Acetylide osmium complexes

The simplest bonding mode found in trinuclear hydrocarbon-substituted clusters of osmium and ruthenium is the 7 -vinyl coordination in which one carbon center is formally cr-bound to one metal atom in the triangular core and the alkene/alkyne unit is formally vr-bound to an adjacent metal, so that the ligand donates three electrons to the cluster. Vinyl complexes are generally prepared by alkyne insertion into [Os3(/U-H)2(GO)io] or by the oxidative addition of an alkene to [Os3(GO)io(NGMe)2l or [Os3(GO)i2], and may be considered to be intermediates in reactions to other hydrocarbon-containing cluster products. A list of reported 77 -vinyl- and the related 77 -acetylide-substituted complexes is presented in Table 1. The related 77 -vinylidene-substituted clusters, in which one carbon atom of the ligand is cr-bonded to two metal centers and the alkene unit is formally vr-bound to the third metal center, can be prepared by the thermal conversion of an 77 -vinyl cluster (Scheme 3). The 77 -vinylidene formally donates four electrons to the cluster core. 

Addition of acid to the /i3-acetylide complexes (92 M = Ru or Os) results in the formation of 93 (M = Ru or Os) by an acid-induced migration of the hydroxy group from carbon to metal (137). Only the orange osmium derivative could be isolated, and this was structurally characterized as the /i3-di phenylpropadienylidene complex shown. 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

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