The chemistry of the classical uranium(III) complex [U(N″)3] (N″ = {N(SiMe3)2}-) is well-developed; however, there have been surprisingly few studies of its reactivity toward N-heterocycles. Here, we report the separate reactions of [U(N″)3] with pyrazine, 4-azido-pyridine, and the methylene transfer agent N-(9,10-dihydro-9,10-epiminoanthracen-11-yl)methanimine (C14H10N2CH2) to give the respective diuranium(IV) products [{U(N″)3}2(μ-NC4H4N-1κN,2κN')] (1), [{U(N″)3}2(μ-NC5H4N-1κN,2κN')] (2), and [{U(N″)3}2(μ-NC14H10N-1κN,2κN')] (3). The solid-state structures of complexes 2 and 3 respectively revealed bent U-N═C angles of 167.6(5) and 155.3(4)°, short U-N bond lengths of 2.070(5) and 2.104(4) Å, and diagnostic C═Nketimide distances of 1.297(7) and 1.364(7) Å. A combination of structural, spectroscopic, magnetic, and quantum-chemical calculations was required to confidently assign U(IV) ions exclusively in complexes 1-3 and to prove that 2 and 3 are ketimides rather than iminyls or nitrenes. Together, this study underscores the need for comprehensive analysis for the correct interpretation of the electronic structures of actinide complexes.

Diphosphonioalkylidene ligands have been central to developing f-element M═CR2 double bond chemistry, but their wide range of Ccarbene 13C NMR isotropic chemical shift (δiso) values has rendered their nature open to qualitative debate. Here, we report the use of 13C and 17O NMR spectroscopies to quantify the chemical shift anisotropies (CSAs) of two diphosphonioalkylidene complexes─[U(BIPMTMS)(O)(Cl)2] (1, J. Am. Chem. Soc. 2012, 134, 10047-10054; BIPMTMS = {C(PPh2NSiMe3)2}2-) and [Y(BIPMTMS)(I)(THF)2] (2; Organometallics 2009, 28, 6771-6776)─that exhibit disparate solution/solid-state Ccarbene 13C δiso values (386.2/401.9 and 61.0/54.9 ppm, respectively) and for 1 a highly deshielded solution oxo 17O NMR δiso (1333.6 ppm). CSA analysis reveals encoded chemical shift tensor (CST) signatures for the U═C and U≡O bonds of 1 due to strong σ ↔ π* and π ↔ σ* magnetic shielding (σ) couplings; the U═C bond exhibits 13C δ11 and span (Ω) values that are both ∼ 50 ppm larger than any alkylidene complex, and the U≡O 17O δiso is deshielded >200 ppm compared to uranyl 17O NMR data. CSA analysis confirms dominant 5f-orbital bonding and is consistent with an inverse-trans-influence in the C═U≡O linkage of 1. By contrast, the 13C NMR data for 2 exhibit signatures of Y═C double bonding tensioned with C═P ylidene contributions. For the M═CR2 bonds, we find that the 13C δiso (σiso), δ11 (σ11), Ω, and the paramagnetic contribution to the shielding (σp) CSTs correlate strongly to bond order for a range of transition metals, rare earths, and actinides. This work demonstrates that CSA analysis is a powerful method for probing actinide chemical bonding, and it brackets the spectroscopic and bonding variance of diamagnetic diphosphonioalkylidene complexes.

Abstract In recent times all-metal aromaticity has arisen, challenging classical views of aromaticity. All-metal aromaticity invokes metal-metal bonding, but this remains rare for the actinides. Recently, one-electron trithorium superatom clusters exhibiting exalted diamagnetism and hence open-shell Jellium aromaticity have been reported. Those clusters complement closed-shell two-electron congeners, but computational assessments advanced conflicting interpretations. Here, we report one- and two-electron trithorium clusters that all exhibit exalted diamagnetism, demonstrating open- and closed-shell Jellium aromaticities. We show that these Jellium aromats exhibit non-linear magnetic responses, so a foundational assumption of ring current calculations is not met, accounting for the experimental-computational disagreement. This work suggests that while π-aromaticity results from preorganized coherent wave functions, for σ-aromats there may be an electronic reorganization barrier to cohesive wave functions and Jellium aromatic ring currents. This work demonstrates the importance of caution when using ring current calculations to assign aromatic character if the experimental magnetic response is not understood.

