Analysis of the kinetic isotope effects for C-H functionalization demonstrates the decisive role of the halogen substituent on the resultant photochemistry and provides a direct reporter on the nature of the C-H cleavage event. Amination of THF by photolysis of N-chloroamide 13a proceeds with kH/kD = 6(1), which is consistent with values for H-atom abstraction reactions at Ncentered radicals. In contrast, amination of THF by photolysis of N-iodoamide 13e proceeds with kH/kD = 18(3). This large kH/kD value exceeds the classical limit for H-atom transfer reactions between light atoms and is consistent with the large KIEs that are often encountered for amination via metal nitrenoids. Cryogenic photolysis provides both the opportunity to directly evaluate the course of the described photoreactions and evidence for the decisive impact of halogen identity on the resultant reactive nitrogen species. Both UV-vis and EPR spectroscopies indicate that the primary photoproduct obtained from chloroamide 13a is Mn(II) complex 15. The M-N homolysis that would generate complex 13 would also afford an amidyl radical intermediate, which would give rise to the observed amination products. In contrast, photolysis of iodoamide 13e affords Mn(IV) nitrenoid 20, the product expected of homolytic activation of the N-I bond of 13e. Available data is consistent with the formation of an S = 5/2 species (i.e. an Mn(III) iminyl radical complex). The observed preference for Mn-N activation in 13a and for N-I activation in 13e is consistent with the relative N-Cl and N-I bond strengths and indicates the ability to control the course of the photoreaction by careful design of the photocleavable ligands.