Approaching cumulative quintuple bonds in all-metal uranyl analogs

While the progression from single (H₂), double (O₂) and triple (N₂) bonds in diatomic molecules to the discovery of transition metal quadruple (Re-Re) and quintuple (Cr-Cr) bonds has significantly expanded our understanding of covalent interactions, the exploration of cumulative multiple bonds in trinuclear systems has remained largely unexplored. Until now, cumulative bonds beyond triple multiplicity have neither been experimentally isolated nor theoretically predicted. Here, we propose a new bonding model-cumulative quintuple bonds-and report the synthesis and characterization of a novel trinuclear species, [Co-U-Co], which exhibits bonding interactions approaching cumulative quintuple bonds. Advanced quantum chemical calculations reveal that the cumulative multiple bonds consist of one pair of U-Co σ bonds, two pairs of U-Co π bonds, one pair of U-Co δ bonds, a three-center-two-electron δ bond and a δ hyperconjugation. This species represents the first example of an all-metal uranyl analog featuring a linear M-U-M core with cumulative multiple bonds. This discovery not only extends the theoretical boundaries of cumulative multiple bonds but also establishes a transformative precedent for cumulative bonding multiplicity in trinuclear species.