Delocalized [4n + 2]π-aromaticity in cyclic planar unsaturated organic molecules conceptually underpins organic chemistry. Recently, the study of all-metal aromaticity has burgeoned, but although there has been interest in cyclo-{E3} (E = P, As, Sb, Bi) species as cyclopropenium analogues, the formation of cyclo-{Bi3} remains rare. Thus, the potential aromaticity of 2/6π-cyclo-Bi3+/3-, as the heaviest 6p analogue of cyclopropylium, has remained open to different interpretations. Here we report the formation of diuranium and dithorium 6π-cyclo-Bi33- inverse sandwich complexes, complementing the small number of acyclic- and cyclic-Bin (n = 3-5) compounds. The 6π-cyclo-Bi33- ring exhibits substantial ring currents, similar to 6π-benzene, 2π-(C3H3)+ or 6π-(C3H3)3-. Calculations reveal similar ring currents for 6π-cyclo-Bi33-, 2π-cyclo-Bi3+ and 0π-cyclo-Bi33+, demonstrating σ-aromaticity that is dominant over π-aromaticity in cyclo-Bi33-, despite the favourability of describing cyclo-Bi33- with localized rather than delocalized bond descriptions. Confirmation of 6π-cyclo-Bi33- σ-aromaticity provides the heaviest all-metal 6p analogue to π-aromatic (C3H3)+/3-, leading to organic-inorganic aromaticity benchmarking.

The concept of covalency in chemical bonding is well established, but there are few methods to experimentally measure covalency, and none is a panacea. Quantum crystallography X-ray charge density determination could be the most powerful method for experimentally visualizing chemical bonding, but its stringent requirements have prevented routine implementation. Hirshfeld atom refinement (HAR) has begun to emerge as an alternative, potentially more accessible quantum crystallography approach for experimentally visualizing covalency, but its use has remained limited with heavy elements such as actinides due to challenges in partitioning and modeling core and valence electron densities in relativistic regimes. Here, we apply HAR to two extreme test cases of clusters containing three proximate heavy actinides exhibiting multi-center thorium-thorium bonding. We demonstrate exceptional agreement between experimental HAR and purely density functional theory analyses, thus visualizing the thorium-thorium bonding and paving the way to more routine use of experimental HAR visualization of chemical bonding.

Metal-mono(imido) linkages have been known for seven decades, and they are found in transition metal, main group, lanthanide, thorium, and uranium complexes. However, transuranium-mono(imido) complexes remain unknown in any scenario. Here, we present evidence for transient neptunium(V)-mono(imido) complexes. Treatment of [NpIII(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) with N3R (R = SiMe3; 1-adamantyl, Ad) results in N2 evolution and dark purple solutions consistent with the formation of [NpV(TrenTIPS)(NR)] (3NpNR). However, solutions of 3NpNR rapidly turn orange, where for R = SiMe3 the isolated 1:1 products are [NpIV(TrenTIPS){N(H)SiMe3}] (4a) and [NpIV(TrenTIPS-2H){N(H)SiMe3}] (4b, TrenTIPS-2H = {N(CH2CH2NSiPri3)2(NCH2CH2NSiPri2C[Me]=CH2)}3-). The latter contains a dehydrogenated-Pri vinyl functionality accounting for the source of the two amido H atoms. The reaction for R = Ad proceeds similarly, but only [NpIV(TrenTIPS){N(H)Ad}] (5a) could be unequivocally confirmed, though its isolation suggests generality of the imido-to-amido functional group transformation. Complexes 4a/4b exhibit slow relaxation of their magnetization, adding to the small number of transuranium single ion magnets. Experimental and computational analysis suggests that the amido products are formed by C-H activation and two sequential hydrogen atom transfer reactions involving a three-step proton-coupled electron-transfer sequence of H• radical abstraction, electron transfer, then another H• radical abstraction step. In contrast to transient 3NpNR, the 5f2 uranium(IV)-imido complex [K(2.2.2-cryptand)][UIV(TrenTIPS)(NSiMe3)] (8UNSiMe3) is robust, even in boiling THF, suggesting the transience of 5f2 3NpNR is not due to the 5fn-count but the increased effective nuclear charge of neptunium vs uranium. This work highlights divergence of uranium- and neptunium-imido stabilities, emphasizing that the latter is an inherently challenging synthetic target.

