal Complexes of Carbones with the P-C-C Skeleton
Mixed carbene-phosphine stabilized carbones from the working group of Bestmann (1974) and Alkarazo (2009).

Transition Metal Complexes of Carbones with the P-C-C Skeleton
Mixed carbene-phosphine stabilized carbones from the working group of Bestmann (1974) and Alkarazo (2009).

Transition Metal Complexes of Carbones with the P-C-C Skeleton
Mixed carbene-phosphine stabilized carbones from the working group of Bestmann (1974) and Alkarazo (2009).

Transition Metal Complexes of Carbones with the P-C-C Skeleton
Mixed carbene-phosphine stabilized carbones from the working group of Bestmann (1974) and Alkarazo (2009).

Transition Metal Complexes of Carbones with the P-C-Si Skeleton
The neutral compound 12 in Figure 15 is a carbone in which the C(0) atom is stabilized by a donor stabilized silylene and a phosphine ligand. The crystal structure of a related compound to 12 (a cyclopentene instead of a cyclohexene ring) shows a P-C distance of 1.6226(4) Å and Si-C distance of 1.6844(4) Å; the Si-C-P angle amounts to 140.03(3)°.
Addition of CuCl generates the complex 12-CuCl. No spectroscopic or structural details are available [99].

Transition Metal Complexes of Carbones with the P-C-S Skeleton
A series of carbones (13a, 13b) in Figure 16 based on a P-C-S core containing the neutral S(IV) ligands SPh2=NMe ( Figure 16) were reported by Fujii [100].

Transition Metal Complexes of Carbones with the P-C-Si Skeleton
The neutral compound 12 in Figure 15 is a carbone in which the C(0) atom is stabilized by a donor stabilized silylene and a phosphine ligand. The neutral Rh complex 11a-RhCl(CO)2 was obtained from reacting the carbone 11a with [Rh(CO)2Cl]2. Similarly, the complex 11b-AuCl results from reaction of 11b with AuCl(SMe2) (Scheme 20) [98].

Transition Metal Complexes of Carbones with the P-C-Si Skeleton
The neutral compound 12 in Figure 15 is a carbone in which the C(0) atom is stabilized by a donor stabilized silylene and a phosphine ligand. The crystal structure of a related compound to 12 (a cyclopentene instead of a cyclohexene ring) shows a P-C distance of 1.6226(4) Å and Si-C distance of 1.6844(4) Å; the Si-C-P angle amounts to 140.03(3)°.
Addition of CuCl generates the complex 12-CuCl. No spectroscopic or structural details are available [99].

Transition Metal Complexes of Carbones with the P-C-S Skeleton
A series of carbones (13a, 13b) in Figure 16 based on a P-C-S core containing the neutral S(IV) ligands SPh2=NMe ( Figure 16) were reported by Fujii [100]. The crystal structure of a related compound to 12 (a cyclopentene instead of a cyclohexene ring) shows a P-C distance of 1.6226(4) Å and Si-C distance of 1.6844(4) Å; the Si-C-P angle amounts to 140.03(3) • .
Addition of CuCl generates the complex 12-CuCl. No spectroscopic or structural details are available [99].

Transition Metal Complexes of Carbones with the P-C-S Skeleton
A series of carbones (13a, 13b) in Figure 16 based on a P-C-S core containing the neutral S(IV) ligands SPh 2 =NMe ( Figure 16) were reported by Fujii [100].

Transition Metal Complexes of Carbones with the P-C-Si Skeleton
The neutral compound 12 in Figure 15 is a carbone in which the C(0) atom is stabilized by a donor stabilized silylene and a phosphine ligand. The crystal structure of a related compound to 12 (a cyclopentene instead of a cyclohexene ring) shows a P-C distance of 1.6226(4) Å and Si-C distance of 1.6844(4) Å; the Si-C-P angle amounts to 140.03(3)°.
Addition of CuCl generates the complex 12-CuCl. No spectroscopic or structural details are available [99].

