The CH sigma-bond activation of methane and the NH sigma-bond activation of ammonia by (Me3SiO)(2)Ti(NSiMe3) 1 were theoretically investigated with DFT, MP2 to MP4(SDQ), and CCSD(T) methods. The CH sigma-bond activation of methane takes place with an activation barrier (E-a) of 14.6 (21.5) kcal/mol and a reaction energy (Delta E) of -22.7 (-16.5) kcal/mol to afford (Me3SiO)(2)Ti(Me){NH(SiMe3)}, where DFT- and MP4(SDQ)-calculated values are given without and in parentheses, respectively, hereafter. The electron population of the CH3 group increases, but the H atomic population decreases upon going to the transition state from the precursor complex, which indicates that the CH sigma-bond activation occurs in heterolytic manner unlike the oxidative addition. The Ti atomic population considerably increases upon going to the transition state from the precursor complex, which indicates that the charge transfer (CT) occurs from methane to Ti. These population changes are induced by the orbital interactions among the d(pi)-p(pi) bonding orbital of the TiNSiMe3 moiety, the Ti d(z)(2) orbital and the CH sigma-bonding and sigma*-antibonding orbitals of methane. The reverse regioselective CH sigma-bond activation which leads to formation of (Me3SiO)(2)Ti(H){NMe(SiMe3)} takes place with a larger E-a value and smaller exothermicity. The reasons are discussed in terms of TiH, TiCH3, TiNH3, NH, and NCH3 bond energies and orbital interactions in the transition state. The NH sigma-bond activation of ammonia takes place in a heterolytic manner with a larger E-a value of 19.0 (27.9) kcal/mol and considerably larger exothermicity of -45.0 (-39.4) kcal/mol than those of the CH sigma-bond activation. The NH sigma-bond activation of ammonia by a Ti-alkylidyne complex, [(PNP)Ti(CSiMe3)] 3 (PNP = N-[2-(PH2)(2)-phenyl](2)(-)]) ,was also investigated. This reaction takes place with a smaller E-a value of 7.5 (15.3) kcal/mol and larger exothermicity of -60.2 (-56.1) kcal/mol. These results lead us to predict that the NH sigma-bond activation of ammonia can be achieved by these complexes.