Trapping of Hemiquinone Radicals at Mo and P Sites by Phosphide-Bridged Dimolybdenum Species:  Chemistry of Complexes [Mo₂(η⁵-C₅H₅)₂(OC₆H₄OH)(μ-PR₂)(CO)₄] and [Mo₂(η⁵-C₅H₅)₂{μ-PR(OC₆H₄OH)}(CO)₄]^- (R = Cy, Ph)

The phosphide-bridged dimolybdenum complexes (H-DBU)[Mo₂Cp₂(μ-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)(μ-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(μ-PCy₂)(μ-SPh)(CO)₄] or [Mo₂Cp₂(μ-PR₂)(μ-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{μ-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{μ-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{μ-PCy(OH)}(CO)₄]^- displays in solution an unprecedented tautomeric equilibrium with its hydride-oxophosphinidene isomer [Mo₂Cp₂(μ-H){μ-PCy(O)}(CO)₄]^-.