One of the main goals of organometallic chemistry in the last decades was the activation of small molecule in mild reaction conditions. Even though multiple examples of catalytic cycles able to produce fine chemicals from cheap and abundant sources using transition metals have been reported, the chemistry of the f-elements is much under-developed in comparison. Despite the plethora of oxidation states accessible for uranium, its redox chemistry usually involves one-electron processes. The main objective of the work presented in this thesis is the synthesis and characterization of molecular complexes containing f-block elements in low oxidation states able to activate small molecules of industrial relevance such as N2 and CO2. Redox-active ligands, able to store and transfer electrons to the substrates, can cooperate in the electron-transfer with the metal centre and facilitate the reduction and activation of different small molecules. In chapter 2, the use of the tripodal trensal ligand for the synthesis of different uranium (IV) complexes will be presented. Moreover, their reactivity towards CO2 will be studied. Careful modification of the ligand backbone allowed not only insertion of CO2, but also its selective reductive disproportionation into CO32- and CO. Chapter 3 will present the synthesis of the analogue complexes with Sm, Eu, Nd. It will be shown that the identity of the metal center is key for determining where the electrons are stored upon reduction of the complexes and the outcome of their reactivity with CO2. It was previously reported in our group that the use of a dinuclear U(III) complex with a bridging oxo group and potassium counterions allowed the four-electron reduction of dinitrogen. This system, however, failed to generate NH3 upon addition of acids or H2. In chapter 4, the first stepwise reduction of N2 and the identification of all the intermediates in the N-N bond cleavage by molecular complexes of uranium will be presented. Furthermore, the reactivity of the nitrides with acids and CO for the generation of new N-C bonds will be shown. In chapter 5, the synthesis, characterization and reactivity towards N2 of U(III) bridging-oxo complexes bearing tris(tertbutoxy)siloxide ligands using different alkali metal cations will be described. Striking differences in the binding of N2 by these complexes will be reported, and the different factors affecting their reactivity towards N2 will be analysed. Chapter 6 will describe the synthesis of complexes without alkali metal cations and their reactivity towards N2. Furthermore, it will introduce new routes for the synthesis of new dinuclear U(III) bridging oxo complexes and preliminary catalytic studies on the reduction of N2. It was shown previously that variations in the bridging ligand gives rise to striking differences in the activation of N2 by a series of dinuclear U(III) complexes. Chapter 7 will report the synthesis, characterization and preliminary reduction experiments of a bridging-sulfide U(IV) complex with tris(tert-butoxy)siloxide ligands. Uranium nitrides have attracted considerable attention as models for the intermediates of the Haber-Bosch processes and due to their unusual reactivity when compared to transition metals. Chapter 8 will report the synthesis of a heterobimetallic U/Mo bridging nitride complex via a novel partial N-atom transfer route. Moreover, its reactivity towards small molecules will be presented.