Diruthenium azido complexes Ru-2(DPhF)(4)N-3 (1a, DPhF = N,N'-diphenylformamidinate) and Ru-2(D(3,5-Cl-2)PhF)(4)N-3 (1b, D(3,5-Cl-2)PhF = N,N'-bis(3,5-dichlorophenyl)formamidinate) have been investigated by thermolytic and photolytic experiments to investigate the chemical reactivity of the corresponding diruthenium nitride species. E Thermolysis of 1b at similar to 100 degrees C leads to the expulsion of N-2 and isolation of Ru-2(D(3,5-Cl-2)PhF)(3)NH(C13H6N2Cl4) (3b), in which a nitrogen atom has been inserted into one of the proximal aryl C-H bonds of a D(3,5-Cl-2)PhF ligand. A similar C-H insertion product is obtained upon thawing a frozen CH2Cl2 solution of the nitride complex Ru-2(DPhF)(4)N (2a), formed via photolysis at -196 degrees C of 1a to yield Ru-2(DPhF)(3)NH(C13H10N2) (3a). Evidence is provided here that both reactions proceed via direct intramolecular attack of an electrophilic terminal nitrido nitrogen atom on a proximal aryl ring. Thermodynamic and kinetic data for this reaction are obtained from differential scanning calorimetric measurements and thermal gravimetric analysis of the thermolysis of Ru-2(D(3,5-Cl-2)PhF)(4)N-3, and by Arrhenius/Eyring analysis of the conversion of Ru-2(DPhF)(4)N to its C-H insertion product, respectively. These data are used to develop a detailed, experimentally validated DFT reaction pathway for N-2 extrusion and C-H functionalization from Ru-2(D(3,5-Cl-2)PhF)(4)N-3. The diruthenium nitrido complex is an intermediate in the calculated reaction pathway, and the C-H functionalization event shares a close resemblance to a classical electrophilic aromatic substitution mechanism.