The activation of small molecules, such as hydrogen gas, carbon dioxide, ammonia, and white phosphorus, is essential in progressing towards a sustainable chemical industry, offering routes to synthesize complex compounds for medicine and consumer products, and serving as renewable feedstocks with high potential to support a circular economy. Currently, small molecule activation relies mainly on transition metal catalysis, enabling key processes such as hydrogenation, hydroformylation, and nitrogen fixation. 1 However, main-group elements offer a promising alternative due to their abundance and sustainability; 2 for instance, silicon is among the most abundant elements on Earth. Originally, main-group elements were considered unsuitable for small molecule activation because of their limited electronic configurations, few oxidation states, and large HOMO-LUMO energy gaps. In contrast, transition metals, often with partially filled d orbitals, smaller HOMO-LUMO energy gaps, and paramagnetic properties, exhibit a rich range of bonding capabilities, rendering them the preferred catalysts for small molecule activation. Nevertheless, despite the limitations of main-group elements, different types of compounds containing main-group elements have fascinated chemists for decades. To name a few, i) Lewis acids, 5 ii) frustrated Lewis pairs, 6 special combinations of a Lewis acid and a Lewis base sterically frustrated from the formation of classic adducts, iii) diradicaloids, 7 iv) s-block chemistry, 8 v) Nheterocyclic carbenes (NHCs), singlet carbenes with the carbon atom directly attached to at least one nitrogen within a heterocyclic scaffold, vi) dimetallyne and dimetallenes (also known as dimetallylenes or tetrylenes), respectively, the heavier Group 14 tetrel analogs of alkenes and alkynes, vii) metallylenes or tetrylenes, the heavier Group 14 analogs of carbenes (R2E:), viii) metallylones or tetrylones, 11 the heavier Group 14 analogs of carbon(0) compounds called "carbones" (Scheme 1). 12 This review will focus on small molecule activation by metallylenes/tetrylenes: silylenes, germylenes, stannylenes, and plumbylenes and discuss their follow up reactions. The nomenclature for these species differs, with literature using metallylenes and others using tetrylenes. From here onward, we will exclusively use the term metallylenes for consistency with our previous work. Before discussing the reactivity of metallylenes, we will briefly dive into the background of carbene and metallylene chemistry. Please do not adjust margins Please do not adjust margins From the beginning of the 1950s, interest in the reactivity of carbenes rose, 13 but the synthesis of stable carbenes still remained a challenge. Lappert's work in 1976 showed that the more stable heavier germanium(II), tin(II), and lead(II) analogs could be synthesized and isolated. After that, West et al. synthesized the first stable disilylene (a silicon(II) dimer) in 1981, and ten years later the synthesis and isolation of a stable N-heterocyclic carbene by soon accompanied by the synthesis of stable silylenes. These and more achievements have inspired a wealth of synthetic strategies for Group 14 compounds, reshaping the landscape of main-group chemistry.