Although uranium-nitrogen multiple bonding is well developed, there are far fewer uranium-phosphorus and -arsenic multiple bonds, and none for antimony, even in spectroscopic scenarios. Here, we report straightforward syntheses of uranium-stibido, -stibinidiide, -distibene, and -stibinidene derivatives containing single, double, and pseudo-triple bond interactions. Quantum chemical calculations suggest that these uranium-antimony multiple bonds are more covalent than thorium-antimony congeners, due to superior spatial and energy matching of uranium and antimony frontier orbitals, but comparison to isostructural uranium-phosphorus and -arsenic analogues suggests that for uranium moving from phosphorus to arsenic to antimony the spatial overlap term reduces but the orbital energy matching improves. Reduction of the distibene complex results in loss of the antimony-component and multimetallic activation and cleavage of dinitrogen to nitride. This constitutes an uncommon mode of reactivity for uranium that is co-facilitated by the distibene and potassium ions.

Although two examples of sigma-bonded trans-bent [RSbSbR]center dot- (R = bulky organo- or Ga-groups) that formally contain the Sb2 center dot 3- radical trianion moiety are known in p-block chemistry, d- or f-element Sb2 center dot 3- radical trianion complexes, with or without R-substituents, have remained elusive. Here, we report that reduction of a 77:23 mix of [{Th(TrenTIPS)}2(mu-eta 2:eta 2-Sb2)] (3a, TrenTIPS = {N(CH2CH2NSiPri 3)3}3-):[{Th(TrenTIPS)}2(mu-SbH)] (3b) with 1.5 equiv of KC8 in the presence of 1.1 equiv of 2.2.2-cryptand yields the emerald green Sb2 center dot 3- radical complex [K(2.2.2-cryptand)][{Th(TrenTIPS)}2(mu-eta 2:eta 2-Sb2)] (4), providing an f-block Sb2 center dot 3- radical trianion complex, and the heaviest actinide-N2 radical analogue. When the recrystallization conditions are modified, a small crop of red crystals determined to be [K(2.2.2-cryptand)]3[{Th(TrenTIPS)(mu-eta 3:eta 3-Sb3)}2(mu-K)] (5) were also isolated, highlighting the complexity of heavy group 15 homodiatomic reduction chemistry. SQUID magnetometry and EPR spectroscopy suggest that the Sb2 center dot 3- radical trianion in 4 is fairly well isolated, due to electrostatic binding to Th, with pseudoaxial g-values reflecting the distinctive Sb2 center dot 3- radical trianion side-on bridging pi-bonded coordination mode. Spectroscopically validated computational analysis of 3a and 4 confirms the stronger donating capability, and weaker Sb-Sb bond, of Sb2 center dot 3- radical trianion compared to the Sb2 2- dianion form.

