Hydrogen bonds involving transition metal atoms

2 Hydrogen bonds involving transition metal atoms 

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Epstein, L. M., Krylov, A. N and Shubina, E. S., Novel types of hydrogen bonds involving transition metal atoms and proton transfer [MH] —A ), J. Mol. Struct. 322,... 

Each of these reactions involves an increase in the oxidation state of the central atom and the formation of additional bonds to that atom. In this chapter, the discussion of such oxad reactions will be limited to those involving transition metals in complexes. An interesting reaction of this type involving hydrogen is the following ... 

The activation of non-polar bonds by transition metals has been of major interest in the last quarter century. With H2, we have the simplest case possible and therefore the one that has been most extensively treated by theoretical methods. Reactions of H2 also tend to be the fastest among all the substrates considered in this chapter. The structures of polyhydrides formed on H2 addition include classical and non-classical forms, a topic that has excited much controversy because of the difficulty of structural characterization. Hydrides, whether formed by H2 or XH addition, are also involved in a wide variety of useful catalytic reactions from isotope exchange to alkane functionalization. The activation of XH bonds also allows formation of a wide variety of M-X bonds that, apart from their intrinsic interest, are also key intermediates in a very large number of catalytic reactions, such as hydrogenation, hydrosilation and various carbonylation reactions. The activation of X-X bonds, where neither group is a hydrogen atom is much more difficult, and except for cases where these bonds are either weak (e.g., Si-Si) or strained (biphenylene) there has been little work done. However,... 

The theory that the chemisorption bond is a covalence involving partially filled d-orbitals is not subject to quantitative test by calculation. Experimental correlations of heats of adsorption with the percentage d-character of the metal, particularly for adsorption of hydrogen on the transition metals, have led to taking the percentage d-character as a measure of unavailability of electrons in atomic d-orbitals and thus the expected strength of the chemisorption bond. For the alternative view in which the surface bond is... 

Hydrogen also exhibits two less common forms of bonding. Boron hydrides form a wide variety of compounds in which the hydrogen acts as a bridging species involving three-center two-electron bonds. Such species are said to be electron deficient because they do not have sufficient electrons for conventional two-electron covalent bonds. The second, less common, form is that of the coordinated hydrides in which the H" ion acts as a ligand bound to a transition metal atom. 

From intermediate C, the next step in the catalytic cycle involves a simple bond rotation to give D. This event is essential because it establishes the necessary syn relationship between a -hydrogen and the palladium atom. With a / -hydrogcn and the transition metal... 

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