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)(4)P (2), results in the formation of molybdenum(VI) oxalate complexes of the general formula R-2[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)degrees 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) Angstrom) 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. (C) 2000 Elsevier Science B.V. All rights reserved.