Transition metal-hydride complexes stability

Mdssbauer spectra of bonding and structure in, 15 184-187 reactions with diborane, 16 213 stabilization of, 5 17, 18-19 cyanates, 17 297, 298 cyanide complexes of, 8 143-144 cyclometallated bipyridine complex, 30 76 diazene complexes, 27 231-232 dinitrogen complexes, 27 215, 217 diphosphine complexes of, 14 208-219 dithiocarbamates, 23 253-254 -1,2-dithiolene complexes, 22 323-327 hydrogen bonding, 22 327 halide complexes with phosphine, etc., 6 25 hexaflouride, structure, 27 104 hydride complexes, 20 235, 248-281, see also Transition metal-hydride complexes... 

Hydroperoxides result from oxidation of transition metal hydride complexes, but since so many transition metal hydrides possess oxidatively sensitive ligands, such as phosphines or CO, it is less likely that hydroperoxides may be intercepted. An interesting example of a stable hydroperoxide is the [(NH3)sRh(OOH)] cation . The hydride from which this species derives is one of the few that is not stabilized by soft, oxidizable ligands. 

Transition-metal hydride complexes which also contain tertiary phosphines as stabilizing ligands are among the most numerous and stable of presently known hydride complexes. Their relative ease of preparation, and their thermal and oxidative stability, make them suitable materials for carrying out detailed studies of the metal-hydrogen system. As shown in Table III, a considerable range of complexes in this class has been prepared and their infrared and proton magnetic resonance spectra have been studied. The results of these studies are discussed in Section II,B and C, respectively. 

Flowing afterglow techniques are yet another approach to studying ion-molecule reactions of organometallic species (, 52). Here ion-molecule reactions are conducted in a flow of inert buffer gas (usually He), so that all reagents and products are completely thermalyzed. Representative experiments have employed bracket-ing/competition experiments (cf., Equations 45-47) to obtain information such as hydride affinities of metal carbonyls and transition metal formyl complex stabilities (, ). ... 

Transition metal hydrides play a key role in the catalytic homogeneous isomerization of olefins. The pure hydrides such as HCo(CO)4 can function as the catalyst, or transition metals complexed to stabilizing ligands can function as catalysts the catalysis almost certainly proceeds through hydride intermediates in many cases. 

Several transition-metal phosphine complexes, including RuCl2(PPh3)3, are effective for the selective reduction of dienes to monoolefins.It is generally believed that this selectivity arises from the exceptional stability of an intermediate 7r-allyl complex. The key intermediate in this process may be a 7r-allyl hydride complex. Complex 6, as we indicated in the Introduction, is the only known example of such a complex. The hydride ligand is cis to the allyl group. 

Many complexes that contain alkyl ligands bearing -hydrogens readily decompose to form olefins and metal-hydride complexes. Such p-hydrogen elimination is the most common process that limits the stability of met -alkyl complexes, although other elimination processes noted beloTv can occur. Kochi summarized early available information on the mechanism of such P-eliminations, and there is also considerable information in early reviews of the chemistry of alkyl ligands. More recent reviews survey all of the decomposition modes available to transition metal alkyl complexes, but emphasize the work of individual authors. ... 

The research by Beerman also demonstrated the relative stability of the Ti-methyl bond as compared to the relatively less stable Ti-ethyl bond that contains a hydrogen on the beta carbon and can, therefore, undergo beta-hydride transfer to the titanium metal and eliminate an ethylene molecule. This early research eventually lead in the 1970s to the identification of transition metal carbene complexes (M=CH2), which when reacted with olefins provide metallacyclobutanes [29]. 

The NVE is often equal to 18 for transition-metal organometallics and for many inorganic complexes. This 18-electron rule should be better viewed as a strong tendency than a rule, but it is followed by a majority of complexes (despite many exceptions, vide infra). The 18-electron electronic structure often brings a good stability for complexes. For instance, this is the case for the metal carbonyl complexes, possibly the largest family. Transition-metal sandwich complexes are more stable in the 18-electron count than in others. The 18-electron electronic structure is also found most of the time in complexes containing a mixture of carbonyls, hydrocarbons, carbenes, hydrides, etc. - ... 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

Many complexes are known which contain one or more hydrogens directiy attached to a transition metal by an essentially covalent bond [2,3]. The occurrence and stability of these complexes frequently parallels that of metal-alkyl complexes. Thus thermally stable transition metal-hydrides are found when the complexes are kinetically staUe, that is, they often... 

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