Carbon-sulfur bond strength in methanesulfinate and benzenesulfinate ligands directs decomposition of Np(v) and Pu(v) coordination complexes.

Gas-phase coordination complexes of actinyl(v) cations, AnO, provide a basis to assess fundamental aspects of actinide chemistry. Electrospray ionization of solutions containing an actinyl cation and sulfonate anion CHSO or CHSO generated complexes [(AnO)(CHSO)] or [(AnO)(CHSO)] where An = Np or Pu. Collision induced dissociation resulted in C-S bond cleavage for methanesulfinate to yield [(AnO)(CHSO)(SO)], whereas hydrolytic ligand elimination occurred for benzenesulfinate to yield [(AnO)(CHSO)(OH)]. These different fragmentation pathways are attributed to a stronger CH-SOversus CH-SO bond, which was confirmed for both the bare and coordinating sulfinate anions by energies computed using a relativistic multireference perturbative approach (XMS-CASPT2 with spin-orbit coupling). The results demonstrate shutting off a ligand fragmentation channel by increasing the strength of a particular bond, here a sulfinate C-S bond. The [(AnO)(CHSO)(SO)] complexes produced by CID spontaneously react with O to eliminate SO, yielding [(AnO)(CHSO)(O)], a process previously reported for An = U and found here for An = Np and Pu. Computations confirm that the O/SO displacement reactions should be exothermic or thermoneutral for all three An, as was experimentally established. The computations furthermore reveal that the products are superoxides [(AnO)(CHSO)(O)] for An = Np and Pu, but peroxide [(UO)(CHSO)(O)]. Distinctive reduction of O to O concomitant with oxidation of U(v) to U(vi) reflects the relatively higher stability of hexavalent uranium versus neptunium and plutonium.