Catalysts nitrosyl complexes, transition metal

Complexes of Other Metals. Having studied in detail catalysts derived from molybdenum nitrosyl complexes, it was interesting to investigate the effect on catalytic activity of substituting other transition metals in both nitrosyl derivatives and in other related complexes. 

This mechanism would seem to be quite plausible since there is good evidence for the formation of both nitrosyl (51-55) and oxygen-bridge complexes (16 26) with transition metal ions in zeolites. Interestingly, for a Cr3 + Y catalyst (51), the activity was enhanced by HzO and there was no deactivation by S02, indicating that such a catalyst would likely be quite robust under practical feed conditions. 

The discovery that the nitrosyl ligand is capable of binding to transition metals in two isomeric valence forms1 is one of the most dramatic recent developments in organometallic chemistry. Since bent NO donates 2 fewer electrons to the metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis.2 In fact, complexes of nitric oxide are receiving increasing attention as catalysts,3 since they are more reactive than the corresponding carbonyls.4... 

We suggest that the initial step is heterolytic splitting of H2 or Dj, leading to the protonated or deuterated transition metal nitrosyl hydrides or deuterides, respectively. H/D exchange via the second nitrosyl ligand might lead to the mixed hydride-deuteride complex, which in a reverse reaction forms HD, and regenerates the cationic catalyst. 

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

Selective NOx removal is an area where the boundary between sorbent and catalyst tends to disappear. Many different types of sorbents have been investigated for NOx sorption for both cold (near ambient temperature) and hot (200-400 °C) applications. Transition metal oxides appear to be the best. In both temperature ranges, a significant amount of adsorption is achieved usually when assisted by catalytic oxidation of NO to NO2 as an intermediate step. In air near ambient temperature, about 5% of the NO is in the form of NO2. NO2 adsorbs more easily than NO for both chemisorption and physical adsorption. The normal boiling point of NO is -152°C and that of NO2 is 21 °C. Thus, NO2 adsorbs readily near ambient temperature in micropores and mesopores by pore filling. In chemisorption, NO2 readily forms nitrite and nitrate on transition metal oxides. These chemisorbed species can be decomposed or desorbed only at elevated temperatures, e.g., 200-300 °C. Other surface species are also formed, such as NO+ (nitrosyl). These species could be desorbed at near-ambient temperature. The complex chemistry of NO is due to the fact that there is one electron occupying the antibonding orbital of NO, which is empty in most other molecules. 

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