Specific effects of the hydride ligand in the coordination of the hydrogen molecule to metallic centers are studied by means of ab initio MO calculations in the model system [Fe(PH3)4H(H2)]+. The two different isomers of this complex (cis and trans) are optimized, several orientations of the hydrogen molecule being considered for the cis isomer. In contrast to phosphine, the hydride ligand favors the coordination of hydrogen to the metal both in the cis and trans positions. When the hydride is placed in the eis position and oriented suitably, there is a strong attractive effect between it and the molecular hydrogen. Discussion of the obtained results is carried out in comparison with the available experimental data. © 1991, American Chemical Society. All rights reserved.