The synthesis and reactivity properties of new dimeric iridium polyhydrides incorporating the acceptor ligand (C 2 F 5 ) 2 PCH 2 CH 2 (C 2 F 5 ) 2 (dfepe) are reported. Hydrogenolysis of (dfepe)-Ir(η 3 -C 3 H 5 ) (prepared by metathesis of [(dfepe)Ir(µ-Cl)] 2 with allylmagnesium chloride) afforded (dfepe) 2 Ir 2 (µ-H) 3 (H) (3) in high yield as an air-stable red crystalline solid. A triply bridged ground-state geometry for 3 was deduced from low-temperature NMR data and was confirmed by X-ray crystallography. Hydride site exchange mechanisms are proposed which are consistent with VT 1 H and 31 P NMR data. Although 3 is formally coordinatively saturated, hydride bridge dissociation readily occurs and leads to ligand addition reactions. Thus, treatment of tetrahydride 3 with 1 atm of H 2 at 20 °C quantitatively affords the hexahydride dimer [(dfepe)Ir(µ-H) 2 (H) 2 ] 2 (5). In the absence of H 2 , 5 rapidly loses H 2 in solution at 20 °C to re-form 3. The structure of 5 has been determined by X-ray crystallography. 3 also reacts with CO to give (dfepe)Ir(CO) 2 H (6), which loses CO under 1 atm of H 2 to reversibly afford (dfepe)Ir(CO)H 3 (7). The trihydride undergoes thermal H/D exchange with both D 2 (20 °C) and benzene-d 6 (120 °C), presumably via the intermediacy of (dfepe)Ir(CO)H. The tetrahydride 3 also undergoes H/D exchange with D 2 and benzene-d 6 under similar conditions. In the presence of tert-butylethylene, dehydrogenation of cyclopentane by 3 at 120 °C quantitatively affords CpIr(dfepe); a likely intermediate in this process is the dihydride dimer [(dfepe)Ir(µ-H)] 2 . (dfepe)Ir(η 3 -C 3 H 5 ) also reacts directly with cyclopentane at 120 °C to give CpIr(dfepe).