A biomimetic study for S/Se oxygenation in Ni(μ-EPh)(μ-SN2)Fe, (E = S or Se; SN2 = Me-diazacycloheptane-CH2CH2S); Fe = (η5-C5H5)FeII(CO) complexes related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H2ases is described. Mono- and di-oxygenates (major and minor species, respectively) of the chalcogens result from exposure of the heterobimetallics to O2; one was isolated and structurally characterized to have Ni-O-SePh-Fe-S connectivity within a 5-membered ring. A compositionally analogous mono-oxy species was implicated by ν(CO) IR spectroscopy to be the corresponding Ni-O-SPh-Fe-S complex; treatment with O-abstraction agents such as P(o-tolyl)3 or PMe3 remediated the O damage. Computational studies (DFT) found that the lowest energy isomers of mono-oxygen derivatives of Ni(μ-EPh)(μ-SN2)Fe complexes were those with O attachment to Ni rather than Fe, a result consonant with experimental findings, but at odds with oxygenates found in oxygen-damaged [NiFeS]/[NiFeSe]-H2ase structures. A computer-generated model based on substituting -SMe for the N-CH2CH2S- sulfur donor of the N2S suggested that constraint within the chelate hindered O-atom uptake at that sulfur site.