Barium as Honorary Transition Metal in Action: Experimental and Theoretical Study of Ba(CO)⁺ and Ba(CO)⁻

Ba(CO)⁺ and Ba(CO)⁻ have been produced and isolated in a low-temperature neon matrix. The observed C−O stretching wavenumber for Ba(CO)⁺ of 1911.2 cm⁻¹ is the most red-shifted value measured for any metal carbonyl cations, indicating strong π backdonation of electron density from Ba⁺ to CO. Quantum chemical calculations indicate that Ba(CO)⁺ has a ²Π reference state, which correlates with the ²D(5d¹) excited state of Ba⁺ that comprises significant Ba⁺(5d_π ¹)→CO(π* LUMO) backbonding, letting the Ba(CO)⁺ complex behave like a conventional transition-metal carbonyl. A bonding analysis shows that the π backdonation in Ba(CO)⁺ is much stronger than the Ba⁺(5d_σ/6s)←CO(HOMO) σ donation. The Ba⁺ cation in the ²D(5d¹) excited state is a donor rather than an acceptor. Covalent bonding in the radical anion Ba(CO)⁻ takes place mainly through Ba(5d_π)←CO⁻(π* SOMO) π donation and Ba(5d_σ/6s)←CO⁻(HOMO) σ donation. The most important valence functions at barium in Ba(CO)⁺ cation and Ba(CO)⁻ anion are the 5d orbitals.