Preparative and structural studies of halodimolybdates(II). IV. Synthesis and crystal structure of (pyH)3Mo2Br7(H2O)2

(pyH)₃Mo₂Br₇(H₂O)₂ (pyH = Pyridinium cation) was prepared from the solution of (NH₄)₅Mo₂Cl₉ · H₂O in 1:1 HBr by the addition of pyHBr. The compound has monoclinic unit cell: P 2₁/n with a = 10.250(4), b = 11.891(4), c = 11.971(3) Å and β = 112.86(2)°. Z = 2, D calcd. = 2.54, D obsd. = 2.52(2) g cm^?3. The structure has been refined to the unweighted and weighted residuals of 7.8 and 8.7%. The structure contains Mo₂Br₆(H₂O)₂^2?, C₅H₆N⁺ and Br^? ions. Short Mo? Mo distance 2.130(4) Å reflects the strong bond. H₂O is coordinated to molybdenum at 2.19(3) Å. Distribution of H₂O molecules and Br^? ions around Mo₂ is different from (picH)₂Mo₂X₆(H₂O)₂ (X = Br, I) and determined by the two-fold axes perpendicular to the Mo? Mo direction and the plane defined by two H₂O molecules and 2 Br atoms. Three independent Mo? Br distances are 2.574(4), 2.606(5) and 2.569(5) Å. Specific structure of the Mo₂Br₆(H₂O)₂^2? ion has no synthetic consequences and reaction with neutral aromatic nitrogen bases leads to the known Mo₂Br₄L₄ compounds.