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 pi backdonation of electron density from Ba+ to CO. Quantum chemical calculations indicate that Ba(CO)(+) has a (2)Pi reference state, which correlates with the 2D(5d(1)) excited state of Ba+ that comprises significant Ba+(5d pi(1)) -> CO(pi* LUMO) backbonding, letting the Ba(CO)(+) complex behave like a conventional transition- metal carbonyl. A bonding analysis shows that the p backdonation in Ba(CO)(+) is much stronger than the Ba+(5d(sigma)/6s) <- CO(HOMO) sigma donation. The Ba+ cation in the D-2(5d(1)) excited state is a donor rather than an acceptor. Covalent bonding in the radical anion Ba(CO)(-) takes place mainly through Ba(5d(pi)) <- CO-(pi* SOMO) pi donation and Ba(5d(sigma)/6s) <- CO-(HOMO) s donation. The most important valence functions at barium in Ba(CO)(+) cation and Ba(CO)(-) anion are the 5d orbitals.