Ab initio (B3LYP) calculations show that PD...H-ReH 4 (PH 3 ) 3 (PD = Proton donor) interactions follow the order PD = pyrrole > NH 3 > HC=CH> C 2 H 4 > CH 3 -H 0 and decrease with the pK a of the PD. For equivalent pK a 's, NH interacts more strongly than CH. However, intermolecular hydrogen-bonding of the M-H...H-C type is too weak to be detected experimentally in FTIR or UV-vis studies between ReH 5 (PPh 3 ) 3 and PhC=CH, C 6 F 5 H or PhCHCl 2 .