An extended Huckel molecular orbital (EHMO) study is undertaken for the adsorption of C-2 and C-3 hydrocarbon molecules on highly olefin-selective adsorbents, Ag+- and Cu+-exchanged sulfonic acid resins and copper(I) halides, by using a resin model and different crystal faces of halides. Consideration of the HOMO and LUMO energy levels of the adsorbate and adsorbent can give a qualitative indication of both the direction of electron transfer and the relative bond strength. Quantitative EHMO results show that for rr complexation of olefins on resins, the strength of the bond follows the order Ag+ > Cu+ > H+. The bond strengths of olefins on Cu+ exposed on different crystal faces of the halides follow the order (111) > (001) > ($($) over bar$$ 111), favoring Cu+ with a lower coordination number. A strong anion effect on pi-complexation is predicted by EHMO, For C2H4 on the (111) face of the copper(I) halides, the bond strength order is CuCl > CuBr > CuI, > CuSO3C6H5. The order is reversed for the (001) and ($($) over bar$$ 111) faces of the halides. From an analysis of the atomic orbital occupations and the net charges, a quantitative understanding of the K-complexation bond is obtained. The sigma donation to the bond is substantially higher than the d-pi* back-donation. For the example of C2H4 on AgSO3C6H5, the contribution from sigma donation accounts for 84% of the bond, whereas that by the d-pi* back-donation accounts for only 16%. The EHMO results are in general agreement with the available experimental data.