The reaction of a [(PHOX)Ir(COD)](+) complex (COD = 1,5-cyclooctadiene) with dihydrogen was studied by NMR spectroscopy (PHOX = chiral phosphinooxazoline ligand). A single [(PHOX)Ir(H)(2)(COD)](+) isomer was formed as the primary product at -40degreesC in THF. Subsequent reaction with H-2 at -40 to 0degreesC led to a mixture of two diastereomeric [(PHOX)Ir(H)(2)(solvent)(2)](+) complexes with concomitant loss of cyclooctane. The stereochemistry of the three hydride complexes could be assigned from the NMR data. The structures and energies of the observed hydride complexes and the possible stereoisomers were calculated using density functional theory. The substantial energy differences (up to 39 kcal/mol) between the various stereoisomers demonstrate the strong influence of the chiral ligand. The observed stereoselective formation of dihydride complexes can be explained by steric effects of the PHOX ligand combined with a strong electronic influence of the coordinating N and P atoms, favoring addition of a hydride trans to the Ir-N bond.