T-Bonded metal thiophene complexes

Metal complexes containing q -bonded thiophenes, where the ring formally donates 6 electrons and occupies three coordination sites (Lj-type ligand) are the most numerous and stable of the transition metal-thiophene derivatives, examples being available for Cr, Mn, Re, Fe, Ru, Rh, and Ir. Curiously, the synthesis of the first n-thiophene metal complex, viz. Cr(CO)3(q -T) reported by Fischer as early as 1958 [67] represents still today the only example available for a Group 6 metal it-bonded to thiophene its X-ray structure was solved by Dahl in 1965 albeit with a strong rotational disorder for the 

An interesting exception to the case of d complexes is [(ri -T)Rh(PPh3)2]PFj this rare example of a 5-coordinated Rh(I) d ri -T complex was prepared by Sanchez-Delgado et al. through the mild hydrogenation of the coordinated cyclooctadiene in the precursor [(COD)Rh(PPh3)2]PFj, in the presence of excess thiophene (see Eq. 2.20) [85]. 

This trend is interesting in that it is consistent with the one observed for the relation between adsorption constants and HDS activities of thiophenes on Co-MO/AI2O3 catalysts nevertheless, a similar trend was also observed in the case of the T -S bonded complexes [CpfCOljRufri S-Th)] also studied by Angelici [44] (see Section 2.2) and therefore no clear conclusions can be safely drawn from these organometallic model compounds as to apreferred mode of surface adsorption (r S or Tj ) being related to HDS activity. 

Interestingly, these observations can be related with the known inhibition effect of benzene and other aromatic hydrocarbons on HDS of BT over solid catalysts [90] this has been ascribed to a much stronger adsorption of benzene on metal sulfide surfaces as compared to common sulfur heterocycles. 

Thus it appears from the knowledge gained from the chemistry of metal complexes of n-bonded thiophenes that n adsorption on highly (triply) unsaturated metal sites, which has been frequently invoked in the heterogeneous literature, may in itself be a peripheral situation, rather than a crucial phenomenon directly related to the actual HDS reactions taking place, since a number of other pathways involving the more abundant singly or doubly CUS are available under desulfurization conditions. 

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