Trimethylphosphine complexes of molybdenum, tungsten, niobium, and tantalum. X-Ray crystal structures of di-µ-hydrido-dihydridohexakis-(trimethylphosphine)dimolybdenum(II)(Mo–Mo) and of µ-dimethylphosphido-µ-hydrido-tetrahydridopentakis(trimethylphosphine)ditungsten(II, IV)(W–W)

Abstract

The reduction of Mo₂[O₂CMe]₄ by sodium amalgam in tetrahydrofuran (thf) in the presence of excess of PMe₃ under hydrogen (3 atm) yields the dimeric complex Mo₂H₂(µ-H)₂(PMe₃)₆(1), whose structure has been determined by X-ray crystallography. The compound is orthorhombic, space group Pbca, with a= 12.133(6), b= 14.892(4), c= 17.692(5)Å, and Z= 4. The structure has been refined to an R of 0.0362 for 2 388 observed [l > 1.5σ(l)] diffractometer data. The centrosymmetric molecule has an edge-shared (H, H) bioctahedral structure with a Mo–Mo distance of 2.194(3)Å. Of the three phosphine ligands on each metal, one is approximately perpendicular to the Mo₂H₂ plane and trans to the terminal hydride with Mo–P 2.430(3)Å, and two lie in the Mo₂H₂ plane; differences in the Mo–P distances to these phosphines [2.430, 2.417(3)Å] suggest possible asymmetry in the Mo–H–Mo bridges, although the low precision with which the H-atom positions are located does not allow an unequivocal decision. Reduction of the complex W₂Cl₄(PMe₃)₄ in thf by sodium amalgam under hydrogen leads to a species best formulated as W₂H₄(µ-H)(µ-PMe₂)(PMe₃)₅(2). This complex is monoclinic, space group P2₁/c, with a= 9.931 (5), b= 12.023(3), c= 26.605(4)Å, β= 91.66(3)°, and Z= 4. The structure was refined to an R value of 0.0525 for 4 218 observed reflections. The molecule appears to be a pentahydride of the binuclear unit (Me₃P)₂[graphic omitted](PMe₃)₃ although it has not been possible to locate the metal-bonded hydrogen atoms. The two metal atoms give two sets of W–P bond lengths, with those for the metal carrying three terminal phosphines slightly shorter [2.355–2.464(5)Å] than those to the metal carrying two phosphines [2.433–2.502(6)Å]. Interaction of MMe₅(M = Nb or Ta) and PMe₃ gives MMe₅(PMe₃)₂; the niobium compound reacts with hydrogen to give NbH₅(PMe₃)₄.