Alkylidyne ligands

Reactions of alkynes with trinuclear group 6-group 9 clusters incorporating bridging alkylidyne ligands proceeded by coupling the C-donor ligands. 

J.M. Burlitch, Cornell University Is there any evidence for an exchange of alkylidyne ligand of the sort shown below ... 

For a general review of metal-ligand multiple bonds, see W. A. Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Alkylidene, or Alkylidyne Ligands (New York, Wiley, 1988). 

In the last several years tungsten alkylidyne complexes [60], W(CCMe3) (CH2CMe3)3 and W(CCMe3)Cl2(dimethoxyethane) in particular, have been a source of alkylidene complexes bound to oxide surfaces as a consequence of protonation of the alkylidyne ligand by a surface-bound hydroxyl group [112-114]. 

The rhenium perhydrocarbyl complex Re(=CBu )(= CHBu )(CH2Bu )2 reacts with the surface hydroxyl groups of a silica(7oo) (Scheme 2.29) to form a well-defined surface species, monografted on silica and containing one alkyl, one alkylidene and one alkylidyne ligands according to mass balance analysis IR, NMR, EXAFS [79-82] and calculations [83, 84]. 

The structure of 11a has been established by X-ray diffraction. The compounds 11 are 30-cluster valence-electron (CVE) dimetal species, and are therefore electronically unsaturated. The electron distribution within the W(fi-C)fth rings may be represented by various canonical forms. However, that shown, which implies that the W=C bond in the ( 5-C2B9H9R 2)(OC)2-W=CC6H4Me-4 moiety formally contributes three electrons to the rhodium center (16-electron valence shell), accords with other results from our laboratory involving electronically unsaturated dimetal compounds with bridging alkylidyne ligands 18). 

As will be seen in later sections, the complexes in which the heteronuclear metal-metal bonds are bridged by agostic B—H—- metal interactions are intermediates in a variety of reactions involving the carbaborane cages and the /i-alkylidyne ligands. 

Several questions remain. In particular, why is it that only one alkylidyne ligand appears to be susceptible to attack (addition/insertion). Perhaps the answer to this question lies in a combination of steric and electronic factors associated with the initial insertion/addition products. In some cases these lack any formal W—W bond, the tungsten atoms being in a formal +6 oxidation state, whereas in others the nature of the W—W bond and its accessibility toward entering 7t-acid ligands is at present not well understood. Further studies are clearly warranted. 

Alkylidene and alkylidyne ligands require metal-carbon p bonding in addition to s. This is different, however, form... 

COMPOUNDS WITH BRIDGING ALKYLIDENE AND ALKYLIDYNE LIGANDS. 

The results described in this paper suggest that further studies in this area would be very profitable. The heteronuclear metal —metal bonded species will have a rich chemistry at the metal centres, particularly in those cases where bridging alkylidene or alkylidyne ligands are present. 

The reactions of alkylidenes (see Alkylidene) and alkylidynes (see Alkylidyne) of niobium and tantalum are typically characterized by nucleophilic alkylidene and alkylidyne ligands. Cp Ta(CH2SiMe3)2(=CHSiMe3) reacts with PhOH to give alkyl phenoxo Cp Ta(CH2SiMe3)3(OPh), and is hydrolyzed in the presence of a trace amount of H2O to give the 0x0 dialkyl derivative [Cp Ta(CH2SiMe3)20] . ... 

Vogler et al. (193) reported that irradiation of either (CO)2-(PPh3)20s=CPh (160) or (CO)(PPh3)2(Cl)Os=CPh (161) in the presence of HCl gives protonation of the alkylidyne ligand and formation of the alkylidene complex 162 [Eq. (155)]. Both starting complexes show... 

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