Acetylene bonding modes

Fleming, P. R., and Hutchinson J. S. (1988), Representation of the Hamiltonian Matrix in Non-Local Coordinates for an Acetylene Bond-Mode Model, Comp. Phys. Comm. 51, 59. 

The inorganic clusters [Mo3(p3-S)(p2-E)(p2-S)2(H20)9]4+ (E = O, S) add acetylene at room temperature in aqueous 1 M HC1 solution to give the dithiolene (243). The dithiolene ligand adopts an unusual bonding mode, wherein each thiolate sulfur atom bridges two metals (Fig. 19). The cluster... 

Bonding mode M3 is truly acetylenic in character, and is common in dinuclear complexes (17). It has been observed in the trinuclear platinum cluster Pt3((PEt3)4(PhCCPh)2 (151). The three Pt atoms adopt... 

In the context of these structures it is interesting to compare the geometries of alkynes bonded to metal clusters and those bound to metal surfaces. While the more common bonding modes such as M6 and Mil may represent models of acetylenes bonded to well-defined metal planes, the less common modes such as M14 and M19 may be representative of the type of bonding that occurs at dislocations on the metal surface. 

As has been noted elsewhere (17) the environment of the a carbon atom of the acetylide type ligands such as those observed in bonding modes M8, M13, and M18 resembles, to an extent, that of a carbido carbon. As the nuclearity of the cluster with which the acetylene interacts increases, the similarity between the environments also increases. In the case of Ru5(CO)13(p-PPh2)(C=CPh) (431), the structure is closely related to the carbido complex Ru5C(CO)15 (432). 

Acetylenes and ethene insert into the S—S bonds of metal polysulfides [M—S] to yield, with two exceptions (535 and 540), dithiolene and 1,2-ethanedithiolato ligands coordinated in a variety of bonding modes (Table VI). 

Figure 1. Schematic representation of a possible mode of polymerization of a terminal acetylene bonded to a group G involved in a nonbonded interaction.

The Raman spectrum of the carbon film shown in Figure 5.6(b) presents one peak at 1460cm and two sharp peaks detected near 1864cm and 2248 cm are assigned to allenic and acetylenic stretching mode of sp-bond carbon chains [35]. The frequency of the Raman peak in the spectra depends on the excitation wavelength and the length of carbon chains in the carbyne. 

For the alkyne ligand two bonding modes are possible for dimetallic complexes, MAS and MA6, which are known as perpendicular and parallel acetylene complexes, respectively. The bonding mode of MAS is the most common (Table 16). These two modes have been studied in detail by Hoffmann and coworkers . The perpendicular MAS alkyne is seen as a 4-electron donor of 2 Ti-electrons to each metal, whilst the parallel alkyne ligand donates 2 electrons via a bonds. At the time of publication of References 78 and 79 (1982) only a few dimetallic alkyne crystal structures had been determined. However, Hoffmann... 

Acetylenes with electronegative substituents reversibly combine with planar iridium(I) 93, 99) and rhodium(I) complexes (I) to form 1 1 adducts (XXII) (31). These are characterized by strong infrared absorption bands in the 1600-1800 cm region, apparently caused by a stretching mode of the coordinated acetylenic bond. The carbon-carbon bond order in these complexes may approach that of an olefin. In view of a recent... 

These ruthenium complexes were also investigated as catalysts for the dimerisation of phenylacetylene (Scheme 5.13). ° The trimerisation of acetylene is typically a competitive process and when compared to [RuCl2(/)-cymene)]2, catalysts 53-56 afforded higher dimerisation yields. However, the results obtained did not allow a clear distinction between the effects of the different types and bonding modes of the carbene ligands. 

In the related 77 -acetylide-substituted clusters the acetylene or 77 -ethynyl unit can also adopt the two bonding modes as illustrated in Scheme 4,... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

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