The direct oxidative addition of O2 to a mononuclear Cr(I) complex is spin forbidden.

Mononuclear chromium(I) alkyne complex (i-Pr2Ph)2nacnacCr(η(2)-C2(SiMe3)2) (1) reacts rapidly with dioxygen to yield chromium(V) dioxo species (i-Pr2Ph)2nacnacCr(O)2 (2). The mechanism of this oxygen cleavage has been studied experimentally and computationally. Isotope labeling studies rule out a direct four-electron oxidative addition of O2 to one chromium atom, which involves a spin-forbidden transformation. Instead, the reaction likely proceeds via an unsymmetric binuclear chromium bis(μ-oxo) complex. The latter has been independently prepared and structurally characterized. Its reactivity with O2 is consistent with the proposed mechanism.