Transition-metal hydrides reactions with

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

Many more examples have been collected for the reaction of transition metal hydride complexes with 1,3-dienes, which appear to proceed via radical pair mechanisms, even without photochemical activation72-77. The following general mechanism has been assumed to be operative for the reaction of HMn(CO)572,73, HFe(CO)4SiCl374,75, HFe(CO)2Cp76 and HCo(CO)4 (H-[M]) (equation 18)77. 

The analogous reactions of Zn and Cd derivatives arc less well studied. Zinc alkyls ZnRi (R = Et, n-Bu) react with transition-metal hydride complexes, H2M(Cp-(M = Mo, W) ... 

The diversity of the substrates, catalysts, and reducing methods made it difficult to organize the material of this chapter. Thus, we have chosen an arrangement related to that used by Kaesz and Saillant [3] in their review on transition-metal hydrides - that is, we have classified the subject according to the applied reducing agents. Additional sections were devoted to the newer biomimetic and electrochemical reductions. Special attention was paid mainly to those methods which are of preparative value. Stoichiometric hydrogenations and model reactions will be discussed only in connection with the mechanisms. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

Most transition metal ions react with aliphatic amines by hydride abstraction and some second- and third-row metal ions react by dehydrogenation and de-methanation. This area has been adequately reviewed by Eller and Schwarz (9), but to highlight a specific example, the reaction of ammonia with bare Ti+ will be discussed. 

Thermodynamic data on the acidity of organometallic hydrides should help identify situations where apparent reactions of acidic transition-metal hydrides actually result from their conjugate bases. A case in which both species can react but give different products (as was pointed out by Prof. Espenson three years ago (18)) is the addition of hydridocobaloximes, HCo(dmgH)2B, to olefins with electron-withdrawing substituents... 

Another way in which a transition-metal hydride may conceivably interact with a substrate is by rate-determining proton transfer to the substrate itself, reaction 7. 

It should be both possible and profitable to divide all reactions of transition-metal hydrides into these three classes. Even concerted reactions, in which the metal interacts with the substrate at the same time as its hydrogen ligand does, can usefully be divided according to which of the above hydrogen transfer reactions they most resemble. 

Mdssbauer spectra of bonding and structure in, 15 184-187 reactions with diborane, 16 213 stabilization of, 5 17, 18-19 cyanates, 17 297, 298 cyanide complexes of, 8 143-144 cyclometallated bipyridine complex, 30 76 diazene complexes, 27 231-232 dinitrogen complexes, 27 215, 217 diphosphine complexes of, 14 208-219 dithiocarbamates, 23 253-254 -1,2-dithiolene complexes, 22 323-327 hydrogen bonding, 22 327 halide complexes with phosphine, etc., 6 25 hexaflouride, structure, 27 104 hydride complexes, 20 235, 248-281, see also Transition metal-hydride complexes... 

For reduced catalysts in solution, protonation competes with the reaction with CO2 (step 2 of Sch. 2). Protonation of the reduced metal results in the formation of a transition metal hydride, which may ultimately lead to the formation of hydrogen or formate. Protonation can be quite fast and favorable. For example, [Co (r c-Me6[14]4,ll-diene)]+ has binding constants of 2.5 x 10 and 2.5 x 10 pKa = 11.4) for the reaction with CO2 and protons, respectively [53]. Similarly, the rate constants for binding protons... 

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