Procedures for activating and degrading compounds containing carbon-halogen bonds are highly sought after due to the environmental persistence and potential hazards of such compounds. Such activations are challenging because of the high stability of these bonds, particularly those with C-F bonds. Here, we report on the activation of carbon-halogen bonds, including C-F bonds, by the cerium(iii)-triamidoamine complex CeIIITRENTIPS (1, TRENTIPS = tris-(2-(tri-iso-propylsilylamidoethyl)amine)). Under light irradiation, 1 reaches a strongly negative excited state redox potential, and our measurements enable it to be estimated as -3.2 V relative to Cp2Fe0/+. Hence, the photo-reactivity of 1 with carbon-halogen bonds has been established with numerous examples, including Persistent Organic Pollutants (POPs) and fluorinated compounds. The photoactivation of POPs is rapid, but the photoactive nature of the cerium(iv) products precludes complete conversion. This study provides insight into the activation of POPs that may benefit the future design of photodegradation approaches for these highly problematic compounds.

The parent diphosphene (HPPH) molecule is of fundamental interest, but its reactive nature renders it challenging to isolate and study. Metal-stabilization is an attractive approach for studying HPPH, but molecular derivatives are limited to three complexes of p-/d-metals reflecting a scarcity of synthetic methods for rationally preparing HPPH complexes. Here, we introduce f-element HPPH complexes, adding to f-element diazenes (HNNH) that were first reported over thirty years ago. By utilizing 7λ3-phosphadibenzonorbornadiene and uranium(III) reagents we show how parent diphosphene, phosphinidiide, and diphosphorus motifs can all be constructed, developing synthetic approaches for this area. Computed reaction profiles reveal common, initial reaction steps that subsequently diverge depending on the ancillary ligands, radical nature of intermediates, and the 7λ3-phosphadibenzonorbornadiene P-substituent. Calculations demonstrate a surprising prevalence of open-shell radical intermediates, and that the redox chemistry is P-, not U-, centred. This work thus provides insights to inform future synthetic endeavours in this area.

Quantum-confined nanoclusters can be described by the jellium model, which emphasizes closed-shell electron configurations, but an open-shell variation with jellium aromaticity has been proposed. Such clusters are termed superatoms because they behave like an atom, and they exhibit unusual properties. Superatoms feature metal-metal bonding; hence, since their discovery 40 years ago, superatoms have exclusively involved main group or transition metals, with actinides only considered computationally as dopants owing to actinide-actinide bonding being exceedingly rare. Here we report trithorium nanoclusters exhibiting three-centre-one-electron actinide-actinide bonding. Experimental and computational analysis demonstrates Robin-Day Class III 6d-orbital valence delocalization in these clusters. These S = 1/2 clusters are paramagnetic, but in external applied magnetic fields they exhibit exalted diamagnetism, evidencing actinide open-shell jellium aromaticity superatom character. Exalted diamagnetism is not normally associated with a single unpaired electron, but with a 1S1 magic number, the valence delocalization enables exalted diamagnetism, which is aromaticity, via superatom ring currents.

The dominant form of elemental nitrogen on Earth is dinitrogen, but elemental phosphorus is found predominantly as white phosphorus or other singly bonded allotropes. Thus, there is interest in studying diphosphorus derivatives, most notably trapping between metal ions affording diphosphorus in +2, +1 center dot, 0, 1-center dot, 2-, and 4- charge states. However, the diphosphorus radical trianion form had previously remained elusive due to the instability of main-group diatomics with large, odd negative charges. Here, we disclose a crystalline diuranium diphosphorus radical trianion complex with strong antiferromagnetic uranium-phosphorus magnetic exchange coupling parameters of up to-731 cm-1. This value is over five times greater than that of lanthanide analogues and is comparable to or exceeds d-block metal-metal and metal-ligand exchange couplings, despite being based on a 5f metal, which is typically regarded as possessing contracted valence orbitals compared with d-block ions. This highlights exchange-coupled f-element-p-block radical bridged character that can be engendered in molecular magnetism.

