Theoreticaland Spectroscopic Analysis of Square-PlanarTellurium(II) and Selenium(II) Diphenyl Thiourea Complexes in p² Electronic Configurations

In this work, we performed a detailed mathematical ligand field theory analysis of square-planar (<i>D</i>_4h) main group complexes in p² electronic configurations, and the results were subsequently used to generate an energy-level correlation diagram. We synthesized the model p² tellurium(II) diphenyl thiourea coordination complex for further spectroscopic investigation. Dissolution of TeO₂ in concentrated HCl resulted in the formation of a [TeCl₆]^2– complex in acidic media, and subsequent addition of diphenyl thiourea resulted in the reduction of Te⁴⁺ to Te²⁺ (observed by ¹²⁵Te NMR spectroscopy) and the formation of a <i>cis</i>-Te(dptu)₂Cl₂ complex, which was characterized by single-crystal X-ray diffraction (XRD). Using in situ optical/magnetic spectroscopy – specifically, UV–vis and magnetic circular dichroism (MCD) spectroscopy – we observed absorbance bands and polarized transitions consistent with the calculated p-orbital splitting in a square planar (<i>D</i>_4h) ligand field. Using the results of our spectroscopic investigation, we generated a molecular orbital (M.O.) diagram for the <i>cis</i>-Te(dptu)₂Cl₂ complex. We then used the M.O. diagram, in conjunction with the energy level correlation diagram, to assign the electronic transitions observed in the spectra of the <i>cis</i>-Te(dptu)₂Cl₂ complex. The analogous selenium(II) complex, <i>cis</i>-Se(dptu)₂Cl₂, was used to elucidate the observed transitions with minimal contribution from spin–orbit coupling. Our work examined how in situ spectroscopy and complementary ligand field theory analysis can be used to elucidate the electronic structures of main group coordination complexes.