Transition metal hydrides preparation

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. 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Comparisons to methods for transition metal acyl synthesis are instructive. Although many transition metal alkyls can be readily carbonylated to transition metal acyls, the carbonylation of transition metal hydrides to transition metal formyls has not been observed [Eq. (2)] (4, 5). As will be seen, many transition metal formyls are thermodynamically unstable with respect to transition metal hydrides and CO. Thus approach A-i (Scheme 1) is not preparatively useful. (See, however, Addendum, p. 34.)... 

Main group hydrides are of particular interest in themselves but are relevant to coordination chemistry only to the extent that they are used to prepare transition metal hydrides or act as ligands. 

All the metallic transition-metal hydrides can be prepared by direct reaction at elevated temperatures. The method has been used commonly to prepare polycrystalline material according to the exothermic reaction described by the above equation. 

General articles concerning transition metal hydrides,366 their crystallography,368 and on the preparation and properties of borohydride complexes369 are available. An overview on the use of HCo(CO)4 and related cobalt hydrides as catalysts in the hydroformylation of alkenes is available.367... 

A number of compounds having a direct zinc transition metal bond are known for long time, that is, zinc-bis(transition metal) such as Zn[Co(C04)]2 and transition metal zinc halides such as (CO)4Fe(ZnCl)2. However, organozinc-transition metal compounds RZn-TM are comparatively more recent. They can be prepared by hydrocarbon elimination between a diorganozinc derivative and a transition metal hydride (see Hydrides) (equation 35), but dialkyl- and diarylzinc compounds usually fail to react or react very slowly. Furthermore, when they do react they give unstable products that show a very strong tendency to disproportionate and cannot be isolated (equation 36). 

Few examples exist for preparing molecular transition-metal hydrides from the bulk metal. Hydride complexes of Co and Fe are prepared from the bulk metal, H and CO or a phosphine ligand [PFj, or o-C H fPEtjlj] at high T and P ... 

This is a common method for preparing transition-metal hydrides (see Table I) " . Borohydride reduction is complex, e.g., RujfCOjj with NaBH in THF gives over a dozen products, but a few useful mechanistic generalizations can be offered. Syntheses of metal hydrides proceed through intermediate borohydride complexes. Electron-pair bases (an ethereal solvent may be sufficient) are then necessary to complex and remove BHj. In a few examples the borohydride complexes are observable ... 

H. D. Kaesz, Inorg. Synth., 17, 52 (1977) a listing of all earlier preparations of transition-metal hydrides published in Inorg. Synth, as well as an introduction to newer preparations. Many of the preparations involve NaBH. ... 

Examples of Li MH use are found in reviews " " and in"" Table 1. Although LiAlH is a more powerful reducing agent than NaBH, the latter is used frequently in EtOH, while LiAlH requires ether. Comparison of Table 1 with the corresponding table in 1.10.7.1 shows that transition-metal hydrides can be prepared by the use of either reagent. 

Table 1. Preparations of Transition-Metal Hydrides by [AlHJ Reactions...

Table 1. Preparation of Transition-Metal Hydrides from Alcohols ...

When side reactions do not interfere, the use of alcohol and base conveniently yields metal hydride. Each of the first four reactions in Table 1 prepares a complex transition-metal hydride from a metal salt in a few minutes, but only with triphenylphosphine. 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

The hydride ion (H-) is an efficient small ligand in organometallic chemistry. The first transition metal hydrides were prepared using the Hieber base reaction, exemplified in (6.50). The hydroxide adds to the carbon of one CO ligand to produce an intermediate that rapidly loses carbon dioxide, leaving the hydride ion to occupy the coordination site. 

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