Metathesis reactions are among the most important chemical processes in modern organic synthesis. They are used to efficiently break and rearrange carbon-carbon bonds obeying excellent atom economy, both in industrial-scale applications, as well as in chemical and pharmaceutical research. From a mechanistic perspective, olefin metathesis (OM) is based on a [2+2]cycloaddition/-reversion cycle that is catalyzed by proficient and well-defined metal-alkylidene complexes involving a metallacycle intermediate. Cyclopropanation can be an undesired side reaction in this process (Figure ). For their pioneering work in this very field, Chauvin, Schrock and Grubbs were awarded the Nobel Prize in chemistry in 2005. Consequently, the most widely used types of catalysts for metathesis (of any type and flavor) are high oxidation state Molybdenum complexes developed by Schrock and the Ruthenium based systems later developed by Grubbs. Important modifications to the existent systems and contributions to this field have been reported by Hoveyda and Copéret, as well as Buchmeiser, Blechert 10 and Nolan. Even though the current catalysts provide high yields under mild conditions, the development of sustainable, non-toxic and cheap base-metal catalysts is highly desirable. One area of research, that is currently of significant interest, is the substitution of ruthenium with iron, the corresponding 3d transition metal of the first series of the periodic table. Numerous research groups have been working on the design of operational iron systems for OM, both experimentally and theoretically. Within this context, the research groups of Bukhryakov and Milstein have recently reported on new low-valent iron complexes to be active in ring-opening metathesis polymerization (ROMP). Recent advances regarding the chemistry of iron carbene complexes and iron-based cycloaddition can further be attributed to Illuc and Meyer. While iron is well accepted as a surrogate for ruthenium in OM, it is somehow surprising not to find any investigations or at least considerations on the use of manganese for the same purpose.