The reactions of transition metal (TM) hydrides (Cp)Re(H)(NO)(CO), (Cp)Ru(H)(CO)(PH3), and (Cp)Re(H)(NO)(PH3) with poor, moderate, and strong proton donors HR (H2O, HOCF3, and H3O+) are studied using DFT B3PW91. The reaction pathway depends on the relative proton-attracting powers of the TM and hydride H atoms, as well as on the proton donor ability of HR. In the case where these two atoms have comparable basicity, (Cp)Re(H)(NO)(CO) forms an intermolecular H ... H bonding intermediate upon reaction with both poor and strong proton donors H2O and H3O+. This is followed by rearrangement to eta(2)-H-2 by proton transfer over a very small barrier. The reaction of (Cp)Ru(H)(CO)(PH3) with its highly nucleophilic hydride yields H ... H bonding complexes with moderate proton donor HOCF3, whereas the strong donor H3O+ produces only the eta(2)-H-2 structure. Rapid rearrangement of eta(2)-H-2 to cis-dihydride is possible although the trans-dihydride is mon stable. For both types of hydride, a reaction pathway through a H ... H bonding complex is preferred over direct interaction of HR with the TM atom, forming the corresponding dihydride. The latter pathway is favored for (Cp)Re(H)(NO)(PH3), where the TM atom is the more basic. In this case cis- and trans-dihydride complexes form upon reaction with H3O+ without any H ... H and eta(2)-H-2 intermediates. Although the trans-structure is more stable than the cis-, a PH3 ligand favors the cis-direction of H3O+ attack by recoordination of H2O from the hydride atom to a hydrogen of the PH3 group.