Formation of M-H bond

It should be emphasized that these two schemes are principally different. The second mechanism corresponds to a situation in which the formation of M- H bonds precedes the full proton transfer, while dihydrogen bonds can be formed only via direct protonation of the hydridic hydrogen. In addition, the protonation of the metal center can be slow at very fast transformation to the dihydrogen complex, masking detection of the dihydride molecule. Therefore, first, it is necessary to show what hydride center is protonated faster. 

A complex reducing agent was prepared from NaH, RONa and nickel(II) acetate This catalyst (referred to as Nic), similarly to the P-1 and P-2 nickel catalysts, is a selective catalyst in diene reductions. The reactive parts of Nic are metal hydrides and the key step in the hydrogenation is the formation of M—H bonds. The sodium salt of the alcohol added plays an important role as an activating agent in reductions using Nic. Whereas P-1 and P-2 nickels are selective and sensitive to the double-bond structure and show a rather low propensity toward isomerization, Nic has no propensity toward disproportionation. 

Formate production has also been reported for electropolymerized films of [Co(4-vinylterpyridine)2] " on glassy carbon electrodes in dimethylformamide solutions [63]. Interestingly, the product of this same catalytic system in aqueous solutions is formaldehyde [81]. Other heterogeneous systems that produce formate include Cd, Sn, Pb, In, and Zn electrodes in aqueous media [12] (see also Vol VII 5.2.3). It is likely that the pathway to formate formation on metal electrodes follows the sequence of M—H bond formation followed by CO2 insertion to form a M—0C(0)H species followed by desorption from the electrode surface. 

J. O. Metzger, Reactions of Radicals with Formation of C,H-Bond, in Methoden Org. Chem. (Houben-Weyl) 4th ed. 1952-, C-Radicals (M. Regitz, B. Giese, Eds.), Vol. E19a, 147, Georg Thieme Verlag, Stuttgart, 1989. 

A. M. El-Khawaga, H. M. R. Hoffmann, Formation of C-H Bonds by the Reduction of C=C Double Bonds and of Carbonyl Groups with Metals ( Dissolving Metal Reduction ), in Stereoselective Synthesis (Houben-Weyl) 4th ed., 1996, (G. Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann, Eds.), 1996, Vol. E21 (Workbench Edition), 7, 3967—3987, Georg Thieme Verlag, Stuttgart. 

Liquid alcohols display an extremely broad absorption near 650 cm (half-width 200-300 cm"" ). Stuart and Sutherland recognized the importance of this band and have attributed it to the hindered rotation perturbed by the formation of the H bond (1967, 1966). They find this band centered at 670 cm"" for pure liquid methanol, and at 475 cm"" for methanol-. The ratio of these frequencies, 1.41, is characteristic of a motion dominated by hydrogen atom movement. The band is present in CCI4 and CS2 solutions of methanol at concentrations above 1 M and is very much weaker at concentrations below 0.1 M. In spectra of dilute solutions, the absorption is presumed to shift below the limit of observation, to about 350 cm . The over-all spectral behavior is as striking and unusual as that of the stretching mode, as shown in Fig. 3-26. In sharp contrast are the relatively minor spectral... 

Formation of M-H complexes by refluxing metal halides, or complex halides, with alcohols in the presence of stablizing hands occurs via formation of a metal alkoxide followed by j -hydride elimination . This reaction represents the reverse of insertion of an aldehyde or ketone into an M-H bond. 

The physical origin of this difference is a pure quantum effect that comes from the gain in zero vibrational energy of the vibration between the H-bonded state and the free state that enters the enthalpy of formation of the H-bond and is different in H- and D-bonds. This quantity is negative because, as illustrated in Novak s curve of Figure 4.5, the difference between terms 1 and 3 of eq. (7.7) is negative the wavenumbers of bands, which are related to the m s by the following eq. (7.8) that takes into account eq. (1.A3),... 

The basic equations that rule the mechanics of H-bonds are developed in this appendix. They have been already established in the appendix of Ch. 5 but take here a slightly different form that makes the role of the mass m of the H-atom more apparent, in view of predicting effects of an H/D substitution. The formation of an H-bond is the result of an electrostatic interaction between the electrons and the nuclei of two molecules X-H and Y. Molecules are quantum objects that are ruled by an Hamiltonian H that depends on the coordinates r of electrons, q of the H(D)-atom that establishes an H(D)-bond and Q that defines the relative positions of the two molecular components X-H and Y. r stands for aU coordinates of all electrons e. The relative coordinates q and Q of the nuclei are defined in Figure 2.1. Q stands for all three intermonomer coordinates Q, Qg and defined in this figure. The quantum description is necessary for this H-bond, because a classical description fails to describe any chemical bond. This Hamiltonian H writes ... 

Although [M°(por)] (M = Rh, Ir) species are obviously not organometaUic in nature themselves, they reveal an interesting reactivity pattern toward a variety of substrates (olefins, CO/H2, isocyanides, C—H bonds) with formation of M—C bonds. 

The characteristics of an effective chain transfer catalyst are now clear [75]. First, M must be stable under the conditions of polymerization. Second, the metal center must be crowded enough to discourage (i) the dimerization of M", (ii) the formation of a bond between M and the chain-carrying radical R, and (iii) the hydrogenation of R by the transfer of H from M-H. (Dimerization of M will interfere with its ability to abstract H from R formation of M-R bonds will also deplete M transfer of H to R will terminate growing chains.) Finally, the BDE of the M-H bond in the corresponding hydride should be as close as possible to the BDE of the C-H bond in the chain-carrying radical. (If the substrate is methyl methacrylate, the C-H BDE formed by the H transfer in Equation 1.26 will be SOkcalmol- [51, 52].)... 

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