A unique route for the synthesis of indoles from o-tolyl isocyanides using Ru(dmpe)2(H)-(naphthyl) and Ru(dmpe)2H2 as catalysts has been examined. The scope of this method for indole preparation has been examined with a variety of o-tolyl isocyanides, including 3-, 4-, and 5-R-o-tolyl isocyanides (where R = methyl, methoxy, or fluoro), 2,6-xylyl isocyanide, 2,6-diethylphenyl isocyanide, 2-ethylphenyl isocyanide, o-tolyl isocyanide, and 6-ethyl-o-tolyl isocyanide. The mechanism of indole formation has been investigated using kinetic and isotope effect experiments to differentiate key product-determining steps of the cycle. Results are consistent with a mechanism involving irreversible CNR coordination prior to intramolecular oxidative addition of the o-methyl C-H bond. Competitive isotope effect studies using d0- and α,α,α,α′,α′,a′,3,5-d 8-labeled 4-tert-butyl-2,6-xylyl isocyanides indicate virtually no isotope effect (kH/kD = 1.08) when the selection of which bond to active is intermolecular. Use of 4-tert-butyl-2,6-xylyl-α,α,α-d3 isocyanide shows that C-H activation is faster than C-D activation (kH/kD = 2.6) in an intramolecular competition, where the choice of C-H and C-D bonds to activate is within one xylyl isocyanide. The reaction with 2,6-diethylphenyl isocyanide to give 3-methyl-7-ethylindole is first order in [Ru(dmpe)2(H)(naphthyl)] and zero order in [CNR]. While hindered 2,6-disubstituted phenyl isocyanides eliminate free indoles catalytically, less hindered isocyanides give stable indole N-H oxidative addition adducts with [Ru(dmpe)2]. The resulting cis and trans N-H activated complexes are thermodynamically the most stable species in solution. N-H addition products were also formed with substituted indoles, pyrrole, pyrazole, indazole, and pyrrolidine. Blocking the N-H position of indole with a methyl group results in C-H oxidative addition of ruthenium at the 2-position of the ring. trans-RuH(3-methylindole-N)(dmpe)2 crystallizes in orthorhombic space group Pnnm (No. 58) with, a = 17.263(10) A?, b = 10.668(10) A?, c = 13.524(10) A?, V = 2491(6) A?3, and Z = 4. trans-RuH(5-methoxyindole-N)(dmpe)2 crystallizes in monoclinic space group P21/c (No. 15) with, a = 9.065(2) A?, b = 16.379(3) A?, c = 19.196(5) A?, β = 92.06(4)°, V = 2848(2) A?3, and Z = 4. trans-RuH-(5-fluoroindole-N)(dmpe)2 crystallizes in monoclinic space group P21/c (No. 15) with, a = 8.854(4) A?, b = 16.45(1) A?, c = 18.873(8) A?, β = 93.95(4)°, V = 2743(5) A?3, and Z = 4. cis-RuH(N-methylindole-C2)(dmpe)2 crystallizes in monoclinic space group P21/c (No. 15) with, a = 15.08(1) A?, b = 10.173(8) A?, c = 18.09(2) A?, β = 114.49(7)°, V = 2526(8) A?3, and Z = 4.