Ultrafine and highly dispersed metal nanoparticles (NPs) exhibit unique optical, electronic and surface properties, and hold promise for application in various fields, including nanotechnology, optoelectronics, biomedicine, sensing, and catalysis. In the field of heterogeneous catalysis, ultrafine (i.e., particles with an average size of ca. ≤3 nm) metal NPs often show enhanced catalytic properties that are distinct from their larger counterparts due to the effects of electron energy quantization and surface geometry, i.e., specific surface atom arrangement and coordination. For example, ultrafine gold NPs, owing to their large surface area, exposing various surface facets, and a high number of undercoordinated atoms at edge and corner sites, are known to exhibit extraordinary performance in various catalytic conversions, such as CO oxidation, hydrogenation, oxidation of alcohols, coupling reactions, and reduction reactions. However, ultrafine metal NPs are thermodynamically and kinetically unstable and tend to aggregate in order to reduce the surface free energy, often resulting in a drastic loss of catalytic performance over time, and thus limiting their practical application. Typical strategies to stabilize ultrafine NPs consist in immobilization on high surface area supports, embedding into molecular cage-like materials (e.g., metal-organic frameworks, MOFs), or surface protection by molecules or ions that can act as effective capping agents. However, all these strategies have also drawbacks. The supported catalysts often exhibit decreased activity as compared to colloidal NPs, and suffer from nanoparticle migration and aggregation, irregular distribution and regrowth, and weak affinity of metal NPs to supports, resulting in increased leaching. The applicability of MOF-embedded metal NP in catalysis is often limited due to MOF stability issues under catalytic conditions. Finally, the surface capping agents can not only inhibit aggregation but also block the active sites on the NP surface, leading to reduced catalytic activities. A typical example would be the strongly inhibiting effect of low-weight thiolated polyethylene glycol (HS-PEG) as stabilizing ligand on the catalytic activity of Au NPs towards the reduction of 4-nitrophenol. Considering the aforementioned aspects, the development of new stabilizers that can effectively stabilize metal NPs without blocking the active sites and the diffusion of the reactants and products to and from the metal surface is of paramount importance in catalysis.