Synthesis, spectroscopic characterization, and DFT-guided thermodynamic studies of Ni(II) and Cu(II) mixed-ligand complexes for corrosion inhibition of mild steel in HCl medium

Ni(II) and Cu(II) mixed-ligand complexes derived from 2-aminothiophenol and thiourea were synthesized by sequential ethanolic reflux and evaluated as corrosion inhibitors for steel in 0.5 M HCl. The products were characterized using elemental analysis, molar conductivity, FT-IR, UV–Vis spectroscopy, mass spectrometry, Cu(II) ESR, thermogravimetric analysis, and density functional theory calculations. The recalculated elemental composition, spectroscopic data, reported qualitative magnetic behavior, and DFT coordination-core models are collectively consistent with four-coordinate structures, with square-planar environments proposed for both metal centers. The Ni(II) product is reported as diamagnetic, consistent with a low-spin d⁸ configuration, whereas Cu(II) is paramagnetic; for Cu(II), g∥ = 2.22 and g⊥ = 2.07 are consistent with a predominantly d(x²−y²) ground state. Because no single-crystal X-ray structure and no quantitative magnetic-susceptibility/µeff dataset are available, the geometries are treated as spectroscopically supported structural models rather than definitive crystallographic assignments. Gravimetric measurements at 298 and 308 K showed concentration-dependent inhibition, with the Cu(II) complex reaching a maximum reported inhibition efficiency of 97.8%. Langmuir analysis of the reported surface-coverage data gave R² values of 0.9914–0.9988 and ΔG°ads values of approximately −23.4 to −27.3 kJ mol⁻¹ , indicating spontaneous adsorption with mixed physical/electrostatic and more specific interfacial contributions. DFT-based FMO, MEP, NCI-RDG, ELF, and LOL analyses describe differences in the intrinsic electronic structures of the complexes, while no direct Fe-surface adsorption is inferred in the absence of explicit surface calculations.