Transition Metal Complexes of Carbones with the P-C-S Skeleton
A series of carbones (13a, 13b) in Figure 16 based on a P-C-S core containing the neutral S(IV) ligands SPh2=NMe ( Figure 16) were reported by Fujii [100].  Table 11. Collection of transition metal complexes with the carbones 13a and 13b. 31 P NMR signals (in ppm) are given.
Addition of TM fragments to 13a or 13b in Scheme 21 elongates P-C and S-C bond length as reported for 1a. That of [13bH-AuPPh3](OTf)2 in which 13b acts as four-electron donor are elongated to normal single bonds [100].
Addition of TM fragments to 13a or 13b in Scheme 21 elongates P-C and S-C bond length as reported for 1a. That of [13bH-AuPPh 3 ](OTf) 2 in which 13b acts as four-electron donor are elongated to normal single bonds [100].

Transition Metal Complex with a P-C-S Core Possessing a Neutral S(II) Ligand
The carbone 14 in Figure 17 contains a phosphine and a S(II) ligand with a free pair of electrons to stabilize the C(0) atom. However, the bare 14 could not be isolated, but only the protonated cation [14H] + and used as starting material [101].

Transition Metal Complex with a P-C-S Core Possessing a Neutral S(II) Ligand
The carbone 14 in Figure 17 contains a phosphine and a S(II) ligand with a free pair of electrons to stabilize the C(0) atom. However, the bare 14 could not be isolated, but only the protonated cation [14H] + and used as starting material [101]. The transition metal complex [14-CuN(SiMe3)2](OTf) was prepared upon reacting [14H]+ with KHMDS/CuCl. X-ray analysis reveals a Cu-C distance of 1.903(4) Å and the P-C and S-C distances amount to 1.709(5) and 1.677(5) Å, respectively. As found in carbone addition compounds of 13a and 13b the P-C distance is longer than the S-C distance. An acute P-C-S angle of 115.3(2)° was recorded. The 31 P NMR signal is shifted to lower fields at 66.5 ppm [101].

Transition Metal Complex with a P-C-S Core Possessing a Neutral S(II) Ligand
The carbone 14 in Figure 17 contains a phosphine and a S(II) ligand with a free pair of electrons to stabilize the C(0) atom. However, the bare 14 could not be isolated, but only the protonated cation [14H] + and used as starting material [101]. The transition metal complex [14-CuN(SiMe3)2](OTf) was prepared upon reacting [14H]+ with KHMDS/CuCl. X-ray analysis reveals a Cu-C distance of 1.903(4) Å and the P-C and S-C distances amount to 1.709(5) and 1.677(5) Å, respectively. As found in carbone addition compounds of 13a and 13b the P-C distance is longer than the S-C distance. An acute P-C-S angle of 115.3(2)° was recorded. The 31 P NMR signal is shifted to lower fields at 66.5 ppm [101].

Transition Metal Complexes of Carbones with the S-C-Se Skeleton (16)
Compound 16 in Figure 19 is the first carbone containing a Se(II) compound together with a S(IV) one as ligand for stabilization of a C(0) atom. are less shielded than that of the basic carbones [102].

Transition Metal Complexes of Carbones with the S-C-Se Skeleton (16)
Compound 16 in Figure 19 is the first carbone containing a Se(II) compound together with a S(IV) one as ligand for stabilization of a C(0) atom. C NMR signals of the donating C(0) atoms (if available) of all addition compounds of 15a to 15c are less shielded than that of the basic carbones [102]. (16) Compound 16 in Figure 19 is the first carbone containing a Se(II) compound together with a S(IV) one as ligand for stabilization of a C(0) atom.   Table 13). In this and in [16H-Ag-16H] 3+ the donating C(0) acts as a four-electron donor (see Scheme 25) [105]. Table 13. Transition metal complexes with selected bond length (Å) and angles (deg) of the carbone 16. 13 C NMR signal (in ppm) of the central carbon atom.

16-M 13 C NMR C-M C-S C-Se S-C-Se
Ref.

