Reaction of the mixed-metal cluster H2FeRu3(CO)13 with a series of tertiary phosphines and phosphites (L = PPh3, PPh2Me, PPhMe2, PPhEt2, P(OMe)3, P(OEt)3, P(OEt)2Ph, PMe3, P(i-Pr)3) leads to the formation of substituted clusters of the form H2FeRu3(CO)13-nLn (n = 1, 2) in 20-30% yield. Small quantities of trisubstituted clusters have also been isolated from the reactions employing PPh3, P(OMe)3, and P(OEt)3. The infrared spectra of the substituted clusters are similar to that of H2FeRu3(CO)13 and indicate that no severe structural changes occur in the cluster upon substitution. Detailed 31P and NMR studies show that the monosubstituted clusters can exist in two isomeric forms of Cs and C1 symmetries. The ratio of these two isomers for a given phosphorus donor ligand is dependent on both the ligand size and basicity. Large ligands give exclusively the Cs isomer. With small ligands, basicity becomes the controlling factor with the more basic ligands preferring the C1 isomer. Kinetic measurements indicate that the substitution reaction of H2FeRu3(CO)13 with PPh3 proceeds by a first-order, [PPh3]-independent path, presumably involving CO dissociation from H2FeRu3(CO)13 in the rate-determining step, with activation parameters ∆H±* = 25.3 ± 0.9 kcal/mol and ∆S±* = 4.9 ± 3.1 cal/(mol K). The structure of the C, isomer of H2FeRu3(CO)12(PMe2Ph) has been determined by X-ray diffraction. It crystallizes in the space group P1-Ci1 (No. 2) with a = 9.958 (4) Å, b = 9.970 (3) Å, c = 14.705 Å, a = 87.36 (2)°, β = 104.88 (3)°, and = 103.76 (3)° with Z = 2. The structure was refined to R1 = 0.034 and R2 = 0.035 for 5490 independent reflections having I > 3.0σ (I). The overall geometry of H2FeRu3(CO)12(PMe2Ph) is similar to that of the parent H2FeRu3(CO)13 cluster, implying that little distortion occurs upon substitution. The hydrides were located and refined in the H2FeRu3-(CO)12(PMe2Ph) structure and bridge two of the three Ru-Ru bonds in accord with the suggestion made earlier for H2FeRu3(CO)13 on the basis of bond length arguments. The phosphorus atom is attached to the Ru atom that is also ligated by both hydrides and lies 2.11 A below the Ru3 plane.