Reappraisal of the fluxional behavior of Ru3(.mu.-H)(.mu.3-.eta.2-C.tplbond.CBu-tert)(CO)9 and evidence for the concerted rotation of the Ru(CO)2(PMe2Ph) group in the enantiomerization of Ru3(.mu.-H)(.mu.3-.eta.2-C.tplbond.CBu-tert)(CO)8(PMe2Ph)