Kinetics oxidative dihydrogen addition

Electron-richer dM compounds can also be considered as H2-activating alternatives to compounds with the unfavorable dM configuration. In the case of the bis-dppm bridged Rh(I)Ir(-I) complex 14, the d d configuration has been found to result in a metal-metal bonded species in which the coordination around the rhodium center is similar to that in planar homovalent d compounds. [47] The kinetic product of dihydrogen addition to 14 is consistent with the occurrence of a single-metal oxidative addition to the Rh(I) (Scheme 12). This kinetic product is thermally unstable and reductively eliminates methane from the iridium center. The overall reaction constitutes a clear example of bimetallic cooperation, since the oxidative addition to one center provokes a reductive elimination in the other metal. 

Oxidative addition of H2 to iridium(I) complexes yields a variety of six-coordinate iridium(III) dihydrides. Dihydrogen reacts with [Ir(Cl)(CO)(PPh3)2] in benzene at 298 K to produce [Ir(H)2-(Cl)(CO)(PPh3 )2] 411 the reaction is reversible and a kinetic investigation has been reported.412 [Ir(Cl)(PPh3) ] reacts with H2 at 298 K and atmospheric pressure to give [Ir(H)2(Cl)(PPh,)3].413 The four-coordinate iridium(I) complex [Ir(dppe)2]+ reversibly reacts with H2, affording [Ir(H)2(dppe)2]+.414... 

Oxidative addition reactions of dihydrogen , iodine ", alkyl halides and Hg(CN)2 to carbonyl, olefin or phosphine substituted derivatives of rhodium(I) and iridium(I) have been described. In order to determine the effect on the rate of the reaction, the kinetics of the oxidative addition of Hg(CN)2 to Rh(dik)(P(OPh)3)2 has been studied . A second-order rate law coupled to large negative values of the activation entropy suggest an associative mechanism which probably proceeds via a cyclic three-centred transition state (equation 58). Analogous results were obtained with Ir(dik)(cod) . ... 

In spite of the elimination of formic acid in a couple of steps changing the oxidation number of the rhodium metal center from -nl to -i-3 and vice versa, the reaction could take place by an alternative mechanistic pathway via cr-meta-thesis between the coordinated formate unit and the nonclassical bound hydrogen molecule [48, 49]. Initial rate measurements of a complex of the type 13 show that kinetic data are consistent with a mechanism involving a rate-limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species, first with CO2 and then with H2. The calculations provide a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. From these results s-bond metathesis seems to be an alternative low-energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen [48 a]. 

In this chapter, we will focus on oxidative addition of one or two molecules of dihydrogen (H2) to coordinatively unsaturated M(PH3)2C1, M = Rh and Ir. We will examine the performance of first-principles computational methods based on the traditional molecular orbital approach and on density functional theory, with a focus on thermodynamic and kinetic parameters. 

REACTION PROFILES AND KINETICS FOR COORDINATION AND OXIDATIVE ADDITION OF DIHYDROGEN AND OTHER G LIGANDS... 

One of the most significant oxidative additions of dihydrogen occurs to the complex RhQ(PPh3)3 (Equation 6.4) - which is "Wilkinson s hydrogenation catalyst." Kinetic studies - of this oxidative addition reaction show that addition of can occur to the... 

Rate constants for the oxidative addition to the 14-electron intermediate were obtained directly from flash photolysis experiments. Flash photolysis of RhCl(CO)(PPh3)j causes selective dissociation of CO to form the 14-electron intermediate RhCl(PPh3)j or a derivative of it containing coordinated benzene (Scheme 6.2). Oxidative addition of dihydrogen to this intermediate was observed directly by timeresolved UV-vis spectroscopy and was shown to be fast k = 1.0 x 10 M" s and AG = 10 kcal/mol). A small kinetic isotope effect was measured, k /k = 1.4, and this value is consistent with an early, three-centered transition state in which little H-H bond breaking has occurred. 

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