The structures, relative stabilities, and multiplicities of the cationic, late-transition-metal oxides FeO+, CoO+, NiO+, and CuO+ are rationalized on the basis of ab initio computations. The bonding situation in these cations is analogous to that in the dioxygen molecule with a biradicaloid pi-bonding, and hence the electronic ground states of these metal oxide cations correspond to their high-spin variants, FeO+ ((6) Sigma(+)), CoO+ ((5) Delta), NiO+ ((4) Sigma(-)), CuO+ ((3) Sigma(-)). Density functional theory augmented with CASPT2D computations is used to explore the reaction surface of FeO+ + H-2 and to unravel the roots of the extremely low reactivity observed for this system. According to these calculations, the reaction violates spin-conservation rules and involves a curve crossing from the sextet ground state to the excited quartet surface, giving rise to a multicentered, energetically low-lying transition structure, from which the hydride iron hydroxide cation H-Fe-OH+ is formed as the initial oxidation product. The implications of these results with respect to other ion/molecule processes of metal oxide cations with oxidizable organic substrates are discussed.