Photolysis of solutions of M(CO)(6) (M = Cr, W) at low temperature in the presence of hydrogen gas affords Cr(CO)(5)(H-2) (1) and W(CO)(5)(H-2) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T-1 values for the hydride resonances and the observation of a large HD coupling in the HD derivatives. Irradiation of a phosphine-substituted derivative (PMe3)Cr(CO)(5) in the presence of hydrogen gas gave similar results. Thus cis-(PMe3)Cr(CO)(4)(H-2) (3) and trans-(PMe3)Cr(CO)(4)(H-2) (4) were prepared and characterized by H-1 and P-31 NMR spectroscopy. When the photolysis reactions were carried out in methylene chloride, solvent binding competitive with hydrogen binding was observed. This was not observed in less coordinating solvents such as alkanes. Subsequent displacement of solvent by H-2 leads to the clihydrogen complexes. Complexes 1 and 2 are moderately acidic, with deprotonation effected by mild bases.