The phosphide-bridged dimolybdenum complexes ( H-DBU)[ Mo2Cp2( mu-PR2)( CO)(4)] ( R) Cy, Ph; DBU) 1,8-diazabicyclo[ 5.4.0.] undec-7-ene) react with p-benzoquinone to give the hemiquinone complexes [ Mo2Cp2( OC6H4OH)( mu-PR2)( CO)(4)]. The latter experience facile homolytic cleavage of the corresponding Mo-O bonds and react readily at room temperature with HSPh or S2Ph2 to give the thiolate complexes [ Mo2Cp2( mu-PCy2)( mu-SPh)( CO)(4)] or [ Mo2Cp2( mu-PR2)( mu-SPh)( CO)(2)]. In contrast, PRH-bridged substrates experience overall insertion of quinone into the P - H bond to give the anionic compounds ( H-DBU)[ Mo2Cp2 {A- PR( OC6H4OH)}( CO)(4)], which upon acidification yield the corresponding neutral hydrides. The cyclohexyl anion experiences rapid nucleophilic displacement of the hemiquinone group by different anions ER- ( ER) OH, OMe, OC4H5, OPh, SPh) to give novel anionic compounds ( H-DBU)[ Mo2Cp2 {mu-PCy( ER)}( CO)(4)], which upon acidification yield the corresponding neutral hydrides. The structure of four of these hydride complexes [ PPh( OC6H4OH), PCy( OH), PCy( OMe), and PCy( OPh) bridges] was determined by X-ray diffraction methods and confirmed the presence of cis and trans isomers in several of these complexes. In addition, it was found that the hydroxyphosphide anion [ Mo2Cp2 {A-PCy( OH)}( CO)(4)]- displays in solution an unprecedented tautomeric equilibrium with its hydride-oxophosphinidene isomer [ Mo2Cp2( A- H) {A- PCy( O)}( CO)(4)](-).