Anion effects on the adsorption of acetylene by nickel halides

The adsorption of acetylene on different nickel halides has been studied by ab initio molecular orbital calculations. The strength of adsorption on different nickel halides is calculated by ab initio molecular orbital calculation and it follows the order NiF₂>NiCl₂>NiBr₂>NiI₂. The calculated heats of adsorption are 25.97, 20.42, 18.24, and 16.42 kcal/mol, respectively. The calculations are performed on C₂H₂, NiX₂, and bonded C₂H₂-NiX₂, at the HF/3-21G level for geometry optimization and B3LYP/3-21+G** level for a detailed analysis of the electronic distribution using natural bond orbital (NBO) theory. The bonding between acetylene and NiX₂ involves three parts: (1) σ-donation (overlap of the 2p_x orbital of C with the 4s orbital of Ni), (2) electron redistribution (from the 4s orbital to the 3d_xz orbital of Ni, and (3) d-π* back-donation (from the 3d_yz orbital of Ni to the 2p_y*, or π*, orbital of C). The back-donation dominates the bonding. The three steps combined yield the minimum total energies.