Strength of M-H Bonds

Some results from the use of Equation 3.126 were given previously in Table 3.1. Such cycles, used for C-H bonds by Breslow and BordweU, involve thermodynamic param- 

Bond dissociation energies have also been obtained (Table 3.3) from calorimetric measurements of the heat of protonation of neutral complexes with CF3SO3H in dichlo-roethane. These heats correlate linearly with differences in pK (even differences in pK in another solvent, CH3CN) and thus can be used along with measured electrochemical potentials to estimate bond dissociation energies (Equation 3.127). 

M-H bond strengths increase from first to second to third row in the periodic table first second third row. The introduction of electron-donating substituents (i.e., the replacement of a carbonyl by a phosphine or phosphite) has little effect on the M-H bond strength, as the effect on the pX largely cancels that on the oxidation potential. -  

The sum of the total bond strengths for a bridging hydride can be greater than the strength of a terminal M-H bond. Vites and Fehlner have demonstrated that the interaction energy of the three atoms in the Fe(fx-H)Fe unit is about 83 kcal/mol. 

Comparison of M-H bond strengths with M-C bond strengths allows one to predict the energetics of -hydrogen elimination and thus the stability of alkyl ligands. BDE(M-H) 

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The strength of M-H bonds has considerable influence on the rate of ttieir H transfer reactions, although steric effects are substantial. Table 3.4 lists the rate constants, for H transfer from various hydride complexes to Ar C (Ar = p-ferf-butylphenyl) at 25 °C. Replacement of Cp by Cp, which the data in Table 3.1 show to have little effect on the Mo-H bond strength, decreases by a factor of 40. 

The relative bond strengths of M-H and M-CH3 are of great importance in applications of transition metals to catalysis. 

Reviews during late 1988 and 1989 relevant to metal-hydrides include a treatment of H2 oxidative addition and dihydride transfer reactions involving [Ir(X)(CO)L2]. Stereochemical aspects are emphasized. A discussion of polynuclear iridium hydrides has also appeared. The relative bond strengths of M—H... 

Potential sources of error in the determination of M-H bond strengths by this method have been reviewed in Eisenberg, D. C Norton, J. R. Isr.. Chem. 1991,31,55, and the relative and absolute uncertainties in such bond strengths have been discussed in Qtoi, ]. Pulling, M. E. Smith, D. M. Norton, J. R. J. Am. Chem. Soc. 2008,130, 4250. 

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

There is almost no reliable information about the strength of metal-hydrogen bonds in organometallic compounds. Earlier (Section 2.2.) it was established that M-H bonds formed by (spontaneous) -elimination from metal alkyls should be stronger that their precursor, but this is an unsatisfactory and imprecise position. The dissociation... 

D. L. S. Brown, J. A. Connor, M. Y. Leung, M. I. Paz-Andrade, H. A. Skinner. The Enthalpies of Thermal Decomposition of Iron-Olefin Complexes, and the Strengths of Iron-Olefin Bonds. J. Organometal. Chem. 1976, 770,79-89. 

In contrast to solutions, because of the proximity of molecules in the solid state, the existence of M- -H intramolecular hydrogen bonding or its absence is controlled by competition between intermolecular classical bonds O-H- O and intramolecular interactions O-H- -M. It has been found that M- H bonding is more effective when the proton-accepting strength of the metal atom increases. 

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