The cationic oxorhenium (V) complex [Re(O)(hoz)(2)(CH3CN)]-[B(C6F5)(4)] [1; Hhoz = 2-(2'-hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N3Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)(2)][B(C6F5)(4)] [2a-2f; Ar = 4-methoxyphenyl, 4-methylphenyl, phenyl, 3-methoxyphenyl, 4-chlorophenyl, and 4-(trifluoromethyl)phenyl]. The kinetics of formation of 2 in CH3CN are first-order in both azide (N3Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 x 10(-2) to 1.7 x 10(-1) M-1 s(-1). A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant rho = -1.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (Delta H double dagger) determined by the Eyring-Polanyi equation are in the range 14-19 kcal mol(-1) for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, Delta S double dagger = -21 cal mol(-1) K-1, which is in sharp contrast to the near zero Delta S double dagger observed for phenyl azide and 4-(trifluoromethyl)phenyl azide. The Hammett linear free-energy relationship and the activation parameters support a change in the mechanism between electron-withdrawing and electron-donating aryl azides. Density functional theory predicts that the aryl azides coordinate via N-alpha and extrude N-2 directly. For the electron-withdrawing substituents, N-2 extrusion is rate-determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett a values; thus, the Hammett plot appears with a break in its slope.