Acetylene complexes structure

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

W(n5-C5Me4But)Cl4]2, each in 50% yield (36). The identity of W(q5-C5Me4But)(MeCECMe)Cl2 was proven by an x-ray structural study which showed it to be similar to Ta(ri5-C5Me5)(PhC Ph)Cl2 (37) and related species. The W(V) dimer and W(III) acetylene complex probably form by disproportionation of some intermediate W(IV) complex. What we were most interested in was whether any intermediates not containing a n5-C5Me4Bu ligand could be isolated. 

In summary, the detailed electronic character of dihapto metal-acetylene complexes depends strongly on the Lewis-acceptor capacity of the metal. Formal two-versus four-electron rp ligation to a transition metal can lead to breaking of one or both 7T bonds, dramatically altering the structure and reactivity of the alkynyl... 

Fig. 15. The oloso distorted octahedral structure of the cobalt carbonyl acetylene complex Co4(CO)ioC2Et2 (50).

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

The above type of bonding is assumed to occur in other metal-olefin and metal-acetylene complexes (172). Acetylenes have two mutually perpendicular sets of ir-orbitals and are therefore capable of being bonded to one or to two metal atoms both types of complexes are known. When the hydrocarbon is a nonconjugated polyolefin e.g., cyclo-octa-1,5-diene, each C C bond interacts independently with the metal atom. In complexes of conjugated polyolefins, e.g., cyclopentadiene, infrared and nuclear magnetic resonance studies (99) indicate that it is not yet possible to distinguish between structure (IV), in which each C C bond independently contributes two --electrons to the metal-olefin bonding, and structure (V), in which... 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

Functionalities on the silica surface - The ToF-SIMS spectra were recorded of the untreated and treated silicas. Figure 8 represents an untreated silica sample, and Fig. 9 an acetylene-treated one. They show a complex structure of a plasma-polymerized acetylene film on the silica surface. 

In a recent study of the adsorption of acetylene on platinum single crystals by low energy electron diffraction [160], it has been shown that acetylene adsorbs on the (111) planes. These results show that, on a clean Pt (111) surface, acetylene adsorbs at a distance of 1.95 A above the topmost plane of platinum atoms, either in the C2 or, less likely, the Bl mode shown in Fig. 23. No evidence was found for adsorption in the A or A2 modes, which corresponds to a 7r-complex structure or for the B2 mode corresponding to a di-o-complex, although it was stated that such structures may be possible with a less stable overlayer which had been observed. 

A urylene derivative of cobalt, [(CpCo)2 (Bu NCONBu1)] (71) has been obtained by the reaction of N-r-butylsulfurimide, (Bu N S, with [CpCo(CO)2]. The reaction has been suggested to proceed via the formation of an intermediate nitrene complex, [CpCo OJNBu1].257,258 The X-ray crystal structural determination has shown the presence of a short Co—Co distance (2.37 A), which is 0.1A smaller than that reported in the related acetylene complex, Co2(CO)6(C2Ph2).258,26 ... 

The microwave spectrum of the complex between ethylene and ketene (and of deuter-ated derivatives) reveals56 a crossed structure (22), while the ketene/acetylene complex shows a planar geometry57. This difference in geometry is explained by the different quadrupole moments of two unsaturated hydrocarbons. 

Figure 7.49 X-ray crystal structures of 7.68 (a) apohost, (b) 1 1 inclusion compound with acetylene (1 bar) and (c) 1 2 acetylene complex (8 bar). The cavity volume is shown as a van der Waals survace -note that each cavity is discrete. Chloride ligands are shown as spheres and the acetylene molecules are shown in space-filling mode (pictures courtesy of Prof. L. J. Barbour, Stellenbosch University).

A modest number of acetylene complexes has been investigated structurally. Many of the features of olefin complexes are also observed in acetylene complexes, the major difference being the change in geometry of the coordinated acetylene. 

The field of acetylene complex chemistry continues to develop rapidly and to yield novel discoveries. A number of recent reviews 1-10) covers various facets including preparation, structure, nature of bonding, stoichiometric and catalytic reactions, and specific aspects with particular metals. The first part of this account is confined to those facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts will be made to give a consistent interpretation of the reactivities of coordinated acetylene in terms of a qualitative molecular orbital picture. 

---