Highly Reduced Organometallics. 15. Coordinatively Unsaturated Tetranuclear Hydrido Carbonyl Clusters of Molybdenum and Tungsten. Structural Characterization of [(n-C₃H₇)₄N]₄[HMO(CO)₃]₄

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]₄^4- 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)₈^2-, which are then converted in high yields to M₂(CO)₁₀^2-. 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) ų, and z = 2.