Certain transition metal complexes can read to break normally inert carbon-hydrogen (C-H) bonds, but these metal-based processes typically require photochemistry or elevated temperatures. In addition, most are unselective toward complicated functionalized substrates, which has limited their synthetic usefulness. The cationic iridium complex Cp*(P(CH3)(3))lr(CH3)(ClCH2Cl)(BArf-)-B-+ [Cp* = eta(5)-C-5(CH3)(5), BArf = B(3,5-C6H3(CF3)(2))(4)] can thermally activate methane and terminal alkanes at unprecedentedly mild temperatures (10 degrees C). This complex will also induce C-H activation reactions in various functionalized substrates at ambient temperatures. High steric and electronic selectivity is observed, leading invariably to only one reaction product; the initial C-H activation reaction is typically followed by rapid metal-based rearrangements (that is, functionalization).