C-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O-2-activating enzymes. Of crucial interest is the effect of spin state of the Fe-IV(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [(LFeIV)-Fe-NHC(O)(MeCN)](2+) (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol(-1) show pseudo-first-order behavior and large but temperature-dependent kinetic isotope effects (KIE 32 at -40 degrees C). When compared with a topologically related oxo-iron(IV) complex bearing an equatorial N-donor ligand, [(LFeIV)-Fe-TMC(O) (MeCN)](2+) (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k'(2) that are 2-3 orders of magnitude higher. UV-vis experiments in tandem with cryospray mass spectrometry evidence that the reaction occurs via formation of a hydroxo-iron(III) complex (4) after the initial H atom transfer (HAT). An extensive computational study using a wave function based multireference approach, viz. complete active space self-consistent field (CASSCF) followed by N-electron valence perturbation theory up to second order (NEVPT2), provided insight into the HAT trajectories of 1 and A. Calculated free energy barriers for 1 reasonably agree with experimental values. Because the strongly donating equatorial tetracarbene pushes the Fe-d(x2-y2) orbital above d(z2), 1 features a dramatically large quintet-triplet gap of similar to 18 kcal/mol compared to similar to 2-3 kcal/mol computed for A. Consequently, the HAT process performed by 1 occurs on the triplet surface only, in contrast to complex A reported to feature two-state-reactivity with contributions from both triplet and quintet states. Despite this, the reactive Fe-IV(O) units in 1 and A undergo the same electronic-structure changes during HAT. Thus, the unique complex 1 represents a pure "triplet-only" ferryl model.