Reactions between [(Tren(TIPS))(UN)-N-VI] (1, Tren(TIPS) = {N(CH2CH2NSiPr3i)(3)}(3-)) and [M-II(eta(5)-C5R5)(2)] (M/R = Cr/H, Mn/H, Fe/H, Ni/H) were intractable, but M/R = Co/H or Co/Me afforded [(Tren(TIPS))(UN)-N-V-(eta(1):eta(4)-C5H5)Co-I(eta(5)-C5H5)] (2) and [(Tren(TIPS))U-IV-NH2] (3), respectively. For M/R = V/H [(Tren(TIPS))U-IV-NVIV(eta(5)-C5H5)(2)] (4), was isolated. Complexes 2-4 evidence one-/two-electron uranium reductions, nucleophilic nitrides, and partial N-atom transfer.

We introduce the boryloxide ligand {(HCNDipp)(2)BO}(-) (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)(2)}(3)] with 3 equiv of (NBOH)-H-Dipp produced the uranium(III) tris(boryloxide) complex [U(NBODipp)(3)] (1). In contrast, treatment of UCl4 with 3 equiv of (NBOK)-K-Dipp in THF at room temperature or reflux conditions produced only [U(NBODipp)(2)(Cl)(2)(THF)(2)] (2) with 1 equiv of (NBOK)-K-Dipp remaining unreacted. However, refluxing the mixture of 2 and unreacted (NBOK)-K-Dipp in toluene instead of THF afforded the target complex [U(NBODipp)(3)(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN(3) (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBODipp)(3)(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene delta-back-bonding with approximately equal donation into the arene pi(4) and pi(5) delta-symmetry pi* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |m(l),m(s)> states and m(j) projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, H-1 NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

Reaction of the cesium antimonide complex [Cs(18C6)(2)][SbH2] (1, 18C6 = 18-crown-6 ether) with the triamidoamine actinide separated ion pairs [An(Tren(TIPS))(L)][BPh4] (Tren(TIPS) = {N((CH2CH2NSiPr3)-Pr-i)(3)}(3-); An/L = Th/DME (2Th); U/THF (2U)) affords the triactinide undeca-antimontriide Zintl clusters [{An(Tren(TIPS))}(3)(mu(3)-Sb-11)] (An = Th (3Th), U (3U)) by dehydrocoupling. Clusters 3Th and 3U provide two new examples of the Sb-11(3-) Zintl trianion and are unprecedented examples of molecular Sb-11(3-) being coordinated to anything since all previous reports featured isolated Sb-11(3-) Zintl trianions in separated ion quadruple formulations with noncoordinating cations. Quantum chemical calculations describe dominant ionic An-Sb interactions in 3Th and 3U, though the data suggest that the latter exhibits slightly more covalent An-Sb linkages than the former. Complexes 3Th and 3U have been characterized by single crystal X-ray diffraction, NMR, IR, and UV/vis/NIR spectroscopies, elemental analysis, and quantum chemical calculations.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

