A sorbent was prepared by dispersing a monolayer of Ni2+ ions (by spreading a monolayer of NiCl2 . 6H(2)O) on a high-surface-area gamma-Al2O3. This sorbent showed high selectivities for acetylene (C2H2) over other hydrocarbons, by forming a weak pi-complexation bond between Ni2+ and C2H2. For example, at 60 degrees C, the equilibrium amounts of hydrocarbons adsorbed at 1 atm were (in mmol/g) C2H2 = 1.56, C2H6 = 0.043, C2H4 = 0.061, C3H6 = 0.42, and C3H8 = 0.30. The bond energy for the Ni2+-C2H2 complex was approximately 9.3 kcal/mol, and the adsorption was reversible. The stoichiometry of the complex was Ni2+(C2H2)(n) where n = 1-3 at 25 degrees C and subatmospheric pressure. The pi-complexation bond was formed by the donation of the C2H2 pi-electrons to the vacant hybridized dsp(2) orbitals of Ni2+ and the back-donation of electrons from the filled d (or dp-hybrid) orbitals of Ni2+ into the antibonding orbitals of C2H2. This work demonstrates that new sorbents can be designed and prepared by exploiting the weak, reversible pi-complexation bonds, and, consequently, conventional separation/purification processes can be replaced by more efficient adsorption processes.