Experimental and DFT study of the effect of mercaptosuccinic acid on cyanide-free immersion gold deposition

In order to search for an effective alternative to cyanide for gold plating, mercaptosuccinic acid (MSA) was selected as the complexing agent of Au(+) by open circuit potential tests and gold plating compared with 1-hydroxyethylidene-1,1-diphosphonic acid and aminomethylphosphonic acid. For the first time, a novel, stable, slightly acidic and cyanide-free gold plating bath was prepared. Scanning electron microscopy, Tafel tests, and tin dipping tests showed that the Cu/Ni–P/Au coating had a fine and even grain size, no black pad, good corrosion resistance, and good weldability. Quantum chemical calculations based on density functional theory were used to further study complexants and complexes. Molecular electrostatic potential indicates that Au(+) approaches MSA in the direction of C[double bond, length as m-dash]O. Frontier molecular orbital theory, atomic contribution to orbital composition, condensed local softness, and average local ionization energy indicate that the coordination capacity of the S atom in MSA is much stronger than that of other atoms. Fuzzy bond order analysis shows that the S–Au–S coordination structure is the most stable form in the plating solution. UV-visible absorption spectroscopy clarifies that the wavelength is redshifted when MSA–Au(i) ions form.