Treatment of M(CO)3(PMTA) (PMTA = 1,1,4,7,7-pentamethyldiethylenetriamine; M = Mo, W) with K[sec-Bu3BH] in refluxing THE, followed by cation exchange, provides 50-60% yields of deep purple [zt-Pr4N]4[HMo(CO)3]4 and 25-40% yields of deep blue-violet [R4N]4[HW(CO)3]4 (R = Et, n-Pr, n-Bu). Similar reactions of Mo(CO)4(TMED) (TMED = N,N,N’,N′-tetrarnethylethylenediarnine) with K[sec-Bu3BH] give 18-27% yields of [R4N]4[HMo(CO)3]4. While excess K[sec-Bu3BH] reacts with [Et4N]2[H2MO2(CO)8] to provide a 12% yield of [n-Pr4N]4[HMo(CO)3]4, substantially lower yields (ca. 4%) of [Et4N]4[HMo(CO)3]4 are obtained from the reactions of Na4[Mo(CO)4] with excess CH3CN or Mo(CO)6 with NaBH4 in refluxing THF, followed by cation exchange. Treatment of K2[H2W(CO)4] with aqueous [Et4N]Br or [Me3O] [BF4] in CH3CN, followed by cation exchange, provides a 19% or 38% yield of [Et4N]4[HW(CO)3]4. These initial examples of hydrido carbonyl clusters of molybdenum and tungsten are coordinatively unsaturated 56-electron tetramers and are characterized on the basis of elemental analyses and IR and 1H NMR spectra. Analysis of the latter for [HW(CO)3]44- establishes this cluster to be stereochemically nonrigid where there is equivalent coupling of each hydride to all four tungsten atoms from +20 to -40 °C in acetonitrile. Although these clusters are quite resistant to attack by basic reagents, they do readily interact with CO to give initially unsaturated dimers, H2M2(CO)82-, which are then converted in high yields to M2(CO)102-. A single-crystal X-ray structural determination of [n-Pr4N]4[HMo(CO)3]4 shows the presence of an essentially tetrahedral anion in which the carbonyl groups are eclipsed with respect to the M-M edges. The latter structural feature strongly suggests the presence of four face-bridging hydrogen atoms, which were not located directly. The crystals were monoclinic (space group P21) with cell parameters a = 15.467 (6) Å, b = 15.540 (14) Å, c = 15.143 (4) Å, β = 92.37 (3)°, V = 3637 (6) Å3, and z = 2.