The reaction of cis-Re(CO) 4 (PR 3 )Me (R=Ph, Cy) with the Lewis acid B(C 6 F 5 ) 3 was studied by NMR spectroscopy, and was found to provide an equilibrium mixture of the solvent-coordinated complex [cis-Re(CO) 4 (PR 3 )(ClCH 2 Cl)][MeB(C 6 F 5 ) 3 ] and the reactants. Reaction of cis-Re(CO) 4 (PR 3 )Me with HX (X=H, SiEt 3 ) in the presence of B(C 6 F 5 ) 3 at low temperature yielded the σ-bonded complexes [cis-Re(CO) 4 (PR 3 )(η 2 -HX)][MeB(C 6 F 5 ) 3 ] which decomposed at room temperature via intramolecular heterolytic cleavage of the X-H bond to produce MeX and the hydride-bridged dimer [cis-Re(CO) 4 (PR 3 )] 2 (μ-H){MeB(C 6 F 5 ) 3 }. The unstable H 2 binding and cleavage on this and other highly electrophilic organometallic complexes that contain strong π-acceptor CO ligands can be related to the structure and function of metalloenzymes such as Fe-containing hydrogenases that catalyze H 2 2H + +2e - . The latter contain dinuclear organometallic-like active sites with CO ligands, which would promote binding and heterolytic cleavage of molecular H 2 in biological systems. The Fe-Fe and Ni-Fe bonds in hydrogenases are likely sites for protonation to form a bridging hydride as the initial step in the mechanism of H 2 formation, and electrophilic fragments such as [Re(CO) 4 (L)] + strongly prefer to form bridging rather than terminal hydrides. The reaction of olefins and Et 3 SiH with the [cis-Re(CO) 4 (PR 3 )(ClCH 2 Cl)][BAr F ] (BAr F =B[3,5-(CF 3 ) 2 (C 6 H 3 ) 4 - ]) system was also investigated, and the X-ray crystal structure of the dimer {[cis-Re(CO) 4 (PPh 3 )] 2 (μ-H)}{BAr F } was determined.