Quantum chemical calculations using density functional theory at the B3LYP level in combination with relativistic effective core potentials for the metals and TZ2P valence basis sets have been carried out for elucidating the reaction pathways of ethylene addition to MeReO₂(CH₂) (C1). The results are compared with our previous studies of ethylene addition to OsO₂(CH₂)₂ (A1) and OsO₃(CH₂) (B1). Significant differences have been found between the ethylene additions to the osmium compounds A1 and B1 and the rhenium compound C1. Seven pathways for the reaction C1+C₂H₄ were studied, but only the [2+2]_Re,C addition yielding rhenacyclobutane C5 is an exothermic process with a high activation barrier of 48.9 kcal mol⁻¹. The lowest activation energy (27.7 kcal mol⁻¹) is calculated for the [2+2]_Re,C addition, which leads to the isomeric form C5′. Two further concerted reactions [3+2]_O,C, [3+2]_O,O, and [2+2]_Re,O and the addition/hydrogen migration of ethylene to one oxo ligand are endothermic processes which have rather high activation barriers (>35 kcal mol⁻¹). Four isomerization processes of C1 have very large activation energies of >65 kcal mol⁻¹. The ethylene addition to the osmium compounds A1 and B1 are much more exothermic and have lower activation barriers than the C₂H₄ addition to C1. Copyright © 2007 John Wiley & Sons, Ltd.