Spectroelectrochemical and computational studies of tetrachloro and tetrabromo oxo- and nitrido-technetium(V) and their Tc^VI counterparts

The technetium complexes [NBu₄][Tc^VINX₄] and [NBu₄][Tc^VOX₄] (X = Cl or Br) have been studied by electrochemical and theoretical methods. The redox potentials of the halide complexes are strongly affected by the introduction of axial O^2- or N^3- ligands. The Tc^VI/V couple decreases from +1.84 V for [TcOCl₄]^0/- to +0.21 V for [TcNCl₄]^-/2-. In situ spectroelectrochemical techniques were employed to characterise d¹ Tc^VIOCl₄ (by oxidation) and d² [Tc^VNX₄]^2- (by reduction) in solution. However, the oxidation of [TcOBr₄]^- at +1.73 V is chemically irreversible. The featureless visible spectra of the Tc^V species contrast markedly with the wealth of charge-transfer bands found for the Tc^VI chromophores. In-plane {X₄} → Tc^VI (d_xy) transitions are precluded by the low-spin (d_xy)² configuration induced by the axial perturbation (by O^2- or N^3-) of the d_π levels. Density functional calculations were used to analyse the trend in redox potentials and electronic spectra, and, in particular, to resolve additional complexities of the visible CT spectra of the five-co-ordinate d¹ [TcNX₄]^- (X = Cl or Br) dissolved in non-co-ordinating solvents. These relatively complicated spectra closely resemble the visible reflectance spectra measured for the unambiguously five-co-ordinate [NBu₄][TcNX₄] salts in the solid state, whereas the absorption spectra of the nitrido complexes dissolved in aqueous HX suggest the presence of the corresponding six-co-ordinate d¹ trans-[TcNX₄(H₂O)]^- complex.