A theoretical investigation into the first-row transition metal-O₂ adducts

An extensive investigation into various M-O₂ species (M = Cr^I, Mn^I, Fe^I, Co^I, Ni^I, Cu^I) has been conducted using a Density Functional Theory (DFT) approach, generating M^I-O₂, M^II-superoxo or M^III-peroxo species. Two different ligands, 12-TMC and 14-TMC, are used to gauge the effects of the ligand ring-size. In general, theory reproduces the experimental results (where available) well enough to give confidence in the calculations. In addition to the usual calculated features of the individual metal complexes, a statistical analysis has been done by comparing the M-O₂ species across the periodical system. It is found that the O₂ binding energy diminishes with higher metal atomic number, while an end-on structure becomes gradually favored. Also, multi-spin state reactivity becomes more likely for metals above Fe. The spin density on O₂ (and with it the formal oxidation state of the metal) is more dependent on the prevailing spin state of the compound rather than the metal type per se, and the higher flexibility of the larger 14-TMC ring has also been verified. The theoretical methods used are also evaluated regarding their accuracy.