Strategies for limiting, or reversing, the degradation of airsensitive, base metal catalysts for the hydrogen evolution/oxidation reaction on contact with adventitious O-2 are guided by nature's design of hydrogenase active sites. The affinity of oxygen for sulfur and selenium, in [NiFeS]- and [NiFeSe]-H(2)ase, yields oxygenated chalcogens under aerobic conditions, and delays irreversible oxygen damage at the metals by maintaining the NiFe core structures. To identify the controlling features of S-site oxygen uptake, related Ni(mu-E-PhX )(mu-S'(N2))Fe (E = S or Se, Fe = (eta(5)-C5H5)Fe-II (CO)) complexes were electronically tuned by the parasubstituent on mu-EPhX (X = CF3, Cl, H, OMe, NMe2) and compared in aspects of communication between Ni and Fe. Both single and double O atom uptake at the chalcogens led to the conversion of the four-membered ring core, Ni(mu-E-phX)(mu-S'(N2))Fe, to a five-membered ring Ni-O-E-Fe-S', where an O atom inserts between E and Ni. In the E = S, X = NMe2 case, the two-oxygen uptake complex was isolated and characterized as the sulfinato species with the second O of the O2SPh-Nme2 unit pointing out of the five-membered Ni-O-S-Fe-S' ring. Qualitative rates of reaction and ratios of oxygen-uptake products correlate with Hammett parameters of the X substituent on Ephx. Density functional theory computational results support the observed remote effects on the NiFe core reactivity; the more electron-rich sulfurs are more O-2 responsive in the S ax series; the selenium analogues were even more reactive with O-2. Mass spectral analysis of the sulfinato products using a mixture of O-18(2)/O-16(2) suggests a concerted mechanism in O-2 addition. Deoxygenation, by reduction or O atom abstraction reagents, occurs for the 1-O addition complexes, while the 2-O, sulfinato, analogues are inert. The abstraction of oxygen from the 1-O, sulfenato species, is related to oxygen repair in soluble, NAD(+)-treducing [NiFe]-H(2)ase (Horch, M.; Lauterbach, L.; et al. J. Am. Chem. Soc. 2015, 137, 2555-2564).