Transition Metal Carbido Complexes [M]-C
The second part of this review summarizes the research of transition metal complexes with a naked carbon atom as ligand [M]-C. They are often termed as carbides, but the bonding situation is clearly different from well-known carbides of the alkaline and alkaline earth elements E, which are salt compounds of acetylene EnC2. The electron configuration of carbon atom in the 1 D state (2s 2 2px 2 2py 0 2pz 0 ) is perfectly suited for dative bonding with a transition metal following the DCD model [7]

Transition Metal Carbido Complexes [M]-C
The second part of this review summarizes the research of transition metal complexes with a naked carbon atom as ligand [M]-C. They are often termed as carbides, but the bonding situation is clearly different from well-known carbides of the alkaline and alkaline earth elements E, which are salt compounds of acetylene E n C 2 . The electron configuration of carbon atom in the 1 D state (2s 2 2p x 2 2p y 0 2p z 0 ) is perfectly suited for dative bonding with a transition metal following the DCD model [7] in terms of σ donation and π backdonation [M] Molecules 2020, 25, x; doi: FOR PEER REVIEW www.mdpi.com/journal/molecules Transition metal compounds with metal-carbon bonds are the backbone of organometallic chemistry. Molecules with M-C single bonds are already known since 1849 when Frankland reported the accidental synthesis of diethyl zinc while attempting to prepare free ethyl radicals [1,2]. Molecules with a [M]=CR2 double bond (carbene complexes) or a [M]≡CR triple bond (carbyne complexes) were synthesized much later [3][4][5][6]. Two types of compounds with metal-carbon double or triple bonds having different types of bonds are generally distinguished, which are named after the people who isolated them first. Fischer-type carbene and carbyne complexes are best described in terms of dative bonds following the Dewar-Chatt-Duncan (DCD) model [7,8] [9][10][11].
This review deals with transition metal complexes with metal-carbon bonds to two types of ligands, which have only recently been isolated and theoretically studied. One type of ligand are carbones CL2 [12], which are carbon(0) compounds with two dative bonds to a carbon atom in the excited 1 D state L→C ←L where the carbon atom retains its four valence electrons as two lone pairs that can serve as four-electron donors [13,14]. Thus, carbones CL2 are four-electron donor ligands whereas carbenes CR2 are two-electron donors. Carbenes have a formally [15] C|. Carbon complexes [M]-C may thus be considered as carbone complexes [M]-CL 2 without the ligands L at the carbon atoms. A theoretical study showed in 2000 that the 18 valence electron (VE) complex [(CO) 4 Fe(C)] is an energy minimum structure with a rather strong Fe-C bond [106]. However, such 18 VE systems could not be synthesized as isolated species but were only found as ligands where the lone-pair electron at the carbon atom serves as donor (see below). It seems that the electron lone-pair at carbon in the 18 VE complexes [M]-C makes the adducts too reactive to become isolated.
It came as a surprise when Heppert and co-workers reported in 2002 the first neutral adducts with a naked carbon atom as a ligand, which are the formally 16 VE diamagnetic ruthenium complexes [(PCy 3 )LCl 2 Ru(C)] (L= PCy and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene; Cy = Cyclohexyl) [27]. A subsequent bonding analysis of the model compound [(Me 3 P) 2 Cl 2 Ru-C] considered five different models A-E for the Ru-C bonds that are shown in Figure 20 [28]. It turned out that the best description for the bonding interactions is a combination of electron-sharing and dative bonds. An energy decomposition analysis [107] suggested that the model B provides the most faithful account of the bond, where the σ bond and the π bond in the Cl 2 M plane come from electron-sharing interactions Cl 2 M=C whereas the π bond in the P 2 M plane is due to backdonation (Me 3 P) 2

