Acetylene, coordinated

The acetylene coordinates trans to the least o electron donor group, chlorine. Coordination of the C-H bond is a less favorable alternative to coordination of the n system. The o C-H complex is 17.1 kcal.mol 1 less stable than the rc-alkyne complex (Figure 5). From this c C-H intermediate the 1,2 shift is possible with a relatively small activation barrier (+15.5 kcaLmol 1) to yield the vinylidene complex. However this mechanism is in contradiction with the labeling experiment. 

Complex D4 is considered as the active species for both alkyne and olefin coordinations. Starting from the olefin coordinated complex (D5 ), the olefin insertion into the Pt-B bond is unfavorable because of a high activation barrier (22.9 kcal/mol). On the contrary, the acetylene insertion from the acetylene coordinated complex (D5) occurs easily with a small reaction barrier (9.0 kcal/mol). This significant difference in the reaction barriers has been used to explain the inertness of olefins for diborafion reactions. The smaller barrier from D5 to D6 coincides with the highly stable insertion product D6. In contrast, the olefin insertion product D6 is relatively unstable with respect to the olefin coordinated species D5 . 

In this reaction, too, norbornadiene and two molecules of acetylene coordinate to the catalyst and cyclization takes place to liberate cyclopentadiene. 

Calculation has been performed mainly at the HF level with the double-zeta plus polarization quality basis sets and ECP for Pd, P, Si, and Sn. The energetics for R = Me has been recalculated at the MPn (n = 2-4) level at the HF geometries. The overall reactions for various RCCH (R = CN, H, CH3, and OCH3) and H3SiSnH3 with the model catalyst Pd(PH3)2 have been studied. It has been shown that H3Si-SnH3 easily, with a few kilocalories per mole barrier, adds oxidatively to the catalyst Pd(PH3)2 and the resultant complex lies only 5.9 kcal/mol lower than the reactants. The next step, acetylene coordination to the catalyst, causes with the dissociation of... 

One needs to be adaptable in viewing the structures of molecules. Normally 21.74 is considered to be an acetylene coordinated to a trinuclear cluster. There... 

The acetylene coordination to the active catalyst HCo(CO)3 forms the most stable 7i-complex (15a) with the C=C bond in the equatorial plane. The following step of acetylene insertion into Co-H bond leads to the stable vinyl complex (H2C=CH)Co(CO)3 (15b), which has a butterfly geometry with vinyl group in the axial site. In addition, acetylene insertion has an activation free energy of 28.5 kJ/mol, and is an exergonic (—84.9 kJ/mol) and irreversible process, in contrast to the thermal neutral process of alkenes. Therefore, it is expected that the regioselectivity of terminal alkynes RC=CH with HCo(CO)3 leading to linear or branched product should be controlled by this irreversible step. 

Sketch the molecular orbital interactions for acetylene coordinated to a transition metal atom. Comment on their relative importance. 

The complete vibration-rotation Flamiltonian for acetylene-like tetraatomic molecules has been derived by Handy et al. by hand [155] and using a computer algebra program [156]. (Note that in both of the mentioned papers there are some minor errors, see also [144,157,158]). Handy uses as bending coordinates... 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

First, dehydrogenative bonding of acetylene to the catalyst surface will free hydrogen and produce moieties bonded to the catalyst coordination sites. These units are assumed to be the building blocks for the tubules. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

The 3,5-bis(trifluoromethyl)pyrazolate analog [Ir(cod)(/x-3,5-(CF3)2pz)]2 does not enter into oxidative addition with iodine, methyl iodide, or acetylenes. The mixture of pyrazolate and 3,5-bis(trifluoromethyl)pyrazolate gives [(rj -codllrf/x-pz)(/L-3,5-(CF3)2pz)Ir(rj -cod)], which reacts with bis(trifluoromethyl)acetylene in a peculiar manner [83JCS(CC)580], producing 145, where 3,5-bis(trifluoromethyl) pyrazolate is replaced by the ethylene bridge and the rj -coordination mode of one of the cod ligands is converted into the rj -allylic mode. 

The vast majority of phosphine/phosphite-substituted products involve F-ligand ligation at late transition metals. In contrast, phosphite ligands displaced rhenium-coordinated CO or acetylene in [RePt3(/i-dppm)3(CO)3(L)l", - which are the... 

The reaction proceeds with isolated double bonds and electron-rich alkynes. Electron-withdrawing groups in the acetylene moiety decelerated the reaction. A plausible mechanism implies the activation of the olefin by coordination of the metal triflate followed by nucleophilic attack of the acetylene or acetylide (Scheme 31). 

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