Barium as Honorary Transition Metal in Action: Experimental and Theoretical Study of Ba(CO)+ and Ba(CO)<sup/>.

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 2 Π reference state, which correlates with the 2 D(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 2 D(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.