The formal metal(III) cations [CoO2H2](+) and [NiO2H2](+) can be generated by electrospray ionization of aqueous solutions of the corresponding metal(II) nitrates. In contrast to M=Fe, the fragmentations of [CoO2H2](+) and [NiO2H2](+) imply that the hydrated metal-oxide cations, (H2O) MO+, are more favorable compared to the dihydroxo isomers M(OH)(2)(+) than for M=Fe. The experimental data are corroborated by density functional calculations of the [MO2H2](+) surfaces for M=Fe-Ni, which predict Fe(OH)(2)(+) to be more stable than (H2O) FeO+, whereas the opposite applies for M=Ni and both isomers are energetically close for M=Co. (C) 2009 Elsevier B. V. All rights reserved.