Metal–carbon triple bonds come in two broad flavours. Schrock alkylidynes, named after Richard Schrock, arise when a formally quartet metal fragment combines covalently with a carbon fragment to form a strong, high-oxidation-state triple bond. Fischer carbynes, first reported by Ernst Otto Fischer’s group in 1973, are fundamentally different: a singlet carbon fragment donates its electron pair to the metal, while the metal reciprocates with two orthogonal back-bonds into empty orbitals on carbon. This donor–acceptor picture makes Fischer carbynes more polar, more stabilized by heteroatom substituents, and generally less reactive than their Schrock counterparts. Classifying where a given compound sits between these extremes is notoriously subtle, because competing resonance forms and electromers, differing in formal electron counts at the carbyne, blur the boundaries into a continuum rather than a clean dichotomy.

Uranium, the most transition-metal-like of the actinides, has a rich and rapidly growing repertoire of multiple bonds to elements such as nitrogen, oxygen and sulfur, and even uranium–carbon double bonds have been developed in allenylidene and various alkylidene complexes. Triple bonds to carbon, however, remained out of reach for routine synthesis. The only known uranium–carbon triple bonds were X₃U≡CH species (X = F, Cl, Br) trapped at cryogenic temperatures in inert gas matrices, and heterometallic carbide clusters encapsulated within endohedral fullerenes. Neither approach translates to ordinary solution chemistry, leaving a glaring gap between the well-developed transition metal carbyne literature and the actinide world.

The difficulty stems from two intertwined problems. First, practical reagents for transferring a bare carbon atom are scarce in organometallic chemistry. Second, constructing a donor–acceptor linkage between uranium and carbon requires energy matching between the hard uranium frontier orbitals and the soft carbon orbitals, a pairing that is intrinsically suboptimal. The team’s solution was to exploit Ph₃PCN₂, a diazo-phosphorus ylide that has recently emerged as a stable carbon-atom transfer reagent. The researchers reasoned that a reducing uranium centre could extrude the excellent leaving group triphenylphosphine and directly transfer the CNN fragment, sidestepping the multistep syntheses and stubborn carbon–silicon bond cleavages that complicate alternative routes based on organosilyl-diazomethanes.

The synthesis proved remarkably simple. Treating the uranium(III) triamide complex [Uᴺ″₃], where N″ denotes the bulky bis(trimethylsilyl)amide ligand, with two equivalents of Ph₃PCN₂ delivered the bimetallic product [N″₃U(μ-CNN)UN″₃] as a red powder in 61% yield, with triphenylphosphine released as the by-product. The bulky amide ligands, though sterically demanding, can rearrange to allow an unencumbered carbon donor to approach closely enough for a multiple bond to form. Recrystallization from slowly cooling toluene afforded single crystals suitable for X-ray diffraction, revealing a diuranium complex in which each four-coordinate uranium ion is linked end-on to the bridging CNN unit, the whole UCNNU core sitting over a crystallographic centre of inversion.

The structural metrics immediately signalled something unusual. The uranium–carbon distance of 2.379(15) Å is substantially shorter than known uranium–carbon single bonds, such as the 2.450(15) Å U–CH₃ distance in [H₃CUᴺ″₃], yet longer than the covalent uranium–carbon double bonds in related alkylidene complexes, which fall around 2.28 to 2.31 Å. The C–N bond is short, indicating delocalization, the N–N distance sits between single and double bond values, and both the U–C–N and U–N–N angles are close to linear. To go beyond conventional crystallography, the team performed quantum crystallography using Hirshfeld atom refinement, a technique that models the experimental electron density directly, allowing them to visualize the bonding rather than merely infer it from distances.

