A SPECTROPHOTOMETRIC AND ESR STUDY OF MONOMER, MU-OXO DIMER, DI(MU-OXO) DIMER INTERCONVERSION OF NITRIDOTECHNETIUM(VI) COMPLEXES IN SOLUTIONS OF SULFUR AND PHOSPHORUS OXO ACIDS

UV-Vis and ESR spectroscopy have been used to study the behaviour of Cs2[TcNCl5] and the hydrolysis product [{TcN(OH)(OH2)}2(mu-O)2] (1) in concentrated and aqueous solutions of a variety of sulfur and phosphorus oxo acids ranging from the very weakly coordinating CF3SO3H to H2SO4 and the coordinating acids H3PO4 and H4P2O7. The paramagnetic monomeric TcVIN3+ (d1) forms were readily identified by ESR spectroscopy and the diamagnetic dimeric mu-oxo (Tc2N2O4+) and di(mu-oxo) (Tc2N2O22+) forms by their UV-Vis spectra. For solutions of CS2[TcNCl5] the chemical form of TcVIN and the rate of the conversion of monomer --> mu-oxo dimer --> di(mu-oxo) dimer is dependent on the presence of Cl- and the concentration and complexing ability of the acid, with the monomeric form being favoured by the acids of greater complexing power. For solutions of 1 the reverse order of conversion was observed. In 1 M CFSO3H, MeSO3H or toluene-p-sulfonic acid, 1 is converted to the di(mu-oxo) aquanitrido cation [{TcN(OH2)3}2(mu-O)2]2+ while in 7.5 M CF3SO3H it is proposed that the predominant form is the mu-oxo aqua dimer [{TcN(OH2)4}2(mu-O)]4+. In H2SO4 and the phosphorus oxo acids the aqua cations are coordinated by the acid anions. The monomeric aqua cation, [TcN(OH2)5]3+ , does not appear to be a viable species, as is also the case for MoO3+ (aq), and spontaneously dimerises in 7.5 M CF3SO3H. In anhydrous CF3SO3H or H2SO4 monomeric TcVIN with CF3SO3- Or HSO4- coordination is observed. A characteristic feature of the UV-Vis spectra of monomeric TcVIN is a strong absorption at 266-300 nm assigned to piN --> Tc charge transfer. The mu-oxo dimers show an intense visible absorption at 470-510 nm.