Group VI metals

Dimerization reaetions of 1-azirines with several transition metal eomplexes have been studied (76TL2589). Reaetion of 2-arylazirines (289) with an equimolar amount of a Group VI metal earbonyl gives 2,5-diarylpyrazines (290) in good yield. On the other hand, these eompounds are eonverted to 2-styrylindoles (291) with rhodium earbonyl eompounds or with dieobalt oetaearbonyl in benzene. 

With respect to CO complexes, the luminescence spectra of a series of Group VI metal carbonyls and substituted carbonyls were obtained in frozen gas matrices at 12K. In addition, the IR spectra of HCo(CO>4 and HCo(CO)3 (proposed as an intermediate in hydroformylation) were observed in an argon matrix. ... 

Mikulski CM, Harris N, Iaconianni FJ, et al. 1980. Group-VI metal hexacarbonyl reactions with diisopropyl methylphosphonate. Inorganic Nuclear Chemistry 16(2) 79-89. 

The Group VI metal carbonyls demonstrate good activity in the WGSR, but differ significantly from ruthenium carbonyl in several ways. Tables IV and V summarize some WGSR experiments with chromium and tungsten carbonyls in a tetrahydrofuran-water solvent system. 

The possible intermediacy of the formate ion (eqs. 6 and 18) in the WGSR has been considered (2,6,10), but its involvement has not been clearly demonstrated. The Group VI metal carbonyl complexes are effective in the decomposition of formic acid (as sodium formate), as shown in Table VII. Some heterogeneity is observed in those reactions carried out under nitrogen pressure, but in no case was CO detected. The similarity in rates for WGSR... 

Figure 4. Proposed mechanism for the group VI metal carbonyl-catalyzed homogeneous WGSR...

The Activity of the Group VI Metal Carbonyls in Decomposition of Formic Acid as Formate... 

For the model olefin ethene, we again investigated various epoxidation mechanisms (Figure 7) [67]. As before for the group VI metals, insertion was found to exhibit significantly higher activation barriers. 

The ligand behavior of the arsenic-based compound 9 (Figurel3) structurally related to P4S3, is of interest in this context. It was shown that 9 reacts with group VI metal hexacarbonyls under UV irradiation to give the adducts nCH3C(CH2As)3]j M(CO)5 n (N = 1,... 

The isomerisation of the pentenyl and butenyl phosphine [PhzP-(CH2) CH=CH2 (n = 2,3) (see above and ref 43)] complexes of rhodium has been held to go through a ji-allyl hydride intermediate. Such an intermediate would be equally applicable to the reactions of the group VI metals with ap. Fig. 43b. 

Although both mechanisms offer apparently equally convincing explanations for the isomerisations being caused by the group VI metals and not by platinum, there is no experimental evidence that enables us to distinguish between them, and neither explains satisfactorily why cts-pp is formed at a much higher rate. 

The strong jt-bonding character in compounds of pentacarbonyl (group-VI metal) (0) with thiocarbene can be seen from the metal-carbon bond distances. For chromium compounds they were found to be ... 

Title Aluminum Phosphate-Supported Group VI Metal Amide Catalysts for Oligomerization of Ethylene... 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

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