Rhenium triple bond

The multiple bond to the metal in a rhenium(V) compound is best represented by the MO description of it being a triple bond, consistent with the symmetry of the px and p orbitals on the ligand see 1 (1). In keeping with that, these bonds are found to be relatively short and quite strong. The structure has also been presented as 1 a valence bond formulation that facilitates the counting of oxidation states but does not provide an accurate representation. 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

The cycloaddition of imines to the metal-carbon triple bond in the cationic rhenium carbyne complex 263 was reported by Geoffroy and... 

CH3SCH2CH2SCH3 (DTH) yields Re2Cl5(DTH)2, which has the structure57 shown in Fig. 26-D-10. Here a reduction has occurred and the. electron distribution is such that there is only a triple bond between the rhenium atoms. The rotational configuration is then staggered instead of... 

C. The characteristic infrared stretch of the ReN triple bond has not been found due to interference from Ph modes the ReCl stretch occurs at 323 cm (Nujol mull). Despite the color of the complex in the solid state and the yellow color of a CH2CI2 solution, no distinctive features appear in the visible region of the spectrum. The dipole moment of 1.6 (3) D is consistent with the X-ray crystal structure obtained elsewhere that reveals a distorted square pyramid with an apical nitrido ligand and mutually trans chloride ligands in the basal plane. The triphenylphosphine ligands are distorted toward the empty rhenium coordination site. 

The triple bond in ortHo-benzyne can be stabilized by complexation with transition metals. Aryne-metal complexes were originally proposed as intermediates in the decomposition of various aryl derivatives of early transition metals, and the first fuUy characterized mononuclear ortho-benzyne complex, TaMe2(q -C5Me5) (q -CjH4), was prepared. Although this method does not appear general for all transition metals, various complexes of zirconium, rhenium, and niobium have been characterized. More recently, complexes of nickel and platinum have also been... 

Other metal complexes of titanium, zirconium, vanadium, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver and gold to synthesize a wide variety of interesting new compounds. It utilizes Hoffmann s [51] isolobal relationship between the C=C bond and the metal-carbon triple bond. 

Furthermore, an Re Re triple bond was also invoked for [CpReH]2(p--H)4, but theoretical calculations indicate that the rhenium-rhenium interaction is described by a (ct/8) (8 ) (8 ) configuration [87] which corresponds to a direct Re-x-Re antibond (Fig. 24). Thus, whenever there are more bridging hydride ligands than... 

Fig. 24 Three postulated isomeric rhenium hydride complexes for which formal Re s Re triple bonds were invoked to satisfy the 18-electron nile (right hand side) the corresponding description of their bonding in terms of 3-center-2-electron bonds and direct Re-Re bonds (left hand side)...

Manganese and Rhenium.—Re2(t -allyl)4 is not isostructural with its Cr and Mo analogues, possessing only terminal allyls and having a very symmetrical structure calculations suggest an approximate R Re triple bond. Two products of the reaction of phospholes with Mn2(CO)io have been shown crystallographically to have structures (164) and (165). ... 

Nucleophilic addition of 5-substituted tetrazoles (501) to ethynyl-phosphonate (500) proceeded regio- and stereoselectively to yield (Z) p-amino-p-tetrazolylvinylphosphonates (502) and (503) via attack of the N-1 (predominantly) or N-2 atom of the tetrazole at the C-2 atom of the triple bond of the phosphonate (500), respectively (Scheme 149)/ Treatment of p-ketophosphonates (504) with terminal alkynes (505) in the presence of an rhenium catalyst and under neutral conditions afforded 2i -l,2-oxaphosphorin 2-oxides (507). The reaction proceeded via cleavage of a C-C a-bond of the p-ketophosphonate (504), regio- and stereoselective insertion of the allq ne (505), followed by cyclisation of the resulting 5-phosphonyl a,p-unsaturated ketone (506) (Scheme 150). ... 

Rhenium occupies a very important place in the development of the chemistry of complexes containing metal metal double, triple and quadruple bonds. This chemistry involves coordination complexes that contain pairs or trinuclear clusters of rhenium atoms and has been surveyed in a recent monograph entitled Multiple Bonds Between Metal Atoms .5 This source, which covers the literature up to early 1981, has also been invaluable in preparing the present review and can be consulted for a more complete and detailed coverage of these classes of complexes. 

---