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 di fferent nickel halides is calculated by ab initio molecular orbital calcula tion and it follows the order NiF2 > NiCl2 > NiBr2 > NiI2. The calculated h eats of adsorption are 25.97, 20.42, 18.24, and 16.42 kcal/mol, respectivel y. The calculations are performed on C2H2, NiX2, and bonded C2H2-NiX2, at t he 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) sigma-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) orbi tal of Ni, and (3) d-pi* back-donation (from the 3d(yz) orbital of Ni to th e 2p(y)*, or pi*, orbital of C). The back-donation dominates the bonding. T he three steps combined yield the minimum total energies.