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DOI: 10.1039/A802559B (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1998, 2303-2314

Spectroelectrochemical and computational studies of tetrachloro and tetrabromo oxo- and nitrido-technetium(V) and their TcVI counterparts[hair space]‡

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John Baldas, Graham A. Heath, Stuart A. Macgregor, Klaus H. Moock, Sandra C. Nissen and Raphael G. Raptis

Abstract

The technetium complexes [NBu4][TcVINX4] and [NBu4][TcVOX4] (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 O2– or N3– ligands. The TcVI/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 TcVIOCl4 (by oxidation) and d2 [TcVNX4]2– (by reduction) in solution. However, the oxidation of [TcOBr4]– at +1.73 V is chemically irreversible. The featureless visible spectra of the TcV species contrast markedly with the wealth of charge-transfer bands found for the TcVI chromophores. In-plane {X4} → TcVI (dxy) transitions are precluded by the low-spin (dxy)2 configuration induced by the axial perturbation (by O2– or N3–) of the dπ 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 d1trans-[TcNX4(H2O)]– complex.

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