Trapping of Hemiquinone Radicals at Mo and P Sites by Phosphide-Bridged Dimolybdenum Species:  Chemistry of Complexes [Mo<sub>2</sub>(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>(OC<sub>6</sub>H<sub>4</sub>OH)(μ-PR<sub>2</sub>)(CO)<sub>4</sub>] and [Mo<sub>2</sub>(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>{μ-PR(OC<sub>6</sub>H<sub>4</sub>OH)}(CO)<sub>4</sub>]<sup>-</sup> (R = Cy, Ph) AlvarezCeledonio M. AlvarezM. Angeles AlonsoMaría GarcíaM. Esther RuedaM. Teresa RuizMiguel A. HersonPatrick 2006 The phosphide-bridged dimolybdenum complexes (H-DBU)[Mo<sub>2</sub>Cp<sub>2</sub>(μ-PR<sub>2</sub>)(CO)<sub>4</sub>] (R= Cy, Ph; DBU = 1,8-diazabicyclo[5.4.0.]undec-7-ene) react with <i>p</i>-benzoquinone to give the hemiquinone complexes [Mo<sub>2</sub>Cp<sub>2</sub>(OC<sub>6</sub>H<sub>4</sub>OH)(μ-PR<sub>2</sub>)(CO)<sub>4</sub>]. The latter experience facile homolytic cleavage of the corresponding Mo−O bonds and react readily at room temperature with HSPh or S<sub>2</sub>Ph<sub>2</sub> to give the thiolate complexes [Mo<sub>2</sub>Cp<sub>2</sub>(μ-PCy<sub>2</sub>)(μ-SPh)(CO)<sub>4</sub>] or [Mo<sub>2</sub>Cp<sub>2</sub>(μ-PR<sub>2</sub>)(μ-SPh)(CO)<sub>2</sub>]. In contrast, PRH-bridged substrates experience overall insertion of quinone into the P−H bond to give the anionic compounds (H-DBU)[Mo<sub>2</sub>Cp<sub>2</sub>{μ-PR(OC<sub>6</sub>H<sub>4</sub>OH)}(CO)<sub>4</sub>], which upon acidification yield the corresponding neutral hydrides. The cyclohexyl anion experiences rapid nucleophilic displacement of the hemiquinone group by different anions ER<sup>-</sup> (ER = OH, OMe, OC<sub>4</sub>H<sub>5</sub>, OPh, SPh) to give novel anionic compounds (H-DBU)[Mo<sub>2</sub>Cp<sub>2</sub>{μ-PCy(ER)}(CO)<sub>4</sub>], which upon acidification yield the corresponding neutral hydrides. The structure of four of these hydride complexes [PPh(OC<sub>6</sub>H<sub>4</sub>OH), 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 [Mo<sub>2</sub>Cp<sub>2</sub>{μ-PCy(OH)}(CO)<sub>4</sub>]<sup>-</sup> displays in solution an unprecedented tautomeric equilibrium with its hydride-oxophosphinidene isomer [Mo<sub>2</sub>Cp<sub>2</sub>(μ-H){μ-PCy(O)}(CO)<sub>4</sub>]<sup>-</sup>.