The reaction of UCl4 or ThCl4(DME)2 with 1,4,7,10-tetrazacyclodecane-1,4,7,10- tetraacetic acid (H4DOTA), and 6 equiv of trimethylamine, in DMSO results in the formation of [AnIV(k8-DOTA)(DMSO)] (An = U, Th), which can be isolated in moderate yields after work-up. Both Complexes are the first structurally characterized actinide DOTA complexes to feature the k8 binding mode for the DOTA ligand. In addition, we isolated a few crystals of [U(k4-H2DOTA)(DMSO)4][Cl]2. Crystallographic characterization of this material reveals that the [H2DOTA]2- ligand in [U(k4-H2DOTA)(DMSO)4][Cl]2 is only coordinated to U4+ by its four carboxylate arms, generating an overall k4 binding mode. Similar complexes have been previously proposed as intermediates of H4DOTA complexation pathway, but this intermediate had not been structurally characterized until now.Reaction of [Li(THF)]4[L] (L = Me8-calix[4]pyrrole]) with 0.5 equiv of [UVIO2Cl2(THF)2]2 results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]2[L∆] , concomitant with formation of reduced uranium oxide by-products. [Li(THF)]2[L∆] can also be generated by reaction of [Li(THF)]4[L] with 1 equiv of I2. I hypothesize that formation of [Li(THF)]2[L∆] proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]2[cis- UVIO2(calix[4]pyrrole)]. To test this hypothesis, I explored the reaction of [Li(THF)]2[L∆] with either 0.5 equiv of [UVIO2Cl2(THF)2]2 or 1 equiv of [UVIO2(OTf)2(THF)3], which affords the isostructural uranyl complexes, [Li(THF)][UVIO2(L∆)Cl(THF)] and [Li(THF)][UVIO2(L∆)(OTf)(THF)], respectively. In the solid state, [Li(THF)][UVIO2(L∆)Cl(THF)] and [Li(THF)][UVIO2(L∆)(OTf)(THF)] feature unprecedented uranyl-η5-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, [Li(THF)][UVIO2(L∆)Cl(THF)] and [Li(THF)][UVIO2(L∆)(OTf)(THF)]exhibit some of the smallest O−U−O angles reported to date (162.0(7) and 162.7(7)°; 164.5(5)°, respectively). Importantly, the O−U−O bending observed in these complexes suggests that the oxidation of [Li(THF)]4[L] does indeed occur via an unobserved cis-uranyl intermediate. The reaction of [AnCl(NR2)3] (An = U or Th; R = SiMe3) with NaCCH and tetramethylethylenediamine (TMEDA) results in the formation of [An(C≡CH)(NR2)3] (An = U, Th), which can be isolated in good yields after work-up. Similarly, reaction of 3 equiv of NaCCH and TMEDA with [AnCl(NR2)3] results in the formation of [Na(TMEDA)][An(C≡CH)2(NR2)3] (An = U, Th), which can be isolated in fair yields after work-up. Reaction of [U(C≡CH)(NR2)3] with 2 equiv of KC8 and 1 equiv of 2.2.2-cryptand in THF results in formation of the U(III) acetylide complex, [K(2.2.2- cryptand)][U(C≡CH)(NR2)3]. Thermolysis of [U(C≡CH)(NR2)3] or [Th(C≡CH)(NR2)3] results in formation of the bimetallic dicarbide complexes, [{An(NR2)3}2(μ,η1:η1-C2)] (An = U, Th), whereas reaction of [U(C≡CH)(NR2)3] with [Th{N(R)(SiMe2)CH2}(NR2)2] results in formation of [U(NR2)3(μ,η1:η1-C2)Th(NR2)3]. The 13C NMR chemical shifts of the a-acetylide carbons in [Th(C≡CH)(NR2)3], [Na(TMEDA)][Th(C≡CH)2(NR2)3], and [{Th(NR2)3}2(μ,η1:η1-C2)] exhibit a characteristic spin-orbit induced downfield shift, due to participation of the 5f orbitals in the Th-C bonds. Magnetism measurements demonstrate that [{U(NR2)3}2(μ,η1:η1-C2)] displays weak ferromagnetic coupling between the U(IV) centers (J = 1.78 cm–1). The reaction of [AnCl(NR2)3] (An = U, Th, R = SiMe3) with in situ generated lithium-3,3- diphenylcyclopropene results in the formation of [{(NR2)3}An(CH=C=CPh2)] (An = U, Th) in good yields after work-up. Deprotonation of [{(NR2)3}U(CH=C=CPh2)] or [{(NR2)3}Th(CH=C=CPh2)] with LDA/2.2.2-cryptand results in formation of the anionic allenylidenes, [Li(2.2.2-cryptand)][{(NR2)3}An(CCCPh2)] (An = U, Th). The calculated 13C NMR chemical shifts of the Cα, Cβ, and Cγ nuclei in [{(NR2)3}Th(CH=C=CPh2)] and [Li(2.2.2- cryptand)][{(NR2)3}Th(CCCPh2)] nicely reproduce the experimentally assigned order, and exhibit a characteristic spin-orbit induced downfield shift at Cα due to involvement of the 5f orbitals in Th–C bonds. Additionally, the bonding analyses for [Li(2.2.2- cryptand)][{(NR2)3}U(CCCPh2)] and [Li(2.2.2-cryptand)][{(NR2)3}An(CCCPh2)] show a delocalized multi-center character of the ligand p orbitals involving An. While a single-triple- single-bond resonance structure (e.g., An-CoC-CPh2) predominates, the An=C=C=CPh2 resonance form contributes, as well, more so for uranium analog. I also report the synthesis, characterization, and reactivity of the bis(diisopropylamino)cyclopropenylidene (BAC) adducts of [M(NR2)3] (M = Ce, U; R = SiMe3), namely, [(NR2)3M(BAC)] (M = Ce, U). Photolysis of [(NR2)3Ce(BAC)] with a 365 nm LED source results in formation of the methylenecyclopropene species, [(iPr2N)2C3C(NiPr2)(CCNiPr2)], via the formal dimerization and rearrangement of two BAC fragments. [(iPr2N)2C3C(NiPr2)(CCNiPr2)] can also be generated under catalytic conditions by performing the photolysis of BAC in the presence of 10 mol% [Ce(NR2)3]. Whereas heating [(NR2)3U(BAC)] results in the formation of [(NR2)2U{N(R)(SiMe2)(2,3-(NiPr2)- C(H)C=CC(H))}], via the formal ring opening and insertion of the BAC ligand. The reaction of [Cp3ThCl] with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [Cp3Th(3,3-diphenylcyclopropenyl)], in good yields. Thermolysis of [Cp3Th(3,3-diphenylcyclopropenyl)] results in isomerization to the ring-opened product, [Cp3Th(3-phenyl-1H-inden-1-yl)] via a hypothesized carbene intermediate. By comparison, reaction of [Cp3UCl] with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [Cp2U(h2-triphenylethylene)] via a hypothesized U(VI)-carbyne intermediate. Furthermore, reaction of [Cp3U(THF)] with 2 equiv 3,3-diphenylcyclopropene results in the formation of [Cp3U(3,3-diphenylcyclopropyl)], via formal hydrogen atom abstraction. These transformations represent several new modes of reactivity of 3,3-diphenylcyclopropene with the actinides, improving our ability to use this reagent as a carbene source. A combined DFT and 13C{1H} NMR analysis of [Cp3Th(3,3-diphenylcyclopropenyl)] shows a spin–orbit induced downfield shift at Cα due to participation of the 5f orbitals in the Th–C bond.