Carbonyls, metal Carbyne ligand

However, these reactions jure associated with terminal rather than bridging carbonyl groups. In metal "ketonic compounds, the center for attack is normally the metal. This may be rationalized in terms of additional delocalization of the positive charge from the carbon to the metal center and implies a "carbyne contribution to the carbonyl bridge, of the form discussed above for the methylated products, again emphasizing the possibility that in systems of this type reaction may occur at either the metal or ligand center... 

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

Carbyne ligands may bridge two (p) or three (p3) metal centres, providing a total of 3VE to the overall electron count. Earlier synthetic routes to such complexes involved the reactions of carbonyl metallates with 1,1,1-trihaloalkanes, or the cleavage of alkyne ligands coordinated to clusters (Figure 5.46). 

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). 

Figure 3. The metal core of the cluster Au2Ru3(/(-H)(/(,-COMe)-(/(-Ph2PCH2PPh2)(CO)9l (1), showing the two inequivalent methylene proton environments and the inequivalence of the two phenyl rings attached to each phosphorus atom in the ground-state structure of the cluster. The carbonyl, hydrido and methoxy-carbyne ligands have been omitted for clarity (reprinted by permission of the Royal Society of Chemistry from ref. 23).

In addition to the ring opening/carbonylation pathways illustrated in Eqs. 2-5, photooxidation of metal carbynes can result in other types of ligand rearrangement. An example is the cyclization of butenyl carbyne complex 8 depicted in Eq. 6. Deuterium... 

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