Romero and Piers recently reported a mechanistic study in which they detailed the intramolecular exchange of adjacent methylene groups of a ruthenacyclobutane and the intermolecular degenerate exchange of free ethylene with a second-generation Grubbs-type catalyst, a 14-electron ruthenacyclobutane, (NHC)Cl2Ru(CH2CH2CH2). These authors measured activation parameters for the intramolecular and intermolecular exchange processes (Delta G = 12.3 kcal mol(-1) and Delta H = 13.2(5) kcal mol(-1) and Delta S = -15(2) eu, respectively). The current study applies density functional theory calculations to address their proposed exchange mechanisms. For intramolecular exchange, the formation of a structure with coordinated ethylene proceeds from the cyclobutane species via a direct rotational bond-breaking transition state, which produces an ethylene which is essentially perpendicular to the remaining methylene unit; and the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (Delta G (CH2Cl2) double dagger,-50 degrees C = 14.4 kcal mol(-1)) is close to the observed free energy of activation. For intermolecular exchange, ethylene binds to the metal of the ruthenacyclobutane complex, a rotational bond-breaking transition state produces a bis-ethylene, methylene intermediate, from which the ethylene, formally of the metallocyclobutane, dissociates. For intermolecular exchange, the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (Delta G (CH2Cl2)double dagger,-50 degrees C = 18.6 kcal mol(-1)) is close to the observed free energy of activation (Delta G (CD2Cl2)double dagger,-50 degrees C = 16.9 kcal mol(-1)). A dissociative mechanism could also be operative at elevated temperatures.