The coordination of selenophene in a variety of transition metal complexes has been investigated and compared to the analogous thiophene compounds. A series of [eta][superscript]5 coordinate selenophene complexes: ([eta][superscript]5-Seln)Cr(CO[subscript]3, [([eta][superscript]5- Seln)Mn(CO[subscript]3] O[subscript]3SCF[subscript]3, [([eta][subscript]5- Seln)Ru([eta][superscript]5- C[subscript]5Me[subscript]5)] O[subscript]3SCF[subscript]3, and [([eta][superscript]5- Seln)Ir([eta][superscript]5- C[subscript]5Me[subscript]5)] O[subscript]3SCF[subscript]3 where Seln = selenophene (Sel), 2-methylselenophene (2-MeSel), or 2,5-dimethylselenophene (2,5-Me[subscript]2Sel), have been made and characterized by elemental analysis, [superscript]1H, [superscript]13C, [superscript]77Se NMR and IR spectroscopies. Nucleophiles (H[superscript]-, CN[superscript]- and P(n-Bu)[subscript]3) react with [[eta][superscript]5- Seln)Mn(CO)[subscript]3] O[subscript]3SCF[subscript]3 and [([eta][superscript]5- Seln)Ru([eta][superscript]5- C[subscript]5Me[subscript]5)] O[subscript]3SCF[subscript]3 to give products that are analogous to the thiophene complexes. The reaction of [([eta][superscript]5- Seln)Ir([eta][superscript]5- C[subscript]5Me[subscript]5)] O[subscript]3SCF[subscript]3 with H[superscript]- gives a ring open compound (C,Se-2,5-Me[subscript]2Sel)Ir([eta][superscript]5- C[subscript]5Me[subscript]5) that is spectroscopically similar to the structurally characterized (C,S-2,5-Me[subscript]2T)Ir([eta][superscript]5- C[subscript]5Me[subscript]5). The molecular structure of ([eta][superscript]5-2,5-Me[subscript]2Sel)Cr(CO)[subscript]3 was determined and is compared to the previously known complex ([eta][superscript]5-2,5-Me[subscript]2T)Cr(CO)[subscript]3.;[eta][superscript]1(Se) coordination has been demonstrated in the series of compounds [([eta][superscript]5- C[subscript]5H[subscript]5)(CO)(P(C[subscript]6H[subscript]5)[subscript]3)Ru([eta][superscript]1(E)- L)] BF[subscript]4. The compounds have been characterized by elemental analysis IR, [superscript]1H, [superscript]13C, and [superscript]77Se NMR spectroscopies. The molecular structure of [([eta][superscript]5- C[subscript]5H[subscript]5)(CO)(P(C[subscript]6H[subscript]5)[subscript]3) Ru([eta][superscript]1(Se)-2-MeSel)] BF[subscript]4 has been determined and is analogous to the known thiophene complex. Equilibrium constants (K) for the displacement of Seln have been measured with the trend: T(1.0) < Sel(23.8) < 2-MeSel(100) < 2,5-Me[subscript]2Sel(175). The [superscript]77Se NMR chemical shift data of [eta][superscript]1(Se) and [eta][superscript]5-bound selenophene shows an upfield shift due to coordination of unbound selenophene ([delta] 605) with distinct range for the [eta][superscript]1(Se) complexes, [delta]500-400, and the [eta][superscript]5 complexes [delta]350-150. The chemical shift of individual complexes depends on the charge of the metal and the other ligands in the complex;Thiophene, benzothiophene and selenophene carbon-hydrogen bond activation has been demonstrated in the complex [Cp(NO)(P(C[subscript]6H[subscript]5)[subscript]3)Re([eta][superscript]1(E)- L)] BF[subscript]4 by a series of deprotonation and reprotonation reactions to give [Cp(NO)(P(C[subscript]6H[subscript]5)[subscript]3)Re(2-L-ylcarbene)])[superscript]+. The molecular structure of Cp(NO)(P(C[subscript]6H[subscript]5)[subscript]3)Re(2-BTylcarbene)] O[subscript]3SCF[subscript]3 was determined with a Re-C bond distance (1.992(7)A) intermediate between a Re-C single bond (2.178(6)A) and a Re=C double bond(1.949(6)A). The carbene is very stable and is not attacked by nucleophiles. The 2-methylthienylcarbene rearranges to give the [eta][superscript]1(S) bound complex. A new mechanism for H/D exchange of thiophene during hydrodesulfurization is proposed using the thienyl and carbene intermediates.