Coordinated dihydrogen complexes

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

The proposed intermediate m this scheme represents an alternate mode of coordination for two H ligands—as a coordinated H2 molecule. Although the intermediate in Eq. 15.28 has not been isolated, a number of other dihydrogen complexes have been The first was prepared in I983 32... 

The high concentration of dihydrogen gas in our experiments has allowed us to stabilize complexes which previously had only a fleeting existence at ambient temperatures. For example, the species W(CO)5(H2) has a lifetime in conventional solvents of somewhat less than one second at room temperature. Under our supercritical conditions, however, the lifetime may be extended to more than three minutes. Furthermore, the stability conferred upon this molecule by our supercritical system, and the unique spectroscopic transparency of scXe have allowed us to detect the very weak v(H-H) band of coordinated dihydrogen using only a conventional FTIR spectrometer and a powerful UV lamp (4) as shown in Figure 3. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

A large number of H2 coordination compounds have been identified and characterized. Table 11.5.1 lists the H- H distances (dim) determined by neutron diffraction for some of these complexes. The H-H distances ranges from 82 to 160 pm, beyond which a complex is generally regarded to be a classical hydride. A true dihydrogen complex can be considered to have dim < 100 pm, and the complexes with dim > 100 pm are more hydride-like in their properties and have highly delocalized bonding. 

The mechanism of silane alcoholysis has been the focus of discussions over many years.10a 14 15 17 25 30 A general mechanism is outlined below. This mechanism begins with oxidative addition of the metal into the Si-H bond to form either a r 2 complex (la) or a silyl hydride (lb) (Figure 5). The alcohol then coordinates to the silicon forming a new complex (II) which can lose silyl ether to form a metal dihydrogen complex (HI). The catalyst is regenerated when another silane displace molecular hydrogen from the catalyst. There are several minor variations of this mechanism however this basic mechanism is believed to hold for many catalytic systems. 

Greg Kubas of Los Alamos National Laboratory has determined how hydrogen interacts with metals. The important part of his work is that hydrogen, a substrate that is redox inactive substrate and not Brpnsted acidic, transforms upon complexation whereupon the coordinated H2 becomes acidic. The deprotonation of a metal dihydrogen complex generates oxidizable species and in this way, H2 is connected to electrons and heterolytic activation. Rauchfuss explained that Kubas discovery has helped guide his team s effort to connect H2 binding to this redox-active iron metal. 

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