In the previous sections, we have sometimes touched on common challenges faced in metallylene chemistry related to small molecule activation and subsequent reactions. Nevertheless, this section summarizes the current challenges faced in the field. To establish a catalytic cycle for metallylenes, several main challenges must be addressed: (i) synthesizing stable low oxidation state metallylenes with an open coordination site, (ii) enabling activation of small molecules, (iii) facilitating the elimination of a product, and (iv) regenerating the active catalyst. We did not extensively discuss the synthesis of stable metallylenes before, but it is crucial to stabilize the low oxidation state of the tetrel atom, which often requires ligands with bulky groups. Side reactions of the metallylene often may occur. One major side reaction is the dimerization of metallylenes to dimetallylenes. Dimerization can occur due to the formation of an E=E bond through HOMO-LUMO interactions between the lone pair orbital of one metallylene with the vacant p-type orbital of the other metallylene, Scheme 8a. Although dimetallylenes can activate small molecules, 10 their reactivity is different, since mostly both tetrel atoms interact with the small molecule. The behavior of metallylenes is thus significantly influenced by dimerization. As shown in Scheme 8, in contrast to the planar structure of an alkene (the dimerization product Please do not adjust margins Please do not adjust margins of two triplet carbenes), the dimerization products of its heavy analogues are not planar. Accordingly, E=E bonds in metallylenes differ significantly from a C=C bond and can be broken in solution. Various digermylenes and distannylenes, despite bearing bulky ligands, are dimers in the crystalline state. 104 However, upon dissolution in even apolar solvents such as benzene or cyclohexane, the monomeric metallylenes were observed, indicated by a color change. Additionally, a silylene with alkyl substituents was shown to exist in equilibrium with its dimer in solution. 105 Scheme 8. The interactions in a dimetallylene formed by dimerization of metallylene.