Although reversible covalent activation of molecular hydrogen (H-2) by transition-metal complexes is a common reaction, H-2-mediated sophisticated reversible arrangements of organometallic frameworks have not yet been described. Herein, we report unusual organometallic transformations in solution that can be accomplished by uptake or release of H-2. An efficient route for synthesizing air- and moisture-stable 16-electron M2L2-type metallacycles under very mild conditions has been developed. The new organometallic metallacycles favor the binding of small ligands such as MeCN, Cl-, CO, and pyridine. The reaction of a coordinatively unsaturated 16-electron M2L2-type macrocyclic complex featuring thione ligands with 1 atm of H-2 leads to the isolation of a 18-electron M2L3-type cylinder, along with hydride species. Remarkably, the obtained mixture underwent loss of H-2 in a facile manner upon heating to re-form the starting M2L2-type complex. A possible mechanism is proposed for the reversible transformations.