Mo(PMe3)(6) cleaves a C-S bond of benzothiophene to give (kappa(2)-CHCHC6H4S)MO(PMe3)(4), which rapidly isomerizes to the olefin-thiopheno late and 1-metallacyclopropene-thiophenolate complexes, (kappa(1), eta(2)-CH2CHC6H4S)Mo(PMe3)(3)(eta(2)-CH2PMe2) and (kappa(1), eta(2)-CH2CC6H4S)Mo(PMe3)(4). The latter two molecules result from a series of hydrogen transfers and are differentiated according to whether the termini of the organic fragments coordinate as olefin or eta(2)-vinyl ligands, respectively. The reactions between Mo(PMe3)(6) and selenophenes proceed differently from those of the corresponding thiophenes. For example, whereas Mo(PMe3)(6) reacts with thiophene to give eta(5)-thiophene and butadiene-thiolate complexes, (eta(5)-C4H4S)Mo(PMe3)(3) and (eta(5)-C4H5S)Mo(PMe3)(2)(eta(2)-CH2PMe2), selenophene affords the metallacyclopentadiene Complex [(kappa(2)-C4H4)Mo(PMe3)(3)(Se)](2)[Mo(PMe3)(4)] in which the selenium has been completely abstracted from the selenophene moiety. Likewise, in addition to (kappa(1), eta(2)-CH2CC6H4Se)Mo(PMe3)(4) and (kappa(1), eta(2)-CH2CHC6H4Se)Mo(PMe3)(3)(eta(2)-CH2PMe2), which are counterparts of the species observed in the benzothiophene reaction, the reaction of Mo(PMe3)6 with benzoselenophene yields products resulting from C-C coupling, namely [kappa(2), eta(4)-Se(C6H4)(CH)(4)(C6H4)Se]Mo(PMe3)(2) and [mu-Se(C6H4)(CH)C(CH)(2)(C6H4)](mu-Se)[Mo(PMe3)(2)][Mo(PMe3)(2)H].