Transition elements common reactions involving

Mercury-transition metal bonds have been described for all members of Groups V-VIII of the transition series except, apparently, technetium. They commonly involve a low oxidation state of the transition element and are particularly numerous for the chromium, iron and cobalt families.1 In addition, mercury-titanium bonded species have been postulated as unstable reaction intermediates.2... 

Iron, the most common transition element, has the electron configuration [Ar]4s 3(i. The two electrons in the highest energy level, 4s, are the ones most likely to be involved in the chemical reactions of iron. However, iron is a transition element and, like other transition elements, it has a partially filled d sublevel. Electrons in the d sublevel may also be involved in reactions. In this MiniLab, you will study some reactions of iron compounds and relate the results to the electron configuration of iron. 

Virtually every element in one form or another acts as catalyst for some chemical reaction. Since strenuous precautions are seldom taken to remove traces of ions, such as Cu2+ and Cl-, or other contaminants, notably oxygen and water, from reagents commonly used, many quite ordinary reactions may well be catalyzed to some extent by extraneous materials. In this Chapter we discuss certain catalytic reactions involving transition-metal complexes, mainly those in homogeneous solution that involve the formation of metal-to-carbon bonds. 

A wide range of p-block elements and transition metals have been incorporated into silicon-oxygen ring systems (heterocyclosiloxanes), primarily with a view to their use as precursors to Si-O polymers incorporating another element. The most common synthetic approaches to six-membered heterocyclosiloxanes containing another p-block element involve cyclocondensation reactions between 1,3-dichloro- or 1,3-dihydroxytetraalkyl/aryldisiloxane... 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Metals with a d f" configuration, group 3 metals, lanthanides, and actinides, are usually classified as f-elements. Because they are highly electropositive, they form polarized bonds with p-block elements, including carbon and nitrogen. So far, two reaction mechanisms have been established for d f metals cr-bond metathesis, a 2o—2o process, and 1,2-addition, a [2ct—2jt] process (2cr stands for the two electrons involved in the transition state that come from a tr bond and 2jt indicates the two electrons involved in the transition state that come from a Jt bond) (Scheme 1). " Oxidative addition, another type of reaction mecharusm that is common for late transition metals, is absent from the chemistry of rare-earth metals or actinides. This is partly because of the lack of valence electrons, i.e., a d electronic configuration however, even for uranium, which has multiple accessible oxidation states, no genuine oxidative addition reactivity has been reported. The subject of C—H bond activation mediated by f-elements has been dis-cussed by several recent reviews. ... 

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