Bond energies transition-metal-hydrogen

In this contribution we shall present several applications of the new method, which we shall refer to as LSD/NL, to the calculation of bond energies of transition metal complexes. We shall focus on trends along a transition period and/or down a transition triad. The following subjects will be discussed a) metal-metal bonds in dimers of the group 6 transition metals b) metal-ligand bonds in early and late transition metal complexes c) the relative strength of metal-hydrogen and metal-methyl bond in transition metal complexes d) the metal-carbonyl bond in hexa- penta-and tetra-carbonyl complexes. 

Included in the published account of the plenary lectures presented at the International Conference on Chemical Thermodynamics held during 1986 is an interesting article on metal-ligand bond energies in organometallic compounds, which includes data on metal carbonyls. Relevant n.m.r. data are to be found in two different sources,and ion-pairing effects on the structures and reactivities of metal carbonyl anions have been described. A timely review of the photochemistry of M-M bonds deals almost exclusively with metal carbonyl derivatives these also feature in articles on transition metal-hydrogen bonds. ... 

DFT calculations offer a good compromise between speed and accuracy. They are well suited for problem molecules such as transition metal complexes. This feature has revolutionized computational inorganic chemistry. DFT often underestimates activation energies and many functionals reproduce hydrogen bonds poorly. Weak van der Waals interactions (dispersion) are not reproduced by DFT a weakness that is shared with current semi-empirical MO techniques. 

Hydrogen gas chemisorbs on the surface of many metals in an important step for many catalytic reactions. A method for estimating the heat of hydrogen chemisorption on transition metals has been developed (67). These values and metal—hydrogen bond energies for 21 transition metals are available (67). 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

It is known that on transition metals, dissociation of H2 occurs readily, producing hydrogen adatoms that recombine and desorb as H2 at temperature between 300 and 900 K [9] On the other hand, a substantial activation barrier for H-H bond cleavage makes difficult the dissociation of H2 on noble-metal surfaces [9]. The effect of Pt or Pd catalysts is known to increase activity through the reduction of the activation energy [10,11]. Therefore, strategic addition of a catalyst has the potential to reduce the peak desorption temperature. 

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