Spectroscopic Characterization of Aqua [ fac-Tc(CO)₃]⁺ Complexes at High Ionic Strength

Understanding fundamental Tc chemistry is important to both the remediation of nuclear waste and the reprocessing of nuclear fuel; however, current knowledge of the electronic structure and spectral signatures of low-valent Tc compounds significantly lags behind the remainder of the d-block elements. In particular, identification and treatment of Tc speciation in legacy nuclear waste is challenging due to the lack of reference data especially for Tc compounds in the less common oxidation states (I-VI). In an effort to establish a spectroscopic library corresponding to the relevant conditions of extremely high ionic strength typical for the legacy nuclear waste, compounds with the general formula of [ fac-Tc(CO)₃(OH₂)_{3- n}(OH) _n]^{1- n} (where n = 0-3) were examined by a range of spectroscopic techniques including ⁹⁹Tc/¹³C NMR, IR, XPS, and XAS. In the series of monomeric aqua species, stepwise hydrolysis results in the increase of the Tc metal center electron density and corresponding progressive decrease of the Tc-C bond distances, Tc electron binding energies, and carbonyl stretching frequencies in the order [ fac-Tc(CO)₃(OH₂)₃]⁺ > [ fac-Tc(CO)₃(OH₂)₂(OH)] > [ fac-Tc(CO)₃(OH₂)(OH)₂]^-. These results correlate with established trends of the ⁹⁹Tc upfield chemical shift and carbonyl ¹³C downfield chemical shift. The lone exception is [ fac-Tc(CO)₃(OH)]₄ which exhibits a comparatively low electron density at the metal center attributed to the μ₃-bridging nature of the ^-OH ligands causing less σ-donation and no π-donation. This work also reports the first observations of these compounds by XPS and [ fac-Tc(CO)₃Cl₃]^2- by XAS. The unique and distinguishable spectral features of the aqua [ fac-Tc(CO)₃]⁺ complexes lay the foundation for their identification in the complex aqueous matrixes.