Synthesis and structures of ammonium and tetraphenylphosphonium salts of μ-oxo-diaquadioxalatotetraoxodimolybdenum(VI). An interesting example of intramolecular hydrogen bonds within the dimeric anion

The reaction of molybdenum(VI) oxide with oxalic acid and RCl where R=NH4 (1) and (C6H5)4P (2), results in the formation of molybdenum(VI) oxalate complexes of the general formula R2[Mo2O5(C2O4)2(H2O)2]. As determined by X-ray structure analysis both salts are built up of the dimeric anions containing Mo2O5 core. Molybdenum atoms exhibit distorted octahedral coordination. Whereas in the ammonium salt the MoOMo bridge is linear, the one in the corresponding tetraphenylphosphonium salt is angular with an angle of 162.42(15)° at the oxygen atom. This angular structure of 2 is also caused by additional strong intramolecular hydrogen bonds (2.719(4) and 2.661(4) Å) between coordinated water molecules and oxalato-oxygens. Both complexes were characterized by chemical analysis, thermogravimetry, UV and IR spectroscopy as well as by conductivity measurements.