The monosubstituted [H 4 Ru 4 (CO) 1 1 L] derivatives where L=P(C 6 F 5 ) 3 , PMe 2 Ph, P(OMe) 3 and P(OEt) 3 (compounds 1 to 4, respectively) were synthesized, the crystal structure of 1, 2 and 4 were determined by X-ray diffraction (that of compound 3 had been determined previously but not fully reported so we obtained it again) and their dynamical behavior studied by 1 H, 3 1 P and 1 9 F NMR. The structures of compounds 1, 3 and 4 show the common tetrahedral structure with four long (hydride bridged) and two short metal-metal bonds with the P-donor ligand transoid to an unbridged Ru-Ru bond. The structure of 2 is different since the M-M bonds to the Ru atom bonded to the phosphine, are all hydride-bridged. The variable temperature NMR studies of 1, 2 and 3 show dynamical behaviour similar to what had been reported for compound 4. However we propose that some of the structures involved in the equilibrium have the phosphorus substituent transoid to a bridged Ru-Ru bond.