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* 2 Ru, CpRu(PMe 3 ) 2 I, CpRu(L−L)H, CpRu(PPh 3 ) 2 H, Cp* 2 Os, 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 (Δ H HM ) and redox potentials ( E 1/2 ) for the oxidation of the neutral metal complexes (ML n ). Excellent correlations were found between −Δ 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 −Δ 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.