OsH(Ph)(CO)L-2 (L = PtBu2Me, Ph = C6H5) is synthesized and studied by spectroscopic and DFT (B3LYP) calculations. It forms 1:1 adducts with CO or MeCN, but D-2 causes reductive elimination of C6H6 Reaction with C6D6 gives exclusively OsD(C6D5)(CO)L-2. Lack of conversion of OsD(C6H5)(CO)L-2 to OsH-(C6H4D)(CO)L-2 under the same conditions indicates that zerovalent Os(eta(2)-C6H5D)(CO)L-2 is not kinetically relevant. DFT calculations confirm that an eta(2)-arene complex is not accessible and also show that the dissociative reductive elimination of arene is a high energy process. Reaction of OsH(Ph)(CO)L-2 with fluorinated arenes, HArF gives benzene and OsH(Ar-F)(CO)((PBu2Me)-Bu-t)(2) (i.e., attack on the HArF C-H bond), with the fastest attack involving more heavily fluorinated arenes (but C6F6 does not react) and the attack occurring preferentially at the C-H bond ortho to fluorine. DFT calculations show that the observed isomer is thermodynamically controlled. These results, isotope-labeling results, and rate suppression and exchange involving added free phosphine lead to a mechanism where reagent arene associates with OsH(Ph)(CO)L-2, but benzene loss demands preliminary dissociation of phosphine from such an adduct.