Large d–d luminescence energy variations in square-planar bis(dithiocarbamate) platinum(II) and palladium(II) complexes with near-identical MS₄ motifs: a variable-pressure study

We present the variable-pressure luminescence spectra of crystals of isostructural palladium(II) and platinum(II) complexes with bis-N-benzyl-N′-3-methylpyridyldithiocarbamate (bmpDTC) ligands. The d–d luminescence band maxima E_max for these complexes are compared to others with different peripheral substituents on the dithiocarbamate ligands in the solid state. The comparison reveals significant variations of E_max despite very similar metal coordination geometries. E_max varies by 3000 cm⁻¹ and 1300 cm⁻¹ among four dithiocarbamate complexes of platinum(II) and palladium(II), respectively. Variations of E_max with pressure reveal the effects of intermolecular M⋯H–C interactions on several complexes. ΔE_max/ΔP values are negative for the bmpDTC complexes, unprecedented in the dithiocarbamate family. Static orientation and pressure-induced movement of the C–H bonds involved in intermolecular interactions have a significant effect on E_max and ΔE_max/ΔP, with a stronger impact on platinum(II) complexes than on their palladium(II) analogs.