The new dihydrogen complexes trans-[MH(H-2)L(2)]BF4, L = P(C6H4-4-R)(2)CH2CH2P(C6H4-4-R)(2), R = CF3, M = Fe, Ru, and Os, R = CH3, M = Fe, R = OMe, M = Ru, are prepared by reaction of the dihydride complexes MH(2)L(2) With HBF4. The H-H bond length of the spinning H-2 ligand does not change significantly as a function of R (from CF3 to CH3 when M = Fe and from CF3 to OCH3 for M = Ru) according to H-1 NMR T-1 and (1)J(HD) measurements while there is a lengthening for the Os complexes. The rate constants for H atom exchange (reflecting the ease of homolytic splitting of H-2) increase with the increasing donor ability of R for a given metal as do the pK(a) values (reflecting a decrease in ease of heterolytic splitting). The electrochemical properties of some complexes MH(Cl)L(2) and MH(2)L(2) are reported. As expected H-2 acidity decreases as the parent hydride becomes easier to oxidize with this change in R (same M). The trend in dihydrogen pK(a) values as a function of the metal, Fe < Os < Ru, is distinctively different to the trend in pK(a) values of the dihydride complexes M(H)(2)(CO)(4), Fe < Ru < Os, and [M(C5H5)(H)(2)(PPh(3))(2)](+), Ru < Os, The high H-H bond energy of the RU(2+) complexes trans-[RuH(H-2)L(2)]BF4 is probably the reason why they are less acidic than corresponding Os2+ complexes. A consideration of the pK(a) values correctly indicated that a RuH(2)L(2)/[RuH(H-2)L(2)](+) mixture would be more effective at H/D exchange between D-2 and HO(t)Bu than the dihydrogen complex alone.