© 2015 American Chemical Society. Metallacyclobutanes have been observed as intermediates in the alkene metathesis reaction, with the 5-coordinated metal center, in a trigonal bipyramidal or a square-planar coordination. Previous calculations have shown that only the trigonal bipyramidal form is directly on the reaction pathway of alkene metathesis. The square-pyramidal form, which is obtained by a trigonal-bipyramidal (TBP)-square-based pyramidal (SP) interconversion at the metal, is an intermediate that can be responsible for catalyst deactivation. This computational study aimed at establishing the factors that control the properties of the two metallacyclobutanes (structural and 13C NMR features, stability, and TBP-SP interconversion) that can influence the efficiency of the metathesis reaction. Metallacyclobutanes resulting from the addition of ethene to a large set of methylidene complexes where the metal fragment is M(E)(X)(Y) (M = Mo or W; E = alkyl imido, aryl imido, or oxo), (X and Y) = alkyl, pyrrolyl, alkoxy and fluoroalkoxy, and large monoaryloxy) have been studied by density functional calculations (B3PW91, and M06). From the study of these numerous complexes that include in particular all characterized complexes, properties of the metallacyclobutanes could be derived: Metallacyclobutanes with W are more stable than those with Mo relative to reactants; electron withdrawing ligands stabilize the two isomeric forms of the metallacyclobutane but more the TBP than the SP form, and conversely, electron donating ligands destabilize both forms but more the TBP isomer. The energy barrier for the TBP-SP interconversion is found to be lower than that for the productive ethene metathesis pathway for W complexes but is generally higher for Mo species. These facts rationalize the experimental evidence and accounts in particular for the high efficiency of the Mo catalysts.