Hydride complexes specific metals

The purpose of this chapter is to survey very briefly the general features of the occurrence, preparations and properties of hydrides. Specific compounds will be covered under each element in Volumes 2-4. We have concentrated on transition metal hydrides but also briefly mention some features of main group hydride chemistry. Useful reviews have appeared on various aspects of metal hydride complexes. One by M. L. H. and J. C. Green in Comprehensive Inorganic Chemistry has very useful lists of compounds but dates from 1973. Teller and Bau" have covered the structural data on metal hydrides and give extensive tabulations of structural data. Humphries and Kaesz have considered cluster hydrides, especially in terms of their reactivity. Hlatky and Crabtree have reviewed polyhydrides. In each of these reviews, the authors have extensively tabulated the relevant data. We shall try to avoid duplication by emphasizing areas not previously covered. 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

The kinetics of this reaction have been studied in detail and a hydroxy-carbonyl is specifically proposed as an intermediate consistent with the kinetic data. Decomposition of this intermediate hydroxycarbonyl may proceed by -elimination of the platinum hydride product since the hydroxycarbonyl is a 16-electron coordinatively unsaturated complex. Another well-known example of metal hydride formation from CO and H20 is the reaction of iron carbonyl in aqueous alkali (55) (36). 

The above results are consistent with a steric specific syn 1,2-addition-elimination of metal hydride intermediate which is formed fast in a pre-equilibrium [MH] [MD] and adds to the olefinic substrate to form the metal alkyl intermediate (equation 261). The /1-hydride elimination of the most stable rotamer (equation 262) is the RDS in the rearrangement, leading to a metal hydride-product complex, which starts a new cycle faster than uncoordinated metal hydride. The protonated catalyst, 434, produces a precursor... 

Synthesis of carboorganosiloxane copolymers and polymers is based on hydride polyaddition of organohydrosiloxanes to organoalkenyl silanes [69], Recently, this reaction is of great interest in the field of obtaining complex monomers [70], as well as in the field of study of addition mechanisms on various Pt, Pd, Co and metal carbonyl catalysts [71-74] and specificity of actions of one or other catalytic systems [75], Besides carboorganosiloxane oligomers and linear polymers, other compo-unds were also synthesized by this method [76 -78],... 

Theoretically, in a simple kinetic resolution the ee value should not exceed 32 % at this specific conversion. In addition to the rhodium complex, this reaction requires acetophenone as stoichiometric hydride acceptor, phenanthroline as coligand and potassium hydroxide as base. An ee value of 98 % at 60 % conversion (theoretical value 67 %)is achieved with [Rh2(OAc)4] without an added base after 3 days. Surprisingly, the enzyme tolerates potassium hydroxide in amounts up to 20 mol% at elevated temperatures however, the enantiomeric excesses are somewhat lower than those obtained in an ordinary kinetic resolution. Unselective, base- or metal-catalyzed acylation might be the reason for the somewhat lower ee value. 

Specific examples of alkene insertions are the reactions of cationic hydrides [Cp2ZrH(L)]+ to give metal alkyls which may either be stabilized by agostic interactions, as in (21-XLII), or free of such interactions (21-XLIII), depending on L.180 The insertion of isobutene into M—C bonds of electron-deficient zirconium complexes was found to be reversible.109... 

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