A series of square-planar [M(L)₂]²⁻ complexes (M=Ni(II), Pd(II), Pt(II); L=oxalate, 1,2-dithiooxalate, 1,2-diselenooxalate) were systematically investigated using density functional theory (DFT) and energy decomposition analysis (EDA) to elucidate their geometric preferences and bonding characteristics. Geometry optimizations and vibrational analyses confirmed D₂h-symmetric structures, with E,E′ (where E=S, Se) coordination modes consistently more stable than O,O′-analogues for the chalcogen-substituted ligands. EDA revealed that electrostatic interactions dominate the metal–ligand bonding across all complexes, although their relative contributions decrease from oxalate to diselenooxalate and from Ni to Pt, indicating increasing covalent character. Symmetry-resolved orbital analysis showed b1g-type Pd←L σ-donation as the leading covalent interaction, particularly prominent in softer chalcogen donors. The results provide a detailed quantitative picture of how ligand donor softness and metal identity modulate bonding in this class of chelate complexes based on oxalate and its heavier chalcogen analogues.
