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A series of ^(.El-coordinated (E = S or Se) thiophene, benzo [b]thiophene and selenophene complexes [Cp(NO)(PPh3)Re(?/* 1(E)-L)]+, Cp = C5H5, L = thiophene (T), 2-methylthiophene (2-MeT), 2,5-dimethylthiophene (2,, benzo[b]thiophene (BT), 3-methylbenzo[b]thiophene , selenophene (Sel), 2-methylselenophene (2-MeSel), and 2,5-dimethylselenophene (2,5-Me2Sel) are prepared by the reaction of [Cp(NO)(PPh3)Re(ClC6H5)]+ with the appropriate ligand. The T, 2-MeT, BT, 3-MeBT, Sel, 2-MeSel complexes are deprotonated at C(2) by strong, non-nucleophilic bases to give the neutral Cp(NO. The pK& of the base required to effect this deprotonation increases with the L ligand in the complex in the following order: Sel < T < BT. The 2-Tyl, 2-BTyl and 2-Selyl complexes react with either HBF4-Et20 or HO3SCF3 at -42 °C to give the corresponding carbene complexes [Cp(NO)(PPh3)Re(2-L-ylcarbene)]+ resulting from protonation at C(3). The molecular structure of [Cp(N0)(PPh3)Re(2-BTylcarbene)]03SCF3, as determined by an X-ray diffraction study, exhibits a Re=C bond distance of 1.992(7) A. The carbene complexes do not react with nucleophiles; however, those nucleophiles that are sufficiently basic deprotonate C(3) to give back the L-yl compound. The pK& values of bases that are strong enough to cause deprotonation increase with the L-ylcarbene ligand in the order: Selylcarbene ~Tylcarbene < BTylcarbene. The carbene complexes [Cp(NO)(PPhs)-Re(2-(5-MeTylcarbene)]+ and [Cp(NO)(PPh3)Re(2-(5-MeSelylcarbene)]+ are unstable and rearrange to their more stable isomers [Cp(NO)(PPh3)Re(r/1(S)-2-MeT)]+ and [CpfNOjfPPhs)-Re(V(Se)-2-MeSel)]~. A new mechanism for H/D exchange of thiophene on hydrodesulfurization catalysts is proposed based on deuterium labeling studies of these thiophene complexes.

The series of rj^-selenophene transition metal complexes (t|5-Seln)Cr(C0)3 (1-3), [(RI5-Seln)Mn(C0)3]S03CF3 (4-6), [(t|5-Seln)RuCp*]S03CF3 (7-9), and [(ri5-Seln)IrCp*](BF4)2 (10-12), where Seln = selenophene(Sel), 2methylselenophene (2-MeSel), or 2,5-dimethylselenophene(2,5-Me2Sel), were synthesized and characterized by ^H, and "^"^Se NMR and IR spectroscopy. The molecular structxire of (t|5-2,5-Me2Sel)Cr(CO)3 (3) was determined. Reactions of KT|^-Sel)Mn(C0)3]S03CF3 (4) with nucleophiles (Nuc = H~, CN~) give the neutral addition products [(Sel*Nuc)Mn(C0)3] (Nuc = H~ (4a), Nuc = CN~ (4b)) in which three carbon atoms and the Se are bonded to the Mn. The reaction of [(ri^-SeDRuCp^lSOsCFs (7) with H~, however, results in cleavage of the C-Se bond to form a butadiene selenide complex ((ri5-SeCH=CH-CH=CH2)RuCp*) (7a). Still another type of product results from the reaction of [(TI5-2,5-Me2Sel)IrCp*](BF4)2 (12) with two equivalents of H~; in this case, the H~ acts as a reducing agent to give the ring-opened complex (C, Se-2,5-Me2Sel)IrCp* (12a). All of these reactions are similar to those of the analogous t|5-thiophene complexes. The "^"^Se NMR chemical shift values for the tj^-Seln ligands in complexes 1-12 fall within a range of 225 ppm; they are influenced