[M(diphosphine)(2)](2+) complexes (where M = Ni and Pt) react with hydrogen in the presence of bases to form the corresponding hydrides, [HM(diphosphine)(2)](+). In seven cases, equilibria have been observed from which the hydride donor ability (DeltaGdegrees(H-)) of the hydrides can be calculated. For six of these complexes, the DeltaGdegrees(H-) values calculated using heterolytic activation of hydrogen are compared with those based on thermodynamic cycles using pK(a) measurements and electrochemical half-wave potentials. The agreement between these two methods is good (within 1 kcal/mol). The reactivity of the various [M(diphosphine)(2)](2+) complexes toward hydrogen parallels their measured hydride acceptor abilities.