The energetics of selected dicationic iron oxides and hydroxides FeO
m
H
n
2+ (m = 1, 2; n ≤ 4) are probed by
charge-stripping mass spectrometry in conjunction with ab initio calculations employing the B3LYP/6-311+G*
level of theory. Specifically, Fe+, FeO+, FeOH+, Fe(H2O)+, [Fe,O2,H2]+, (H2O)FeOH+, and Fe(H2O)2
+ and
their respective dications are examined. In most cases, reasonable agreement between experiment and theory
is found, and discrepancies can be attributed to interferences in the experimental study. Nevertheless, some
shortcomings of the theoretical approach are obvious. Combination of experimental and theoretical results
leads to adiabatic ionization energies of the monocationic iron compounds: IEa(FeO+) = 18.3 ± 0.4 eV,
IEa(FeOH+) = 17.0 ± 0.4 eV, IEa(Fe(H2O)+) = 14.3 ± 0.5 eV, IEa((H2O)FeOH+) = 15.6 ± 0.5 eV, and
IEa(Fe(H2O)2
+) = 12.6 ± 0.4 eV. In the case of [Fe,O2,H2]+, structural isomerism and isobaric interferences
give rise to a composite charge-stripping peak and prevent the experimental determination of the ionization
energy. Interestingly, the computational results suggest a reversed order of stabilities for the mono- and
dicationic [Fe,O2,H2]+/2+ isomers, i.e., Fe(OH)2
+ > (H2O)FeO+ > Fe(H2O2)+ versus Fe(H2O2)2+ > (H2O)FeO2+ > Fe(OH)2
2+. The ion energetics are used to assess the effects of ligation on the stabilities of the iron
dications. While the covalent Fe−O and Fe−OH bonds decrease with increasing oxidation state of the metal,
the interactions with water are dominated by electrostatic contributions. On average, solvation by water lowers
the second ionization energy of the iron compounds studied by as much as 1.6 ± 0.3 eV