Oxidative addition dihydrogen

Lewis base complexes, 83 Dichromates, 941, 943 Dihydrogen oxidative addition niobium(II) complexes, 678 niobium(III) complexes, 660 tantalum(II) complexes, 678 tantalum(III) complexes, 660... 

Figure 2.75 Oxidative addition of dihydrogen to Vaska s compound.

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

The transfer reaction utilizes a sacrificial alkene to remove the dihydrogen from the pincer or anthraphos complex first, before the oxidative addition of the target alkane. The elementary reaction steps are slightly different from the thermal reaction, which is discussed in the next section, both in their order and their direction. For simplicity, we describe the symmetric reaction where the sacrificial alkene is ethylene and the reactant is ethane (21b). The elementary reaction steps for the mechanism of this transfer reaction involve IVR, IIIR, VIR, VI, III and IV, where the superscript R stands for the reverse of the elementary steps listed in Section III. These reverse steps (IVR, IIIR, and VIR) involve the sacrificial alkene extracting dihydride from the metal to create the Ir(I) species 8, while steps VI, III and IV involve oxidative addition of target alkane, p-H transfer and olefin loss. 

Extensive computational calculations have been performed by using molecular mechanics (MM) [79], quantum mechanics (QM) [80], or combined MM/QM methods [81]. As major contributions, these theoretical studies predict the greater stability of the major isomer, explain the higher reactivity of the minor diastereomer, introduce the formation of a dihydrogen adduct as intermediate in the oxidative addition of H2 to the catalyst-substrate complexes, and propose the migratory insertion, instead of the oxidative addition, as a turnover-limiting step. 

The nature of the M-H bond-forming step, (ii), in a given catalytic cycle depends strongly on the reducing agent used. Dihydrogen [13, 14, 17, 20, 24, 29] and hydrosilane [78, 81, 82] react mostly by oxidative addition [193, 209, 210]. For example, the product of the reaction in Eq. (23) - which is involved in an... 

The first step consists of the substitution of one of the ligands (L) of 18 by dioxane (39) in an oxidative addition (a) (Scheme 20.16). / -Elimination of 40 releases 2,3-dihydro-dioxine (41) and the 16-electron dihydrogen rhodium complex (42) (b). Alkene 43 coordinates to the vacant site of 42 (c) to give complex 44. A hydride insertion then takes place (d), affording complex 45. After a reductive elimination (e) of the product 46, the coordination of a ligand reconstitutes the Wilkinson-type catalyst (18). 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

Oxidative addition of dihydrogen commonly involves transformation of a d8 square planar metal complex into a d6 octahedral metal complex, or similar transformations involving d2 — d°, d10 —> d8 etc. The oxidative addition of... 

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