Spectroscopy and magnetometry pinned down the oxidation states. Infrared absorptions at 1,962 and 1,321 cm⁻¹, matched well by density functional theory frequency calculations, correspond to collective vibrations of the UCNNU unit and indicate depletion of the C–N bond order together with a uranium–carbon interaction of multiplicity greater than one. Ultraviolet–visible–near-infrared spectra showed the characteristic intraconfigurational f–f transitions of uranium(IV), unusually intense at around 400 M⁻¹ cm⁻¹, suggesting mixing of metal and ligand frontier orbitals. SQUID magnetometry gave an effective magnetic moment of 4.66 μ_B per complex at 300 K, declining to 0.78 μ_B at 1.8 K, a profile consistent with 5f² uranium(IV) ions bound to strong axial donors, while the featureless X-band EPR spectrum confirmed the presence of non-Kramers uranium(IV) centres.

The electronic structure analysis is where the Fischer-type character emerges most clearly. Natural localized molecular orbital calculations revealed a two-electron C→U σ-donation, weak two-electron C→U π-donations, and two orthogonal one-electron U→C π-back-bonds into the π* orbitals of the CNN ligand, an overall σ²π¹π¹ arrangement characteristic of Fischer-type carbyne bonding. Quantum theory of atoms in molecules analysis located bond critical points for both U–C and U–N interactions with intermediate covalency, and near-zero bond ellipticities consistent with triple-bond symmetry. The experimental deformation density and electron localizability index maps from the Hirshfeld refinement independently confirmed π-symmetric electron density accumulation along both the U–C and U–N axes, with the U–N linkage, clearly imido-like, serving as an internal benchmark. The N″₃U=N= fragment effectively plays the role of the tied-back heteroatom-protecting group familiar from amino-carbynes in transition metal chemistry.

Contextual comparisons sharpened the picture. Computed cerium and thorium analogues, which remain synthetically inaccessible because cerium(III) is not reducing enough to activate the diazo reagent and no thorium(III) triamide exists, show only σ-bonding to the CNN unit with low bond orders, underscoring uranium’s special capacity for back-bonding. A previously known uranium CNN complex supported by cyclopentadienyl ligands exhibits only a single weak U→C back-bond, because the multi-hapto ligands monopolize the frontier orbital capacity. Estimated carbon-13 NMR chemical shifts for the new complex approach the range typical of classical Fischer carbynes, above 210 ppm, tracing a progression from electron-rich σ-bonded to electron-deficient σ-bonded to fully σ²π¹π¹-bonded CNN complexes.

A preliminary reactivity survey reinforced the Fischer classification in a satisfying way. Like classical Fischer carbynes, the new complex proved sluggish: many substrates reacted slowly or not at all, with observed chemistry dominated by ancillary ligand activation, halide substitution, carbonylation with molybdenum hexacarbonyl, and simple adduct formation with acetonitrile or tert-butyl isocyanide. Reduction with potassium graphite and a cryptand produced a mixed-valent uranium(III/IV) product in which the extra electron localizes at the carbon-bound uranium, driving increased U→C π-back-bonding and a higher computed bond order, exactly the behaviour expected of an electron-rich Fischer-type carbyne. Together, the results establish diazomethanediide ligands as latent Fischer-type carbynes spanning a continuum of metal-dependent bonding modes, and they signpost a potentially general strategy for constructing isolable uranium–carbon triple bonds, opening a new frontier for comparing actinide and transition metal bonding half a century after Fischer’s original discovery.

Subject of Research: Synthesis and bonding analysis of the first crystalline uranium Fischer-type carbyne complex featuring a uranium-carbon triple bond

Article Title: A crystalline uranium Fischer-type carbyne

Article References: Seed, J. A., Yu, J.-K., Meurer, F., Brookfield, A., Tuna, F., Wooles, A. J., Bodensteiner, M., Hansmann, M. M., & Liddle, S. T. (2026). A crystalline uranium Fischer-type carbyne. Nature Chemistry. https://doi.org/10.1038/s41557-026-02260-0

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02260-0

Keywords: uranium, carbyne, Fischer carbyne, actinide chemistry, metal-carbon triple bond, organometallic chemistry, diazo reagent, X-ray crystallography, density functional theory, quantum crystallography, Hirshfeld atom refinement, f-element chemistry