The electrochemical behaviour of the complexes [Re(CO)(3)X(α-diimine)], X = Cl or Br, α-diimine = 1,4-di-tert-butyl-1,4-diaza-1,3-diene, was reinvestigated using cyclic voltammetry accompanied by IR and UV-vis spectroelectrochemistry. While the reduction results in the loss of halide, as necessary for the electrocatalytic activity of related diimine compounds, a reversible oxidation could be observed for the chloro complex 1 (X = Cl). The conversion of 1 to 1(+) in CH(2)Cl(2) or CH(3)CN is accompanied by high-frequency shifts (Δν = 73-114 cm(-1)) of the three carbonyl stretching bands, by a considerable change in carbonyl stretching modes, and by changed absorption in the visible region. DFT calculations support the spectroelectrochemical results and suggest an unusually large g anisotropy (g(1) 1.38, g(2) 2.06, g(3) 3.20) which explains the absence of detectable EPR signals for 1(+) under normal X band conditions. Frontier orbitals calculated by DFT for 1 reveal two close lying occupied orbitals (HOMO, HOMO-1) with Re-Cl character and a diimine based LUMO.