The System RuCl 2 (PCy 3 ) 2 C ([Ru]C)
By far the most known complexes with carbido ligands that have been synthesized and structurally characterized are ruthenium adducts. The progress in the chemistry of ruthenium carbido complexes was reviewed in 2012 by Takemoto and Matsuzaka [108]. In the following, we summarize the present knowledge on ruthenium carbido complexes which has been reported in the literature.
The X-ray analysis of [Ru]C in Figure 21 exhibits a Ru-C distance of 1.632(6) Å. A signal at 471.8 ppm was attributed to the ligand carbon atom [109]. A general route to carbon complexes is described in [110].  Table 14) [29,109] [111]. A series of Pt, Pd, Rh, Ir, Ag, Ru complexes were presented by Bendix with X-ray data and 13 C NMR shifts of the ligand carbon atom ranging between 340 and 412 ppm [112]. Sulfur containing TM complexes with the metals Pd, Pt, Au, and Cu stem from the same laboratory. The sulfur ligands are ttcn = 1,4,7-trithiacyclononane and S4(MCp*)3 (see Figure 22) [113].  Table 14.   Table 14) [29,109].  [111]. A series of Pt, Pd, Rh, Ir, Ag, Ru complexes were presented by Bendix with X-ray data and 13 C NMR shifts of the ligand carbon atom ranging between 340 and 412 ppm [112]. Sulfur containing TM complexes with the metals Pd, Pt, Au, and Cu stem from the same laboratory. The sulfur ligands are ttcn = 1,4,7-trithiacyclononane and S 4 (MCp*) 3 (see Figure 22) [113].  Table 14) [29,109] [111]. A series of Pt, Pd, Rh, Ir, Ag, Ru complexes were presented by Bendix with X-ray data and 13 C NMR shifts of the ligand carbon atom ranging between 340 and 412 ppm [112]. Sulfur containing TM complexes with the metals Pd, Pt, Au, and Cu stem from the same laboratory. The sulfur ligands are ttcn = 1,4,7-trithiacyclononane and S4(MCp*)3 (see Figure 22) [113].  Table 14.   Table 14.

The System RuCl2(PCy3)(NHC)C ( NHC [Ru]C)
The X-ray analysis of NHC [Ru]C in Figure 23 exhibits a Ru-C distance of 1.605(2) Å. A signal at 471.5 ppm was attributed to the ligand carbon atom. No addition compounds were described so far [27].

The System RuCl 2 (PCy 3 )(NHC)C ( NHC [Ru]C)
The X-ray analysis of NHC [Ru]C in Figure 23 exhibits a Ru-C distance of 1.605(2) Å. A signal at 471.5 ppm was attributed to the ligand carbon atom. No addition compounds were described so far [27].

The System RuCl2(PCy3)(NHC)C ( NHC [Ru]C)
The X-ray analysis of NHC [Ru]C in Figure 23 exhibits a Ru-C distance of 1.605(2) Å. A signal at 471.5 ppm was attributed to the ligand carbon atom. No addition compounds were described so far [27].

The System (NHC)Cl 3 RuC − ( NHC [Ru] − C)
Treating the carbene complex (NHC)Cl 2 (PCy 3 )Ru=CH 2 in Figure 24 at 55 • in benzene generated the neutral complex depicted in Figure 25. X-ray analysis revealed a Ru 1 -C distance of 1.698(4) Å and the Ru 2 -C distance of 1.875(4) Å with a Ru-C-Ru angle of 160.3(2) • . In the 13 C NMR the bridging C atom resonates at the typical value of 414.0 ppm [114].