We report the synthesis and characterisation of thorium(IV), uranium(III), and uranium(IV) complexes supported by a sterically demanding triamidoamine ligand with N-diphenyl-tert-butyl-silyl substituents. Treatment of ThCl4(THF)(3.5) or UCl4 with [Li-3(Tren(DPBS))] (Tren(DPBS)={N(CH2CH2NSiPh2But)(3)}(3-)) afforded [An(Tren(DPBS))Cl] (An=Th, 1Th; U, 1U). Complexes 1An react with benzyl potassium to afford the cyclometallates (Tren(cyclomet)(DPBS)) [An{N(CH2CH2NSiPh2But)(2)((CH2CH2NSiPhBuC6H4)-C-t)}] (An=Th, 2Th; U, 2U). Treatment of 1An with sodium azide affords [An(Tren(DPBS))N-3] (An=Th, 3Th; U, 3U). Reaction of 3Th with potassium graphite affords 2Th. In contrast, 3Th reacts with cesium graphite to afford the doubly-cyclometallated (Tren(d-cyclomet)(DPBS)) ate complex [Th{N(CH2CH2NSiPh2But)((CH2CH2NSiPhBuC6H4)-C-t)}(2)Cs(THF)(3)] (4). In contrast to 3Th, reaction of 3U with potassium graphite produces the uranium(III) complex [U(Tren(DPBS))] (5), and 5 can also be prepared by reaction of potassium graphite with 1U. The loss of azide instead of conversion to nitrides contrasts to prior work when the silyl group is iso-propyl silyl, underscoring how ligand substituents profoundly drive the reaction chemistry. Several complexes exhibit T-shaped meta-C-H & sdot;& sdot;& sdot;phenyl and staggered parallel pi-pi-stacking interactions, demonstrating subtle weak interactions that drive ancillary ligand geometries. Compounds 1An-3An, 4, and 5 have been variously characterised by single crystal X-ray diffraction, multi-nuclear NMR spectroscopy, infrared spectroscopy, UV/Vis/NIR spectroscopy, and elemental analyses.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D3h-symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR and UV-Vis-NIR spectroscopies, SQUID magnetometry and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, whilst two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M–Si bonds, and dissociation of THF from 1-M in solution to give the locally C3v-symmetric complexes [M{Si(SiMe3)3}3(THF)n] (n = 0 or 1), which show restricted rotation of M–Si bonds on the NMR timescale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution, and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Procedures for activating and degrading compounds containing carbon-halogen bonds are highly sought after due to the environmental persistence and potential hazards of such compounds. Such activations are challenging because of the high stability of these bonds, particularly those with C-F bonds. Here, we report on the activation of carbon-halogen bonds, including C-F bonds, by the cerium(III)-triamidoamine complex CeIIITRENTIPS (1, TRENTIPS = tris-(2-(tri-iso-propylsilylamidoethyl)amine). Under light irradiation, 1 reaches a strongly negative excited state redox potential, and our measurements enable it to be estimated as 3.2 V relative to Cp2Fe0/+. Hence, the photo-reactivity of 1 with carbon-halogen bonds has been established with numerous examples, including Persistent Organic Pollutants (POPs) and fluorinated compounds. The photoactivation of POPs is rapid, but the photoactive nature of the cerium(IV) products precludes complete conversion. This study provides insight into the activation of POPs that may benefit the future design of photodegradation approaches for these highly problematic compounds.

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium-carbon bonds remain rare being limited to pi-bonded carbocycle and sigma-bonded isonitrile and hydrocarbyl derivatives. Thus, plutonium-carbenes, including alkylidenes and N-heterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu((BIPMH)-H-TMS)(I)(mu-I)](2) (1Pu, (BIPMH)-H-TMS = (Me3SiNPPh2)(2)CH) and the diphosphonioalkylidene-plutonium complexes [Pu(BIPMTMS)(I)(DME)] (2Pu, BIPMTMS = (Me3SiNPPh2)(2)C) and [Pu(BIPMTMS)(I)(I-Me4)(2)] (3Pu, I-Me4 = C(NMeCMe)(2)), thus disclosing non-actinyl transneptunium multiple bonds and transneptunium NHC complexes. These Pu-C double and dative bonds, along with cerium, praseodymium, samarium, uranium, and neptunium congeners, enable lanthanide-actinide and actinide-actinide comparisons between metals with similar ionic radii and isoelectronic 4f(5) vs 5f(5) electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbital-energy and spatial-overlap terms increase from uranium to neptunium; however, on moving to plutonium the orbital-energy matching improves but the spatial overlap decreases. The bonding picture that emerges is more complex than the traditional picture of the bonding of lanthanides being ionic and early actinides being more covalent but becoming more ionic left to right. Multiconfigurational calculations on 2M and 3M (M = Pu, Sm) account for the considerably more complex UV/vis/NIR spectra for 5f(5)2Pu and 3Pu compared to 4f(5)2Sm and 3Sm. Supporting the presence of Pu=C double bonds in 2Pu and 3Pu, 2Pu exhibits metallo-Wittig bond metathesis involving the highest atomic number element to date, reacting with benzaldehyde to produce the alkene PhC(H)=C(PPh2NSiMe3)(2) (4) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.