Acetylene ligands

The acetylide anion 3 is likely to form an alkynyl-copper complex by reaction with the cupric salt. By electron transfer the copper-II ion is reduced, while the acetylenic ligands dimerize to yield the -acetylene 2 ... 

The structure of Ni4(CO)4(CF3C2CF3)3 79> in which each acetylenic ligand can be considered to provide 6 electrons to the cluster is also shown in Fig. 16. 

Ni(azobenzene)]had been postulated as containing a ti-N=N bond a structural study confirms this. The structure is shown in (118) the N—N bond length of 1.385(5) A is longer than in the free ligand and very close to the single-bond length. The structure of [Ni(PhC=CPh)(CNBu )2] has also been reported and the acetylene ligand is also bonded side-on to nickel. The co-ordination... 

Rh, Ir) (188, 400) and H2M3(CO) (RC CR) (M = Ru, Os) (7dJ). The latter complexes were also the subject of a detailed study (163) which showed four fluxional processes to be present (1) hydrogen exchange, (2) local CO scrambling, (3) total CO scrambling, (4) rotation of the acetylene ligand. 

Incorporation of a third acetylene molecule takes place by CO replacement and without interference with the metallacyclopentadiene ring (170, 371, 379). In the cluster, then, the three acetylene ligands rearrange to a triacetylene ligand of unknown structure before the benzene is liberated (371, 379). 

Jd] or [92] (7). All other new trirhodium clusters contain acetylene ligands and have either the formula (GpRh)3(RG=GR) with the basic structure 99 or (GpRh)3-(RG=GR)(GO) with the basic structures [70(9] or [707], They are formed from... 

The acetylenic C-C bond distance of 1.423(15) A in 130 is longer than in 127, consistent with the transformation of the acetylene ligand from a formal 2-electron donor to a 4-electron donor. 

The terminal acetylene derivatives Osg(CO)i6(RCCH) (R = Me, Et, Ph) have been shown to react with CO to give Osg(CO)i7(RCCH), (89), in which the acetylene ligand is still intact and sits on the base of the capped pyramidal osmium polyhedron. This compound was found to convert by the action of heat to an isomer in which the proton from the acetylene has been transferred to the metal array. The process is accompanied by an opening up of the metal cluster (Scheme 34). 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

Acetylenes are adaptable ligands in that there are two orthogonal n orbitals which can be used for bonding to the metal. These are indicated by (25) as radial and tangential n orbitals. The radial orbital has a a type overlap with the metal however, that for the tangential one is of the n sort. Thus the acetylene ligand can be viewed as a two or four electron donor. Consider then the case oftris(acetylene)W(CO), shown... 

Considering the unusually attractive ligand properties of cyclobutadiene i ), what could prevent two coordinated acetylene ligands from fusing to a cyclobutadiene ligand Transformation 26- 27, however, encounters... 

Cyclobutadiene-metal complexes were obtained from reactions of the corresponding acetylenes with metal complexes i9U,2i) Orbital symmetry principles would suggest that these complexes are either formed via stepwise processes or involved the intervention of bimetallic species (32). The stepwise routes are particularly attractive. Acetylene ligands can reasonably be expected to undergo a [( 2s 2 -f-ji2s] cycloaddition with the metal center generating the metalocyclodiene intermediate 33. Cyclobutadiene can then be extruded from the metal center with the aid of another metal. 

Sometimes, for example, a bridging acetylene ligand may occupy two coordination sites but not on the same metal atom. 

In these complexes the acetylene ligand can be readily substituted by some other ligands. The following general order of stability has been established 15). 

With a strong trans donor (for example, PPh3) one should expect D-type substitution of the acetylenic ligand to take place evidence for this is obtained from the kinetic data of the reaction... 

---