A molecular mechanics model for the eta(2) interaction between olefins and transition metals, utilizing the MMP2 force field, is defined. The model uses a dummy atom displaced from the C=C bond axis with corrections for the changes in connectivity occasioned by the additional atom. Steric sizes for a range of olefins are calculated from this model using the ligand repulsive energy (E(R)) methodology. E(R) values for 45 olefins in the Cr(CO)(5) environment and 38 olefins in the CpRh(CO) environment are calculated and compared with the steric sizes of phosphines, phosphites, amines, and sulfides. There is a good correlation between the calculated E(R) values and the second-order rate constant for the coordination of olefins to HCo[PPh(OEt)(2)](3).