Solution Homolytic Bond Dissociation Energies of Organotransition-Metal Hydrides

Article abstract of DOI:10.1021/ja00199a034

The homolytic bond dissociation energies (BDEs) of the mononuclear metal carbonyl hydride complexes (η⁵-C5H5)M(CO)3H (M = Cr, Mo, W), (η⁵-C5Me5)Mo(CO)3H, (η⁵-C5H5)W(CO)2(PMe3)H, (η⁵-C5H5)M(CO)2H (M = Fe, Ru), H2Fe(CO)4, Mn(CO)4PPh3H, Mn(CO)5H, Re(CO)5H, and Co(CO)3LH (L = CO, PPh3, P(OPh)3) have been estimated in acetonitrile solution by the use of a thermochemical cycle that reguires knowledge of the metal hydride pK_a and the oxidation potential of its conjugate base (anion).The BDE values obtained by this method fall in the range 50-67 kcal/mol.In mostcases, these results agree well with literature data.Our data provide strong support for the common assumption that the M-H bond energies are greater for third-row and for second-row metals than for first-row metals, the difference being 5-11 kcal/mol.Effects of neither phosphine or phosphite substitution nor permethylation of the cyclopentadienyl ring on the M-H bond energies could be detected within the error limits of the method.The results are discussed in relation to previous M-H BDE estimates and metal hydride reactivity patterns.