Vinylidene complexes from metal acetylides

Table XIII contains data for organometallic complexes that do not fall into one of the preceding categories. The nonlinearities of the chloro complexes were used in conjunction with those of terminal acetylenes to demonstrate the importance of electronic communication between ligated metal and acetylide ligand in derivative metal acetylide complexes formed from combining these precursors.49133 134 Evaluation of vinylidene, as with sesqui-fulvalene and carbene, is only experimentally straightforward following metal complexation. The vinylidene cation [Ru(C=CHC6H4-4-N02)...

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. 

Protonation of metal-acetylide complexes affords the corresponding vinylidene complexes e.g. 20 and 99, Figure 1.48). Proceeding from 20 to 99 leads to a lowering of (3 values, by a factor of five. As the vinylidene complexes can be easily deprotonated to give back to the alkynyl precursors, and this sequence can be repeated, these complex pairs can provide an interesting protically switchable NLO system. 

Several hundred examples of vinylidene complexes have been prepared. Vinylidene complexes have been prepared by rearrangement of alkyne complexes, additions of acid or base to acetylide complexes, by deprotonation of carbyne complexes, by dehydration of acyl complexes, and by ot-hydrogen shifts from vinyl complexes. Syntheses from alkjme and from acetylide complexes are most common. A complex of a terminal alkyne and a transition metal can exist as an alkyne complex or as a vinylidene complex. Although the free vinylidene is much higher in energy than the free alkyne, the vinylidene complex is often more stable tlnan the alkyne complex. Vinylidene complexes are most often obtained with late transition metals because this tautomer possesses less repulsion between the filled (i-orbitals of the metal and the filled ir-orbitals of the ligand. 

Equations 13.10-13.12 show three examples of the synthesis of vinylidene complexes by reactions of metal-acetylide complexes with acid or base. The molybdenum(II) acetylide complex in Equation 13.10 reacts with acid to protonate the p-carbon and generate a cationic vinylidene complex. In this case, the vinylidene complex is thermodynamically unstable. Warming to 0 °C leads to rearrangement of this species to the tautomeric alkyne complex. In contrast, the more electron-rich molybdenum-acetylide complex in Equation 13.11 containing three phosphite donors generates a vinylidene complex upon addition of a proton from alumina to the 3-carbon of the acetylide. The vinylidene form of the complex is apparently more stable than the alkyne complex in this case. 

Double cyclization of iodoenynes is proposed to occur through a Rh(I)-acetylide intermediate 106, which is in equilibrium with vinylidene lOS (Scheme 9.18). Organic base deprotonates the metal center in the course of nucleophilic displacement and removes HI from the reaction medium. Once alkenylidene complex 107 is generated, it undergoes [2 + 2]-cycloaddition and subsequent breakdown to release cycloisomerized product 110 in the same fashion as that discussed previously (Scheme 9.4). Deuterium labeling studies support this mechanism. 

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