Metal-hydrogen bond dissociation enthalpies in series of complexes of eight different transition metals

Homolytic M-H bond dissociation enthalpies (BDEs) of the mononuclear cationic metal hydride complexes HML(n)(+), where ML(n) = Cr(CO)(2)(dppm)(2), Mo(CO)(2)(L-L)(2), W(CO)(3)(PR(3))(3), W(CO)(2)(dppm)(2), W(CO)(3)(tripod), W(CO)(3)-(triphos), Cp*Re(CO)(2)(PR(3)), Fe(CO)(3)(PR(3))(2), Fe(CO)(3)(L-L), Cp*Ru-2, CpRu(PMe(3))(2)I, CpRu(L-L)H, CpRu(PPh(3))(2)H, Cp*Os-2, CpOs(PR(3))(2)Br, CpOs(PPh(3))(2)Cl, CpOs(PPh(3))(2)H, CpIr(CO)(PR(3)), CpIr(CS)(PPh(3)), (C(5)Me(n)H(5-n))Ir(COD), Cp*Ir(CO)(PR(3)), and Cp*Ir(CO)(2), have been estimated by use of a thermochemical cycle that requires a knowledge of the heats of protonation (Delta H-HM) and redox potentials (E(1/2)). for the oxidation of the neutral metal complexes (ML(n)). Excellent correlations were found between -Delta H-HM and E(1/2) within related series of complexes. The BDE values obtained by this method fall in the range 56-75 kcal/mol. For related complexes of a given metal, the energy required for homolytic M-H bond cleavage (BDE) increases linearly as -Delta H-HM for heterolytic M-H bond cleavage increases. For analogous complexes with different metals, the M-H BDE values are greater for third-row than second-row and first-row metals, the difference being 1-12 kcal/mol. Other trends in BDE values are also discussed.