Covalent Bonding in Copper(II) Complexes Having N and S Atoms in a trans Arrangement

Bonding in the chromophore trans‐CuN₂S₂ has been investigated using ESR and electronic spectra of single crystals of Cu(II) doped (∼2%) trans‐bis (thiosemicarbazide) Ni(II) sulfate trihydrate and trans‐bis (thiosemicarbazide) Cu(II) nitrate. The unpaired electron is described, in the ``hole'' formalism, in terms of molecular orbitals of D_2h symmetry. Crystal polarized spectra of the undiluted nitrate allow a reasonable, if not unambiguous, assignment of d–d bands. Combination of magnetic and optical results gives all bonding parameters, apart from those for the nonmagnetic ${\mathrm{A}}_g{\mathrm{(d}}_{{\text{3z}}^2{\mathrm{-r}}^2})$ orbital. All bonds, and particularly $\mathrm{Cu}–\mathrm{S}$ , are highly covalent in character. The $\mathrm{\sigma \hspace{0.17em}}\mathrm{Cu}–\mathrm{S}$ bond is roughly twice as covalent as $\mathrm{\sigma \hspace{0.17em}}\mathrm{Cu}–\mathrm{N}$ bond. The out‐of‐plane π bonds, and in‐plane σ bonds, show covalence similar to that observed in D_4h (CuN₄) chromophores and $\sim {\mathrm{D}}_{\text{4h}}$ (more strictly C_4v) (CuS₄+S). This suggests that bonds between two trans ligands and the copper ion are not influenced by the second pair of bonds in square planar complexes.