C-H activation of benzene at 26 degrees C by (eta 5-C5Me5)W(NO)(CH2CMe3)(eta(3)-CH2CHCHMe) results after 4 h in the production of five new organometallic complexes, only two of which are isomers of the desired (eta(5)-C5Me5)W(NO)(C6H5)(eta(3)-CH2CHCHMe) compound. In contrast, the identical reaction involving the eta 5-C5Me4H analogue affords only the phenyl complexes during the first 24 h, thereby facilitating their isolation in good yields. This striking difference in reactivity can be attributed to the lesser steric demands of the eta 5-C5Me4H ligand that result in its complexes reacting at a significantly slower rate.