A Tale of Two Stabilities: How One Boron Atom Affects a Switch in Bonding Motifs in CeO₂B _x^- ( x = 2, 3) Complexes

Boronyl (B≡O) ligands have garnered much attention as isoelectronic and isolobal analogues of CO and CN^-, yet successful efforts in synthesizing metal boronyl complexes remain scarce. Anion photoelectron (PE) spectroscopy and density functional theory calculations were employed to investigate two small CeO₂B _x^- ( x = 2, 3) complexes generated from laser ablation of a mixed Ce/B pressed powder target. The spectra reveal markedly different bonding upon incorporation of an additional B atom. Most interestingly, CeO₂B₂^- was found to have a Ce(I) center coordinated to two monoanionic boronyl ligands in a bent geometry. This result was unexpected as previous studies suggest electron-rich metals are most suitable for stabilizing such ligands; furthermore, it is one of the first examples of an experimental metal-polyboronyl complex. Introducing another boron atom, however, favors a much different geometry in which Ce(II) coordinates an O₂B₃^3- unit through both the O and B atoms, which was evident in the markedly different PE spectra.