The switching of single-site pathway in SACs to multi-site pathway in nanoclusters or particles is a fundamental reason for the modulated performance during structural dynamism. Beyond dynamic clustering, it is also possible for SACs to retain spatial isolation and correlate with the adjacent sites via inter-site metal-metal interaction. Since the number and type of the neighboring atoms in the first and second coordination shells significantly influence the local geometry and charge density of metal centers, such correlated SACs may enjoy unique structural flexibility for dynamic response under working conditions. As shown in sterically congested scaffolds, and pharmaceuticals with highly specific and stable activity. Our recent studies also revealed the dynamic process of ultrahigh loading Cu1-C3N4 SACs in the bimolecular nitrile-azide cycloaddition by operando XAS, triggering the dinuclear pathway when intermetal distance is reduced to the typical diffusion length of the reactants (< 1 nm). A distance threshold around 5.3 Å between adjacent Ni-N4 and Cu-N4 moieties is revealed to trigger effective inter-metal interaction in correlated NiCu SACs for promoted CO2RR activity and selectivity. This is consistent with the findings in densely populated Fe SACs for ORR. Besides affecting the activity and selectivity, the distance between metal species also determines the collective properties that affect the stability by influencing the propensity towards sintering or redispersion, which will not be further detailed. In pursuit of the molecular level understanding of reaction mechanisms and better designs of SACs, substantial efforts have been focused on determining catalytically active sites in the dynamic catalytic cycle when interacting with substrate molecules. Ex situ characterization of the fresh and spent catalysts (e.g., microscopy, spectroscopy) is generally performed to evaluate the structural and electronic changes. Due to dynamic evolution, such structural features given by these ex situ techniques may not be sufficiently reliable to describe the active sites and structure-property relationship. This necessitates monitoring structural dynamism via operando techniques in the following section.