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Observation of relativistic bond weakening in seaborgium hexacarbonyl
Nature
volume 658, pages 337–341 (2026) Cite this article
The seventh row of the periodic table of the elements has been completed with the discovery of the five heaviest elements from flerovium (Fl, Z = 114) through oganesson (Og, Z = 118)1. Chemical properties of the superheavy elements (SHE, Z ≥ 104) are unique owing to the strong influence of relativistic effects on their valence electron shells2. The stability of the metal–ligand bonding in carbonyl complexes is a property that is predicted to be affected by such effects3,4. Here we present an experimental gas chromatography study on the stability of the Sg(CO)6 complex. The measured formation yields of Sg(CO)6 and W(CO)6 enabled determining the first bond dissociation energy (FBDE) of Sg(CO)6, which is 4(2) kJ mol−1 lower than that of W(CO)6 and corresponds to a value of 188(9) kJ mol−1. Thus, the trend of increasing FBDE values of hexacarbonyls within group 6 of the periodic table is reversed from W to Sg. Our result agrees with state-of-the-art theoretical calculations, which predict a decrease in the FBDE from W(CO)6 to Sg(CO)6 owing to the relativistic destabilization of the 6d atomic orbitals (AOs) in Sg (ref. 4). Also, the adsorption enthalpy value of Sg(CO)6 on gold of \({45}_{-6}^{+4}\,{\rm{kJ}}\,{{\rm{mol}}}^{-1}\) is reported.
Man-made SHE (atomic number Z ≥ 104) are produced in nuclear fusion reactions by irradiating Pb, Bi or actinide targets with heavy ions. The rates of those reactions substantially decrease with increasing Z, reaching production rates of single atoms per week for Og. Also, extremely short lifetimes of SHE isotopes limit the number of chemical systems that can be studied experimentally5,6,7. SHE heavier than seaborgium (Sg, Z = 106) were chemically investigated only in the gas phase8,9,10,11,12,13,14,15.
Investigations of chemical bonding in compounds of the heaviest elements are of particular interest because strong relativistic effects influence their valence AOs. The 7s and 7p1/2 AOs are relativistically contracted and stabilized, leading to the closed shell (6d107s2) and quasi-closed shell (7s27p1/22) configurations in copernicium (Cn, Z = 112) and flerovium (Fl, Z = 114), respectively, and thus to high volatility and inertness of these elements8,9,10,11,12. The stabilized 7p1/2 AO also leads to a weaker reactivity of nihonium (Nh, Z = 113) and moscovium (Mc, Z = 115) with respect to their lighter homologues Tl and Bi, respectively (ref. 13). In the elements of the 6d series, the valence 6d and 7s AOs participate in chemical bonding. The indirect relativistic effect is the destabilization and expansion of the 6d AOs, screened from the nucleus by the spherical inner s and p1/2 AOs. This effect is expected to influence properties of the 6d element compounds2, albeit to a lesser extent, as the central metal atom is surrounded by ligands6.
Among the known SHE compounds, the hexacarbonyl complex of Sg, Sg(CO)6, in which the Sg atom is in the zero-valent state and surrounded by six neutral carbon monoxide (CO) ligands, has attracted much attention14. In particular, the value of the FBDE, that is, the energy needed to detach one CO ligand from Sg(CO)6, is a measure of the compound stability and is expected to be influenced by relativistic effects3,4. Bonding in the hexacarbonyls of lighter homologues of Sg, that is, Cr, Mo, W, was originally investigated theoretically using the density functional theory (DFT) approach16,17,18. Two types of electron density transfer process were shown to be involved in the formation of the metal–CO bond: the σ-forth donation from the molecular orbital Φσ(Eg) of the CO ligands to the d(Eg) AO of the metal atom (M) and the π-back donation from the d(T2g) AO of M to the Φπ(T2g) of CO (ref. 16). The influence of relativistic effects on the M–CO bond strength in hexacarbonyls was shown to increase from Cr to Mo and to W (ref. 18). Nash and Bursten calculated the FBDE of W(CO)6 and Sg(