Olefin metathesis on a silica supported tantalumhydridocarbene complex, the key carbon-carbon making process in alkane metathesis, requires a large number of elementary steps in contrast to the known olefin metathesis pathway, which corresponds to successive [2 + 2]-cycloaddition and cycloreversion steps. The direct pathway is forbidden because it requires the formation of a high energy reaction intermediates, an olefin adduct of trigonal bipyramid (TBP) geometry, where the carbene is trans to an hydride ligand. Extra low-energy steps are therefore necessary to connect the reactants to products, the key being a turnstile interconversion at the metal lacyclobutane intermediates.