From Main Group Hydrides

Hydride complexes, particularly those of the early transition metals, are often prepared from main group hydrides (Equations 3.115, 3.116, 3.117, 3.118, and 3.119 0-Borohydride and aluminohydride complexes are known and have been observed as intermediates in a few reactions that form transition metal hydrides. 

CHAPTER 3 COVALENT (x-TYPE) LIGANDS BOUND THROUGH METAL-CARBON AND METAL-HYDROGEN BONDS 

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Another application of an isomerisation reaction can be found in the production of the third monomer that is used in the production of EPDM rubber, an elastomeric polymerisation product of Ethene, Propene and a Diene using vanadium chloride catalysts. The starting diene is made from vinylnorbomene via an isomerisation reaction using a titanium catalyst. The titanium catalyst is made from tetravalent salts and main group hydride reagents, according to patent literature. 

Carbon-carbon bond lengths for a selection of hydrocarbons obtained from molecular mechanics calculations, Hartree-Fock calculations, local density calculations, density functional calculations, MP2 calculations and semi-empirical calculations are compared with experimental distances in Table 5-3. The same basis sets considered for main-group hydrides are utilized here. Mean absolute errors for each method have also been tabulated. 

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. 

In the case of the main-group hydrides, the basicity of the anions decreases in each group of the periodic system from lighter to heavier elements, whereas with the proton affinities of the neutrals a reversal is observed in the group of the periodic system. 

However, the average hybridization conforms closely to the expected sd (83.3% d-character) of equivalent idealized hybrids, consistent with the overall close match to idealized 63.4°, 116.6° valence angles as noted above. Figure 4.9 displays contour and surface plots of NBO (4.51) to illustrate how such sd -based NBOs differ from corresponding sp -based main-group hydride bonds considered previously. 

The main group duster chemistry discussed in this book can be considered to originate from two important, but apparently unrelated developments in inorganic chemistry in the 1930s. The first was the identification of the neutral boron hydrides by Stock [1]. The second was the observation by Zintl and co-workers [2-5] of anionic clusters formed from potentiometric titrations of post-transition metals (i.e., heavy main group elements) with sodium in liquid ammonia. 

Of course, main-group metal hydrides such as these cannot be incorporated into a useful catalytic scheme since they do not form readily from H2. The following, apparent criteria for a suitable catalyst are based on the above discussion. 

Alkali metal 1-methyl- and 1-phenyl-borinates are also available from bis(borinato)cobalt complexes (see below) on treatment with sodium or potassium cyanide in an aprotic solvent like acetonitrile. Cobalt cyanide precipitates and the alkali borinate remains in solution. After addition of thallium(I) chloride to some complexes, thallium 1-methyl- or 1-phenyl-borinate could be isolated as pale yellow solids, the only main group borinates isolated hitherto. They are insoluble in most organic solvents but readily soluble in pyridine and DMSO. The solids are stable on treatment with water and aqueous potassium hydride, but are decomposed by acids <78JOM(153)265). 

The increasingly electropositive character of main group 4 as tin is reached is evident from the fact that its hydrides are much less stable than... 

Allyl ketones can be prepared by trapping acylpalladium intermediates with main group metal compounds. The allyl ketone 251 can be prepared from the allyl ester 248 by trapping acylpalladium 249 with the alkylzinc compound 250 at room temperature and 1 atm [115]. Reaction of geranyl chloride (252) with the furylstannane 253 under CO pressure afforded the ketone 254, which was converted to dendrolasin (255) [93]. Aldehydes are prepared by trapping with a metal hydride. The /l,y-unsaturated aldehyde 257 is prepared by the carbonylation of the allyl chloride 256 in the presence of Bu3 SnH [116,117],... 

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