The System (NHC)Cl3RuC -( NHC [Ru] -C)
Treating the carbene complex (NHC)Cl2(PCy3)Ru=CH2 in Figure 24 at 55° in benzene generated the neutral complex depicted in Figure 25. X-ray analysis revealed a Ru 1 -C distance of 1.698(4) Å and the Ru 2 -C distance of 1.875(4) Å with a Ru-C-Ru angle of 160.3(2)°. In the 13 C NMR the bridging C atom resonates at the typical value of 414.0 ppm [114].   Table 15) [115].  3.5. The Systems OsCl 2 (PCy 3 ) 2 C and OsI 2 ( The carbido complexes [OsX]C in Figure 27 were studied by X-ray analysis. The most important structural parameter is the Os-C separation, which for X = Cl amounts to 1.689(5) Å [116]. Single-crystal X-ray diffraction reveals that molecular [OsX]C adopts an approximately square-pyramidal core geometry, with the carbido ligand occupying the apical position and a short Os-C bond. In the 13 C NMR spectrum the signal at 471.8 ppm for X = Cl was attributed to the ligand carbon atom. It was synthesized via S-atom abstraction from the thiocarbonyl complex Os(CS)(PCy 3 ) 2 Cl 2 by Ta(OSi-t-Bu 3 ) 3 . The diiodo derivative was synthesized from [OsCl]C upon reacting with 10 eq of Me 3 SiI and exhibits a 13 C NMR signal at 446.14 ppm.  The carbido complexes [OsX]C in Figure 27 were studied by X-ray analysis. The most important structural parameter is the Os-C separation, which for X = Cl amounts to 1.689(5) Å [116]. Single-crystal X-ray diffraction reveals that molecular [OsX]C adopts an approximately square-pyramidal core geometry, with the carbido ligand occupying the apical position and a short Os-C bond. In the 13 C NMR spectrum the signal at 471.8 ppm for X = Cl was attributed to the ligand carbon atom. It was synthesized via S-atom abstraction from the thiocarbonyl complex Os(CS)(PCy3)2Cl2 by Ta(OSi-t-Bu3)3. The diiodo derivative was synthesized from [OsCl]C upon reacting with 10 eq of Me3SiI and exhibits a 13 C NMR signal at 446.14 ppm.

Unique Mo Carbido Complex
A further unique carbido complex was described recently as shown in Figure 30. A signal at 360.8 ppm in the 13 C NMR spectrum was assigned to the ligand carbon atom [123].

Unique Mo Carbido Complex
A further unique carbido complex was described recently as shown in Figure 30. A signal at 360.8 ppm in the 13 C NMR spectrum was assigned to the ligand carbon atom [123].

Unique Mo Carbido Complex
A further unique carbido complex was described recently as shown in Figure 30. A signal at 360.8 ppm in the 13 C NMR spectrum was assigned to the ligand carbon atom [123].  Figure 31 [124]. Similarly, with [W]C-Li(THF) and FeCl(CO) 2 Figure 32. The X-ray analysis of the tetrameric unit revealed Au-C distances of 1.995 and 2.078 Å and the W-C distance is 1.877 Å [122].  Figure 32. The X-ray analysis of the tetrameric unit revealed Au-C distances of 1.995 and 2.078 Å and the W-C distance is 1.877 Å [122].   Figure 33) was prepared from the same starting material and [PtCl2(phen)] (see Table 17) [Error! Reference source not found.].   Figure 33) was prepared from the same starting material and [PtCl 2 (phen)] (see Table 17) [125].   Figure 33) was prepared from the same starting material and [PtCl2(phen)] (see Table 17) [Error! Reference source not found.].

The Systems N 3 MoC and O 3 MoC
The potassium salt of N MOC − in Figure 34 is dimeric with two K + ions bridging two anions and can be transformed with the crown ethers 2.0-benzo-15-crown-5 and 1.0 2,2,2-crypt into the related ion pairs. X-ray analysis of the crown ether salt revealed a Mo-C distance of 1.713(9) Å [26,126].

The Systems N3MoC and O3MoC
The potassium salt of N MOCin Figure 34 is dimeric with two K + ions bridging two anions and can be transformed with the crown ethers 2.0-benzo-15-crown-5 and 1.0 2,2,2-crypt into the related ion pairs. X-ray analysis of the crown ether salt revealed a Mo-C distance of 1.713(9) Å [26,126].    [128]. The complex was also obtained upon reacting Fe(TPP) with Me 3 SiCCl 3 [129]; see also [130]. An X-ray analysis was performed in [131] and later in [130]. The Mössbauer spectrum is published in [132].

