Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.