The phosphido-substituted triruthenium cluster Ru-3(CO)(9)(mu-H)(mu-PPh2) is shown to react with H-2 to form the trihydride cluster Ru-3(CO)(9)(H)(mu-H)(2)(mu-PPh2), which undergoes a number of re-arrangement reactions on heating to yield other phosphido-substituted triruthenium clusters. In the presence of alkyne substrates, heating the system leads to catalytic hydrogenation via CO loss and the formation of a Ru-3(eta(2)-PhC=CHPh)(CO)(8)(mu-H)(PHPh2) resting state, in a reaction affected by the polarity of the solvent. No mononuclear fragments are observed in the catalytic transformation, confirming directly that the phosphido ligand is able to exert a stabilising influence on the cluster core.