Asymmetrically Bridged Carbido Complex Fe=C=M
3.11.1. The Fe=C=Re Core The asymmetrical carbido complex (TPP)Fe=C=Re(CO)4Re(CO)5 in Figure 39 was prepared upon reacting the dichlorocarbene complex (TPP)Fe=CCl2 with 2 eq of pentacarbonylrhenate, [Re(CO)5] − , under release of CO and 2 Cl − ; TPP is tetraphenylporphyrin. Crystals were analyzed by X-ray diffraction and revealed a Fe=C distance of 1.605(13) Å and a C=Re distance of 1.957(12) Å. The Fe-C-Re angle amounts to 173.3(9)°; the Fe-C distance is somewhat smaller than in [(TPP)Fe]2C and the Re-C distance is appreciable longer than in [Re(CO)2Cp]2C. In the 13 C NMR spectrum the central carbido C atom resonates at 211.7 ppm [143].  3.11. Asymmetrically Bridged Carbido Complex Fe=C=M 3.11.1. The Fe=C=Re Core The asymmetrical carbido complex (TPP)Fe=C=Re(CO) 4 Re(CO) 5 in Figure 39 was prepared upon reacting the dichlorocarbene complex (TPP)Fe=CCl 2 with 2 eq of pentacarbonylrhenate, [Re(CO) 5 ] − , under release of CO and 2 Cl − ; TPP is tetraphenylporphyrin. Crystals were analyzed by X-ray diffraction and revealed a Fe=C distance of 1.605(13) Å and a C=Re distance of 1.957(12) Å. The Fe-C-Re angle amounts to 173.3(9) • ; the Fe-C distance is somewhat smaller than in [(TPP)Fe] 2 C and the Re-C distance is appreciable longer than in [Re(CO) 2 Cp] 2 C. In the 13 C NMR spectrum the central carbido C atom resonates at 211.7 ppm [143].

Asymmetrically Bridged Carbido Complex Fe=C=M
3.11.1. The Fe=C=Re Core The asymmetrical carbido complex (TPP)Fe=C=Re(CO)4Re(CO)5 in Figure 39 was prepared upon reacting the dichlorocarbene complex (TPP)Fe=CCl2 with 2 eq of pentacarbonylrhenate, [Re(CO)5] − , under release of CO and 2 Cl − ; TPP is tetraphenylporphyrin. Crystals were analyzed by X-ray diffraction and revealed a Fe=C distance of 1.605(13) Å and a C=Re distance of 1.957(12) Å. The Fe-C-Re angle amounts to 173.3(9)°; the Fe-C distance is somewhat smaller than in [(TPP)Fe]2C and the Re-C distance is appreciable longer than in [Re(CO)2Cp]2C. In the 13 C NMR spectrum the central carbido C atom resonates at 211.7 ppm [143].

Conclusions
The experimental and theoretical research with regard transition metal complexes with carbone ligands [M]-CL2 and carbido complexes [M]-C has blossomed in the recent past and it can be foreseen that it will remain a very active area of organometallic chemistry in the future. The well-known family of transition metal complexes with C1-bonded carbon ligands that comprise alkyl (CR3), carbene (CR2), and carbyne (CR) groups has been extended by carbones (CL2) and carbido (C) ligands. The summary of recent work, which is described in this review, indicates that carbone and carbido complexes are still largely terra incognita and that many new discoveries can be expected.
Author Contributions: Conceptualization and writing of the first draft, W.P. and G.F. Checking and partial visualization L.Z. and C.C. All authors have read and agreed to the published version of the manuscript.

Conclusions
The experimental and theoretical research with regard transition metal complexes with carbone ligands [M]-CL 2 and carbido complexes [M]-C has blossomed in the recent past and it can be foreseen that it will remain a very active area of organometallic chemistry in the future. The well-known family of transition metal complexes with C1-bonded carbon ligands that comprise alkyl (CR 3 ), carbene (CR 2 ), and carbyne (CR) groups has been extended by carbones (CL 2 ) and carbido (C) ligands. The summary of recent work, which is described in this review, indicates that carbone and carbido complexes are still largely terra incognita and that many new discoveries can be expected.