Structure metal-alkyne complexes

The NLO properties of organometallic and coordination complexes are also rich (21, 184, 279-296). Metal-alkyne complexes were first reported 1960 (297) and have recently attracted significant interest because of their potential in materials applications (2, 298). Studies of these types (299) have resulted in the development of structure-NLO response relationships for quadratic optical nonlinearities (p-value), which increase with valence electron count and ease of oxidation of metal. The amplitude is also tunable by ancillary ligand modification and substitution. Select small alkynyl complexes have been shown to exhibit p values at 1064 nm > 2600 x 10 ° esu (299). 

Effect of acetylene -ir-donation on the structures of metal-alkyne complexes. 

Another common class of metal-carbene complexes is that of the vinylidene complexes (whose structure compares to that of allenes), isomers of metal-alkyne complexes. A well-known example is the vinylidene complex [Ru(PPh3)2Cl2( =C=CHPh)], the first unimolecular ruthenium catalyst of olefin metathesis discovered by Grubbs in 1992. - This class is extended to the allenylidenes and cumulenylidenes. ... 

The copper-alkoxo unit, which is usually synthesized in situ, plays a significant role in metal-promoted transformations of organic substrates by copper(I). To determine the reaction form of the Cu-OPh unit, Floriani and co-workers structurally characterized four complexes (772) (pseudotetrahedral Cu-Cu 3.223 AT (773) (pseudotetrahedral), (774) ( anion linear coordination) and (775) (planar trigonal).57 Using 3,3,6,6-tetramethyl-l-thia-4-cycloheptyne as terminal ligand the structural characterization of a copper(I)-alkyne complex (776) (Cu-Cu 2.940 A) was reported.573... 

Complex condensation products are obtained by reaction of the alkyne complex with excess of 3,3-dimethyl-l-butyne, which yields two isomeric products of formulas Ru3(CO)6[HC2C(Me)3-COCH2CMe3][HC2CMe3]2 (125). The X-ray structure of one of those adducts (Fig. 19) shows that both dimerization of two alkyne molecules and the insertion of carbon monoxide into the alkyne metal bonds have occurred. The Ru-Ru distances of 2.820,2.828, and 2.686 A in the ring are of interest. The value of 2.686 A is one of the shortest found in a... 

The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

The first complexes to be described were obtained from reactions between MCI2CP2 and metallated alkynes and initially formulated as the dimers M(/r-C=CR)Cp 2. Further studies have clarified the nature of these complexes together with those formed from RC CC CR (R = Me, Bu, Ph, SiMe3) and MCp2. The several structural types obtained contain 1 /1,1 /2, 2/1, and 2/2 ratios of MCp2 to diyne, as shown in Scheme 48. The nature of the complexes formed depends on diyne substituent and metal. 

The simplest alkyne complexes, the metal acetylenes, resemble those of ethylene. For example, there are analogues of Zeise s salt in which an acetylene molecule ts bound to platinum(l)) and occupies a position like that of ethylene in Zeise s salt. In addition, there are L,Pt(RC=CR) complexes that have structures paralleling that of LjWHjC CHj) (Fig. 15.24). For both of these PtfO) complexes, an approximate square planar arrangement around the metal is found. Alkynes are more electronegative than ulkenes and are therefore better it acceptors. Thus it is appropriate to view them as metal lacyclopropenes 79... 

Contrary to the rich organometallic chemistry of cyclopropylacetylenes with main group elements, tr-bonded transition metal complexes of the alkynes are rare. Nevertheless, the single example given in the literature is of a stable crystalline compound, which permits insight into the structure of cyclopropylethynyl complexes in the solid state. 

Scheme 1. Alkene-and alkyne-metal complexes may be formulated with metallacyclo-propane (A) or -propene (B) structures, respectively. Similarly, complexation of a metal atom with an alkylidene or alkylidyne group gives rise to a dimetallacyclopropane (C) or -propene (D) ring system.

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

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