Bridging Carbenes and Carbynes

The carbyne ligand is linear, having sp hybridization, and the MsC bond is very short (first row, 1.65-1.75 A second and third rows, 1.75-1.90 A). The - C NMR shows a characteristic low-field resonance for the carbyne carbon at H-250 to -i-400 ppm. 

Acetylenes RCsCH are readily deprotonated. Some carbynes, L MsCH, also having an acidic CH proton, can be deprotonated to give terminal carbide complexes, [L MsCl. For example, Peters and co-workers have deprotonated l(ArN /-Bu )3MosCH] with KCH2Ph. After the counterion was sequestered by complexation to a crown ether, [(ArN r-Bu )3MosC)(K(crown)] was isolated and structurally characterized. Bridging acetylides, L M—CsC—ML , and poly-acetylides, L M- CsC v-ML , are also well known.  

Like CO, caibenes can act not only as terminal (M=CH2) but also as tnidging ligands. When they bridge, a metal-metal bond is usually present as well (11.13 and 11.14). In bridging, caibenes lose some of their unsaturation, and tiieielbre the very high reactivity of their mononuclear analogs. Fischer methylenes are 

One of the most valuable synthetic routes to bridging carbenes involves the use of diazomethane (Eq. 11.42) and related compounds (Eq. 11.43), which are precursors for free carbenes in organic chemistry. 

Diazomethane adds not only to monomeric metal complexes but also to compounds containing metal-metal double bonds, a reaction somewhat analogous to the addition of a free carbene to a C=C double bond to give a cyclopropene. This analogy suggested itself to three groups at the same time, and, remarkably, they tried exactly the same reaction. Eq. 11.44  

Note how loss of CO regenerates the Rh=Rh double bond in what is really a substitution of CO by CH2. Insertion of CH2 into a metal-metal single bond is seen in the synthesis of the platinum A-frame (so-called because the structure resembles the letter A) complex 11.16 in Eq. 11.46, a rare example of a bridging methylene complex without an M—M bond. 

The second general method of bridging carbene complexes involves the analogy between C=C and M=C double bonds. Since many metal complexes will react with C=C double bonds to give alkene complexes, Stone investigated the reactions of the same metal complexes with compounds containing an M=C bond (Eq. 11.47). This is a very powerful method of making a variety of homo- and heterometallic complexes, and can be extended to the M=C triple bond as well. 

Like CO, CR2 can act not only as a terminal but also as a bridging ligand. On the traditional model, when CO or CR2 bridge, a metal-metal bond is usually present (11.12 and 11.13). In bridging, the carbene carbon moves from tricoordinate sp toward tetracoordinate sp. Fischer methylenes are rare, while the bridged form is better known and less reactive. Bridging carbenes can be made from diazomethane (Eq. 11.35). 

Hydride abstraction from a bridging carbene can give an unsaturated and very reactive p -bridging carbyne, having pronounced carbonium ion character. The bonding scheme resembles the one we saw for Fischer carbenes, except that this is a bis-metal-stabilized carbonium ion, 11.14. Carbynes can also bridge three metals, as in the long-known and very stable tricobalt complex 11.15 these are much less reactive than the unsaturated n -carbynes discussed earlier. 

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Extension to trimetallic complexes has led to the preparation of similar bridging carbene and carbyne complexes with Pt3,421 Pt2W422 and PtWFe423 metal frameworks. 

Using the Isolobal Analogy Metal Complexes with Bridging Carbenes and Carbynes... 

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

Alkylidyne Derivatives and Those with Bridging Carbene or Carbyne Ligands... 

Reactions of electrophilic metal carbenes and carbynes with nucleophilic metal centers are used to prepare a large number of clusters, most containing two or more different metals. The metal-metal bond formed is usually bridged by the carbene or carbyne ligand, but ligand transfer can lead to homometallic products. ... 

Electrons from the ligands this depends, naturally on the ligands. For hydrocarbon ligands, the number is equal to the hapto number. Single-bonded ligands (hydride, halide etc) count as 1 (although a bridging halide counts as 2 - a lone-pair donor), while carbenes and carbynes count as 2 and 3, respectively. Lone-pair donors, such as phosphines and CO, count as 2. 

Similar reactions are used with Cp(OC)2MnC(OMe)Ph and M(cod)2 (M = Pt, Pd, Ni) to give heterobimetallic complexes with bridging carbenes in yields of 25-63% . Bridging carbyne complexes can be prepared, as well ... 

Metal Carbenes and Metal Carbynes as Precursors for a Rational Synthesis of Carbido and Hydrocarbon Bridged Complexes... 

METAL CARBENES AND METAL CARBYNES AS PRECURSORS FOR A RATIONAL SYNTHESIS OF CARBIDO AND HYDROCARBON BRIDGED COMPLEXES... 

The isolobal connections of carbynes W(C0)2(CR)Cp with alkynes and carbene Cr[C(OMe)Ph](CO)3 with alkenes are reviewed.They react with late transition metal centres forming heterobimetallic bridging alkylidene and alkylidyne derivatives, there are many such examples and their chemistry towards species such as nucleophiles protons, alkynes, and metal fragment precursors is prolific. 

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