Unveiling the nucleation process for controlling aggregation of metal cations into structure-specific crystal nuclei represents a key step for isolating ultrafine nanoclusters (NCs) with advanced functionality. [1-6] In liquid phase, metal cation nucleation into ultrafine crystal nuclei represents an essential step in the crystallization of nanomaterials. These ultrafine clusters have been regarded as key intermediates during self-catalyzed crystal growth, [7-9] which have played a decisive role in the precise control of the size and structure of the final products. As a result, ingenious control of nucleation conditions can create various nanomaterials with prominent size and geometry-dependent functionality. [10-16] However, it is still a great challenge to isolate ultrafine NCs in liquid phase because the crystal nuclei are extremely prone to aggregation without the stabilizers in the initial nucleation process. Ru metal tends to adopt hexagonal close packed (hcp) structures with low potential energy in the bulk phase at all temperature ranges. [17-19] Recently, more attention has been paid to explore Ru nanoparticles (NPs) in face-centered cubic (fcc) phase with more exposed active sites than their hcp analogue. [20] They usually exhibit much higher catalytic activity than that of hcp Ru NPs in a number of reactions, such as methanol oxidation, hydrogenation, Fischer-Tropsch synthesis, and dehydrogenation, especially for ammonia borane (AB) methanolysis. [21-30] Recently, controlled synthesis of ultrafine fcc Ru NCs has been of significant interest to further greatly improve their catalytic efficiency. [28,29] Up to date, fcc Ru NPs with sizes > 2.0 nm have been confirmed by theoretical and experimental investigations. For example, fcc Pt NPs were applied as core for epitaxial growth to form 6.8 ± 1.5 nm fcc Ru NPs with polyvinylpyrrolidone as a weak stabilizer. [27] In another case, pure fcc Ru NPs with a diameter of 2â5.5 nm were synthesized via a chemical reduction method with Îł-Al 2 O 3 as the support. [28] Most recently, Zhou and co-workers designed a negatively charged porous coordination cages as a template for the synthesis of ultrafine fcc Ru NCs with an average size of 2.5 nm. This particular work exhibits a record high turnover frequency (TOF) of 304.4 min â1 in AB methanolysis. [29] These small fcc Ru NPs are all confirmed as high-performance catalysts due to the exposure of high energy crystal plane. They were all obtained Ultrafine face-centered cubic (fcc) ruthenium nanoclusters (NCs) are of great interest due to their super high catalytic activity. However, it is extremely difficult to prepare â1 nm fcc ruthenium NCs with high energy atoms due to their easy aggregation. Herein, the nucleation process of ruthenium centers by confined pyrolysis of a multivariate metal-organic framework to isolate ultrafine fcc NCs (from single atom to 1.33 nm) via in situ formed stabilizers is unveiled. Systematic investigations demonstrate that preferential nucleation of Ru single atoms to fcc clusters in the i...