Carbyne complexes nucleophilic attack

The octahedral Ru(II) and Os(II) carbyne complexes are isoelectronic with the Group 6 compounds R—0=M(CO)4X prepared by Fischer and co-workers. Many of the Fischer compounds, particularly the cationic complexes, are susceptible to nucleophilic attack at the carbyne carbon (see Section II,B,2), and similar reactivity might be anticipated for the Group 8 carbynes. 

That the neutral d6 Ru and Os carbyne complexes have proved to be unreactive towards nucleophilic reagents suggests the relative electron richness of the MX3(PPh3)2 (M = Ru, Os) fragments when compared with the M(CO)4X residues in the Fischer complexes. The cationic Ru(II) and Os(II) compounds, however, are readily attacked by appropriate nucleophiles, e.g.,... 

These results suggest the presence of two competing pathways to products, which depend upon the location of protonation at the M=C carbyne bond. Charged controlled protonation at the carbyne carbon followed by nucleophilic attack of the CT leads to the butadiene complex. Frontier control of protonation results in attack at the metal center, leading ultimately to the hydride complex164. This has been verified by reaction of the... 

Nucleophilic attack at a carbocationic carbon, similar to that observed in metal -carbynes, has been observed from CNBu additions to the //2-t/2-ace-tylide in the complex Fe2(CO)6(C=CPh)PPh2 (427). The ligand dipole, indicated from the X-ray structural determination of the product 96, makes it susceptible to additions of primary amines, generating the carbene 97. 

The molecular orbital analysis of the nucleophilic addition at the carbyne C atom infers the orbital control of the reaction since the C atom undergoing attack is the most negative one in the carbyne complex. [2 + 2] cycloadditions of [ReCp(CO)2(CPh)]+ with MeN=C(Ph)H, t-BuN=0, and ArN=NAr (Ar = aryl) but not with aUcenes or aUcynes, give the metallacycles. These reactions are driven by the nucleophilic attack of the lone pairs of the N atom at the electrophilic carbyne carbon atom. These metallacycles are... 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

The electron-rich thiocarbyne complex 479 (R = Me, 2,4-dinitrophenyl) is not susceptible to nucleophilic attack at the carbyne carbon, and reaction with PEtj causes carbonylation to give the / -ketenyl complex 480 (257). The same reaction occurs in the complex containing the ligand HC(pz)3 instead of HB(pz)3 (258). 

As with carbene complexes, metal carbynes display a range of reactivity with electrophiles and nucleophiles. Molecular orbital calculations show that even cationic Fischer carbyne complexes are polarized as M, +=C A neutral Fischer- and Schrock carbyne complexes have an even greater negative charge on Ccarbyne.93 If all reactions between carbyne complexes and other species were charge-controlled, we would predict that nucleophiles would always attack at the metal and electrophiles at Ccarbyne. As we should expect by now, the picture is more complicated in practice. 

In Section 23.7, we introduced carbene and carbyne complexes when we discussed a-hydrogen abstraction. Equations 23.39 and 23.40 exemplified methods of preparation. Carbenes can also be made by nucleophilic attack on a carbonyl C atom followed by alkylation (equation 23.88). 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

Nucleophiles react with carbyne complexes to promote (or trap) carbyne-carbonyl coupling products, attack the metal-carbon triple bond, or displace a substituent on the carbyne carbon. Complexes of the form Tp M( = CR)(CO)2 are coordinatively saturated and in the case of Tp = Tp, the metal also enjoys a substantial degree of steric protection. Accordingly, the reaction of these complexes with nucleophiles does not, in general, involve attack at the metal but rather at a coligand. Attempted synthesis of Tp W( = CMe)(CO)2 via reaction of MeLi with the... 

The methoxycarbyne complex Tp W( = COMe)(CO)2 also reacts at the carbyne substituent with nucleophiles attacking the methoxide Me group, delivering Me and generating [Tp W(CO)3] via an Sn2 reaction.The reactivity of the methoxycarbyne complex Tp W( = COMe)(CO)2 thus differs considerably from that of the methylthiocarbyne complex Tp W( = CSMe)(CO)2, which reacts with phosphines at the carbyne carbon to undergo nucleophilic displacement of the methylthiolate substituent to produce phosphoniocarbyne complexes.The... 

The bonding interactions of a carb)me ligand are essentially those of a metal carbene complex, but with an additional ir-bond (Figure 2.16). One orbital of cr-symmetry and two of iT-symmetry overlap with three metal orbitals of appropriate symmetry. When considered trianionic, all three orbitals of the ligand fragment contain two electrons. Theoretical studies of heteroatom-substituted or "Fischer-type" carbyne complexes - indicate that the HOMO predominantly consists of the metal fragment, and the LUMO consists of one of the TT -orbitals of the metal-carbon bond. This result explains the tendency of nucleophiles to attack the carbyne carbon in carbyne complexes, just as they attack the carbene carbon in Fischer carbene complexes. 

Many carbene complexes undergo nucleophilic attack at the carbene carbon. This chemistry is presented in more detail in Chapter 13 on metal-ligand multiple bonds. In brief, cationic carbene complexes tend to imdergo simple addition processes to generate neutral products (Equation 11.8), whereas neutral complexes can react by either addition (Equation 11.9) or by a sequence of addition and elimination reactions (Equation 11.10). Fischer has shown that the aminolysis of alkoxycarbene complexes occurs by initial attack at the carbene carbon by a mechanism similar to that for the aminolysis of organic esters. Although less studied than reactions of carbenes with nucleophiles, reactions of carbyne complexes with nucleophiles are also known, and these reactions generate carbene complexes. ... 

In carbyne osmium compounds such as [Os3(/i-COMe)(/z-H)(CO)io] nucleophilic attack on the carbyne carbon atom also takes place. By carrying out sequential reactions with nucleophiles and electrophiles, it is possible to break the C—O bond (Table 3.14, footnote reference A). The reaction furnishes an isolable carbene complex, [Os3(M-CHOMe)(/i-H)(CO)io]... 

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