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The hydroxylation of C–H bonds can be carried out by the high-valent Co^III,IV₂(µ-O)₂complex2asupported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co^III₂(µ-O)(µ-OH) intermediate (3a). Complex3acan be independently generated either by H-atom transfer (HAT) in the reaction of2awith phenols as the H-atom donor or protonation of its conjugate base, the Co^III₂(µ-O)₂complex1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm⁻¹associated with the symmetric Co–O–Co stretching mode of the Co₂O₂diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for1aand2aby Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of3aallows us to investigate HAT reactions of2awith phenols in detail, measure the redox potential and pK_aof the system, and calculate the O–H bond strength (D_O–H) of3ato shed light on the C–H bond activation reactivity of2a. Complex3ais found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal2ato be 10⁶-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe^III,IV₂(µ-O)₂and Mn^III,IV₂(µ-O)₂analogs, an unexpected outcome that raises the prospects for using Co^III,IV₂(µ-O)₂species to oxidize alkane C–H bonds. 

High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron( iv )–tosylimido complexes [Fe IV (NTs)(MePy 2 tacn)](OTf) 2 ( 1(IV)NTs ) and [Fe IV (NTs)(Me 2 (CHPy 2 )tacn)](OTf) 2 ( 2(IV)NTs ), (MePy 2 tacn = N -methyl- N , N -bis(2-picolyl)-1,4,7-triazacyclononane, and Me 2 (CHPy 2 )tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)NTs and 2(IV)NTs are rare examples of octahedral iron( iv )–imido complexes and are isoelectronic analogues of the recently described iron( iv )–oxo complexes [Fe IV (O)(L)] 2+ (L = MePy 2 tacn and Me 2 (CHPy 2 )tacn, respectively). 1(IV)NTs and 2(IV)NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1 H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe III (HNTs)(L)] 2+ , 1(III)–NHTs (L = MePy 2 tacn) and 2(III)–NHTs (L = Me 2 (CHPy 2 )tacn) have been isolated after the decay of 1(IV)NTs and 2(IV)NTs in solution, spectroscopically characterized, and the molecular structure of [Fe III (HNTs)(MePy 2 tacn)](SbF 6 ) 2 determined by single crystal X-ray diffraction. Reaction of 1(IV)NTs and 2(IV)NTs with different p -substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)NTs and 2(IV)NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)NTs and 2(IV)NTs with hydrocarbons containing weak C–H bonds results in the formation of 1(III)–NHTs and 2(III)–NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.