Observation of the Ba(0) Complex BaNi(CO) ₃ ^– Featuring the Shortest Bond between Barium and Transition Metal

The heteronuclear barium-nickel anion complex BaNi(CO)₃^- is prepared in the gas phase and studied by mass-selected infrared photodissociation spectroscopy as well as quantum chemistry calculations. The complex exhibits a ²A₁ ground electronic state and adopts a C₃_v equilibrium geometry with all three carbonyl ligands coordinated to the nickel center. The Ba-Ni bond distance closely approximates the sum of their covalent radii. Bonding analysis reveals significant covalent interactions between Ba(0) and Ni(CO)₃^-, involving both a Ba(0)-Ni(CO)₃^- covalent half-bond and a Ba(0) → Ni(CO)₃^- dative bonding interaction. This work represents the first observation of a compound containing true Ba(0) in its ground state 6s² electron configuration and features the shortest covalent Ba-transition metal single bond. These findings expand the chemistry of heavy alkaline earth metals in low oxidation states and suggest a potential route for the synthesis of Ba(0) compounds in the condensed phase.