trans-CpW(NO)(CH2EMe3)(H)(PMe3) (E = C, Si) complexes can be prepared by the hydrogenation at 1 atm of the appropriate CpW(NO)(CH2EMe3)2 precursor in the presence of a slight excess of PMe3. (Our designation of a particular geometrical isomer as cis or trans in this family of complexes indicates the relative positions of the hydrocarbyl and the hydrido ligands in the base of a four-legged piano-stool molecular structure.) The use of D2 in place of H2 during these syntheses affords the corresponding trans-CpW(NO)(CH2EMe3)(D)(PMe3) (E = C, Si) complexes. The cis-CpW(NO)(CH2EMe3)(H)(PMe3) isomers are obtainable by C−H bond-activation reactions of the trans precursors. Thus, activation of SiMe4 by trans-CpW(NO)(CH2CMe3)(H)(PMe3) under ambient conditions produces cis-CpW(NO)(CH2SiMe3)(H)(PMe3). Similarly, activations of C6H6 and C6D6 at 20−27 °C by the trans-CpW(NO)(CH2EMe3)(H)(PMe3) complexes produce cis-CpW(NO)(C6H5)(H)(PMe3) and cis-CpW(NO)(C6D5)(D)(PMe3), respectively, and the solid-state molecular structure of the latter complex has been established by a single-crystal X-ray crystallographic analysis. Kinetic, mechanistic, and theoretical investigations of these benzene C−H activation processes are consistent with initial trans to cis isomerization of the reactants followed by intramolecular reductive elimination of EMe4 to form the 16-electron CpW(NO)(PMe3) intermediate. Subsequent oxidative addition of the incoming benzene substrate to this coordinatively unsaturated intermediate produces the final cis hydrido phenyl complex. These single C−H activation processes are the requisite first steps in the development of these organometallic complexes as catalysts for the selective functionalization of hydrocarbons. All new complexes have been characterized by conventional spectroscopic methods.