Synthesis of cis‐Tetrabromobispyridinemolybdates(III) and the crystal structure of cis‐(NH 4 )[MoBr 4 py 2 ] · ⅓3 H 2 O (py = pyridine)

The reaction between (NH 4 )[MoBr 5 · H 2 O] and pyridine in acetonitrile (CH 3 CN) at room temperature results in the mixture of cis‐ and trans‐(pyH)[MoBr 4 py 2 ] which can be separated on the basis of solubility. cis‐M[MoBr 4 py 2 ] · ⅓ H 2 O (M = NH 4 + , Rb + , Cs + ), cis‐(bipyH)[MoBr 4 py 2 ] (bipy = 2,2′‐bipyridil) and cis‐(PPh 4 )[MoBr 4 py 2 ], were prepared from cis‐(pyH)[MoBr 4 py 2 ]. At the temperature of boiling acetonitrile irreversible cis to trans isomerisation takes place. Bromine oxydizes cis isomers at room temperature to trans‐MoBr 4 py 2 . The compounds were characterised by chemical analysis, infrared, UV‐VIS spectroscopy, conductivity measurements and powder diffraction. The crystal structure of cis‐(NH 4 )[MoBr 4 py 2 ] · ⅓ H 2 O has been determined: rhombohedral, R3c, (No. 161), a = 15.809(3) Å, β = 112.79(2)°, Z = 6, D C = 2.29, D O = 2.27(3) g/cm 3 , V = 2 601(1) Å 3 , R 1 = 0.046, R w = 0.068. Average MoBr and MoN(pyridine) distances within the anion are 2.58(2) and 2.20(2) Å. cis‐Rb[MoBr 4 py 2 ] · ⅓ H 2 O and cis‐Cs[MoBr 4 py 2 ] · ⅓ H 2 O are isostructural with cis‐(NH 4 )[MoBr 4 py 2 ] · ⅓ H 2 O.