The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N-2 reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin iron hydride complex, (LFeH)-Fe-tBu (L-tBu = bulky beta-diketiminate), reacts with PhNNPh to completely cleave the N-N double bond, giving (LFeNHPh)-Fe-tBu. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare eta(2)-azobenzene complex. Therefore, this high-spin iron(II) hydride can break N-N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.