Nanoparticle-based devices, materials and technologieswill demand a new era of synthetic chemistry where predictive principles familiarin the molecular regime are extended to nanoscale building blocks. Typicalcovalent strategies for modifying nanoparticle-bound species rely onkinetically controlled reactions optimised for efficiency but with limited capacityfor selective and divergent access to a range of product constitutions. In thiswork, monolayer-stabilized nanoparticles displaying complementary dynamiccovalent hydrazone exchange reactivity undergo distinct chemospecifictransformations by selecting appropriate combinations of ‘nucleophilic’ or‘electrophilic’ nanoparticle-bound monolayers with nucleophilic orelectrophilic molecular modifiers. Thermodynamically governed reactions allow modulationof product compositions, spanning mixed-ligand monolayers to exhaustive exchange.High-density nanoparticle-stabilizing monolayers facilitate in situ reactionmonitoring by quantitative ¹⁹F NMR spectroscopy. Kinetic analysisreveals that hydrazone exchange rates are moderately diminished by surface confinement,and that the magnitude of this effect is dependent on mechanistic details:surface-bound electrophiles react intrinsically faster, but are moresignificantly affected by surface immobilization than nucleophiles.Complementary nanoparticles react with each other to form robust covalentlyconnected binary aggregates. Endowed with the adaptive characteristics of thedynamic covalent linking process, the nanoscale assemblies can be tuned fromextended aggregates to colloidally stable clusters of equilibrium sizes thatdepend on the concentration of a monofunctional capping agent. Just two‘dynamic covalent nanoparticles’ with complementary thermodynamically governedreactivities therefore institute a programmable toolkit offering flexiblecontrol over nanoparticle surface functionalization, and construction ofadaptive assemblies that selectively combine several nanoscale building blocks.