Organic and Hydride Chemistry of Transition Metals

Certain classical coordination complexes (see Coordination Complexes) of iron (e.g. Prussian blue) will be dealt with in other articles (see Iron Inorganic Coordination Chemistry and Cyanide Complexes of the Transition Metals), as will much of the chemistries of iron carbonyls (see Metal Carbonyls) and iron hydrides (see Hydrides) (see Carbonyl Complexes of the Transition Metals Transition Metal Carbonyls Infrared Spectra, and Hydride Complexes of the Transition Metals). The use of organoiron complexes as catalysts (see Catalysis) in organic transformations will be mentioned but will primarily be covered elsewhere (see Asymmetric Synthesis by Homogeneous Catalysis, and Organic Synthesis using Transition Metal Carbonyl Complexes). 

An important example of transition-metal (T-alkyl synthesis by hydrometalation is the synthesis of bis(cyclopentadienyl)alkylzirconium chlorides ( j -Cp)2ZrRCl from alkenes and biscyclopentadienylchlorozirconium hydride ( ACp)2ZrHCl. The facility and general applicability of this hydrometalation, coupled with the synthetic versatility of the resultant alkylzirconium reagents makes this hydrometalation an important synthetic method in organic chemistry. ... 

The study of the reactivity of transition metal hydride compounds towards unsaturated organic molecules, mainly olefines and alkynes, has been traditionally centered in monohydrides. In general, the reactions lead to the insertion products, alkyl or alkenyl, which have a limited chemistry. Furthermore, the generation of subsequent carbon-carbon or carbon-heteroatom bonds requires the presence of other active ligands, such as halogen or carbon monoxide, in the coordination sphere of the metallic center of the alkyl or alkenyl intermediates. 

The rates at which protons can be removed from transition metal hydrides (their "kinetic acidities") generally parallel their thermodynamic acidities pK values). However, the removal of a proton from a metal is much slower than the removal of a proton from an electronegative atom like nitrogen or oxygen. The reverse is also true the protonation of a metal (to form a hydride) is slower than the protonation of a nitrogen or an oxygen examples are shown in Equations 3.113 and 3.114. The low kinetic acidity of transition metal hydrides is much like that of carbon acids in organic chemistry. - - ... 

H transfer to organic radicals is a common reaction of transition metal hydride complexes. The reaction in Equation 3.128, which is parallel to RjSnH reductions in organic chemistry, is frequently used for the preparation of stable derivatives and for quantifying the amount of hydride present (by measuring the amount of CHClj formed). The mechanism is related to that of the radical chain substitution reactions discussed in Chapter 4. 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

Hydride complexes of the transition metals occupy a central role in contemporary chemistry, both because of their importance as catalytic or stoichiometric reagents for fundamental organic transformations (e g., the catalytic hydrogenation of unsaturated systems) and because of their chemical interest per se. 

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