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The electrochemical and photoredox properties of copper(II) thiopyrazolone Schiff base complexes 1–9 with imine and thiolate coordination, showing variable degrees of tetrahedral distortion, were investigated by means of combined electrochemical and spectroscopic techniques in the temperature range of 193–293 K. The cyclic voltammograms of 1–9 in butyronitrile revealed that the reduction and oxidation paths are strongly dependent on the geometry of the CuN2S2 moiety. Due to the strong delocalization of the singly occupied redox orbital (SOMO) the oxidations and reductions occur in a narrow potential range. The one‐electron‐reduced 1(r)–9(r) and oxidized 1(o)–9(o) products were electrogenerated and stabilized inside optically transparent thin‐layer electrochemical (OTTLE) cells at variable temperatures and could be characterized for the first time by UV/Vis spectroscopy. The reduced formal d10 copper(I) species absorb only weakly in the visible region. The oxidized products 1(o)–9(o) show several strong absorptions in the visible region due to the presence of formal d8 copper(III) species. The spectral information allowed assignment of the initial photoproducts. Irradiations in donor media such as THF or EtOH initially produces 1(r)–9(r). No photoreduction was observed in tBuOH which cannot liberate reducing Hα. The primary oxidized species 1(o)–9(o) were formed in chlorinated acceptor solvents (CHCl3) on UV irradation. Fast relaxation to the ground state prevents the photoredox reactions from CT or LF excited states.

A series of tetrahedrally distorted copper(II) complexes with thiolate and imine coordination were synthesized. Schiff bases derived from 4‐benzoyl‐3‐methyl‐1‐phenyl‐2‐pyrazoline‐5‐thione and various diamines were used as tetradentate ligands to obtain tetrahedrally distorted metal chelates with [CuN2S2] complex units. Crystal structures of the complexes 1, 2, 5 and 6 and of ligand H25 have been determined by means of single‐crystal X‐ray structure analysis. The structure data show a strong influence of the diamine building blocks on the tetrahedral distortion of the copper(II) complexes. Results of Extended Hückel LCAO calculations correlate strongly with structural, electrochemical, UV/Vis‐ and EPR‐spectroscopic features obtained experimentally. The calculations confirm for the whole complex series a strong delocalization of the frontier orbitals. The highest fully occupied molecular orbital shows a weak contribution, resulting from thiolate donor atoms, whereas the antibonding singly occupied molecular orbitals (SOMOs) are distributed between the copper(II) centre (ca. 35–40%) and the N2S2 donor set. The SOMO energy significantly lowers with increasing tetrahedral distortion of the coordination sphere. The influence of the tetrahedral distortion of copper(II) complexes on redox potentials, UV/Vis and EPR spectra is discussed.

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