Uranium carbyne crystallised with a 2.379-angstrom triple bond to carbon

Crystallographers report the distance as 2.379(15) angstroms ^[4]. The bracketed figure is the uncertainty in the last digits, so one standard uncertainty either side spans 2.364 to 2.394 angstroms ^[16]. A distance alone does not count bonds. The triple-bond assignment comes from density functional theory calculations combined with Hirshfeld atom refinement, a quantum-crystallography treatment of the experimental diffraction data ^[5].

Two families of metal-carbon triple bond set the terms of comparison. In a Fischer carbyne, a singlet carbon fragment donates to the metal and the metal sustains two orthogonal back-bonds to the carbon ^[13]. In a Schrock alkylidyne, quartet metal and carbon fragments combine into a covalent triple bond ^[13]. The uranium compound follows the Fischer pattern of donation and back-donation, but each of its two back-bonds from uranium holds one electron, and the authors classify the whole as sigma2pi1pi1 Fischer-type bonding ^[5]. On their scheme, possible electron counts form a continuum from classical Fischer to Fischer-type carbynes ^[14].

The authors are candid about how soft such labels are. Carbyne bonding is less clear-cut than the double-bonded alkylidene and carbene cases, they write, the choice of model is somewhat arbitrary, and even the fully back-bonded description is a formalism ^[14]. I think the evidence supports two statements at different strengths. The uranium-carbon contact is measured. The sigma2pi1pi1 picture is a model of that contact, built from calculation and refined experimental density ^[4][5].

Uranium is the most studied actinide and the most transition-metal-like ^[8]. It already forms many multiple bonds to nitrogen-group and oxygen-group elements, and double bonds to carbon in several alkylidene complexes ^[9]. Metal-carbon triple bonds in general have been known for more than half a century and are used in both fundamental and applied synthesis ^[11].

The thing this doesn't tell you is how the uranium-carbon unit behaves in a reaction; the abstract does not report any reactivity. The carbyne carbon belongs to a diazomethanediide ligand, and the work extends the bonding modes known for that ligand ^[12]. Fischer carbynes usually carry a heteroatom substituent. The paper notes that such substituents open alternative resonance forms that weaken the metal-carbon interaction compared with Schrock alkylidynes ^[15]. The authors offer the compound as a foundation, "providing a basis to develop and understand f-element carbyne chemistry" ^[6].