The diiron(II) complex, [(OCO)Fe(MeCN)](2) (1, MeCN = acetonitrile), supported by the bis-phenolate carbene pincer ligand, 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)benzimidazolin-2-ylidene (OCO), was synthesized and characterized by single-crystal X-ray diffraction,H- 1 nuclear magnetic resonance, infrared (IR) vibrational, ultraviolet/visible/near-infrared (UV/vis/NIR) electronic absorption, Fe-57 Mossbauer, X-band electron paramagnetic resonance (EPR) and SQUID magnetization measurements. Complex 1 activates dioxygen to yield the diferric, mu-oxo-bridged complex [(OCO)Fe(py)(mu-O)Fe(O(C=O)O)(py)] (2) that was isolated and fully characterized. In 2, one of the iron-carbene bonds was oxidized to give a urea motif, resulting in an O(C-NHC equivalent to O)O binding site, while the other Fe(OCO) unit remained unchanged. When the reaction is performed at -80 degrees C, an intensively colored, purple intermediate is observed (INT, lambda(max) = 570 nm; epsilon = 5600 mol L-1 cm(-1)). INT acts as a sluggish oxidant, reacting only with easily oxidizable substrates, such as PPh3 or 2-phenylpropionic aldehyde (2-PPA). The identity of INT can be best described as a dinuclear complex containing a closed diamond core motif [(OCO)Fe-IV(mu-O)(2)Fe-IV(OCO)]. This proposal is based on extensive spectroscopic [UV/vis/NIR electronic absorption, Fe-57 Mossbauer, X-band EPR, resonance Raman (rRaman), X-ray absorption, and nuclear resonance vibrational (NRVS)] and computational studies. The conversion of the diiron(II) complex 1 to the oxo diiron(IV) intermediate INT is reminiscent of the O-2 activation process in soluble methane monooxygenases (sMMO). Most importantly, the low reactivity of INT supports the consensus that the [Fe-IV(mu-O)(2)Fe-IV] diamond core in sMMO is kinetically inert and needs to open up to terminal Fe-IV=O cores to