Theoretical and Spectroscopic Analysis of Square-Planar Tellurium(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.