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

Abstract

AbstractBa(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+(5dπ1)→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+(5dσ/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(5dπ)←CO−(π* SOMO) π donation and Ba(5dσ/6s)←CO−(HOMO) σ donation. The most important valence functions at barium in Ba(CO)+ cation and Ba(CO)− anion are the 5d orbitals.