Reactivity patterns, /-element

A further group of very stable (refractory) compounds is formed (as noticed in the comments to Table 5.18) with the elements at the far right part of the Periodic Table. Within the general reactivity pattern of uranium (Fig. 5.14), the seemingly irregular behaviour shown by the U-Ag system (2nd box in the 11th column) may... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Ketones, specifically 2-alkenones and 2-cycloalkenones, have been used extensively as substrates for tandem vicinal difunctionalization, allowing delineation of various reactivity patterns based upon the structural elements present in the enone. Aldehyde substrates have been used less widely comparison... 

The potential that HNCC offer is beginning to be realized. It is clear that they have properties and exhibit reaction patterns that differ markedly from their mononuclear counterparts and even low-nuclearity clusters. It is also apparent that their reactivity patterns may be rationalized in simplistic terms and thereby extended to other systems. However, much work remains to be done, particularly in designed synthesis and studies of reaction mechanisms. The ability of these clusters to combine with important small substrates such as CO and Hj need also to be explored in much more detail. The study of the reactivity of large mixed-metal systems, which, as expected, exhibit enhanced and modified reactivities, equally requires more detailed investigation. In fact it would be useful to have available HNCC, which contain early and late transition metal elements, in order to combine both Lewis basic and Lewis... 

The comparison of propene, allyltrimethylsilane, and isobutene indicates, that introduction of a trimethyl silyl group in /3-position of the developing carbenium center activates more than a methyl group in a-position. Both series of triphenyl element compounds (left and right column Scheme 44) show the reactivity pattern Si < Ge < Sn, but variation of the substituents at silicon and tin was found to largely affect the reactivity of the double bond. While in the allyl series (right column), the trialkylsilanes and -stannanes are 2 to 3 orders of magnitude more reactive than the... 

The thesis that the lanthanide elements offer something unique to organometallic chemistry has been proven in a variety of ways. New classes of complexes, unusual structural tj es, and novel reactivity patterns have been observed with these elements. In addition, the special properties of the lanthanide elements have allowed major contributions to be made to our knowledge of a variety of fundamental organometallic reactions of general interest including the polymerization of alkenes, the activation of C—H and H—H bonds, and the reduction of CO. 

To achieve a clean addition of the elements of hypobromous acid (Br and OH ) it is advantageous to use reagents such as fV-bromosuccinimide 19 as the source of Br in an aqueous medium. So we see, the reactions given in the textbooks to illustrate the characteristic reactivity patterns of functional groups and the synthetic methods elaborated to realize their potential in practice can be vastly different. 

The close structural similarity between metal clusters and elemental metals leads one to wonder at what size do metal clusters possess physicochemical properties generally associated with metals. Furthermore, given the fact that metal surfaces are important in catalysis, there is considerable interest in determining whether large transition metal clusters will be good models for chemical and physical phenomena at metal surfaces. The essential question, stated imprecisely, is how will increasing the metal-core size affect the electronic structure and reactivity patterns of transition metal cluster compounds ... 

The chemistry of transition metals, lanthanides and actinides is significantly influenced by relativistic effects. Qualitatively, these effects become apparent in the comparison of certain structural properties or reactivity patterns for a group of metals, for example, trends in the chemistry of copper, silver and gold. Quantification of relativistic effects can, however, only be achieved by relating the experimental findings to the results of adequate ab initio studies. Reference to theory is required because nonrelativistic properties cannot be probed directly. Thus, elements behave relativis-tically in any kind of experiment, whether one deals with the spectrum of Hj or the properties of transuranium compounds. 

In contrast to the relatively stable C-H, C-O, C-S, C-N and C-Hal bonds, the analogous silicon-element bonds (Si-H, Si-O, Si-S, Si-N, Si—Hal) normally undergo hydrolytic cleavage reactions under physiological conditions (-> Si-OH). This reactivity pattern causes a stability-related limitation of the sila-substitution approach. A further restriction results from the inability of the silicon atom to form stable silicon-element multiple bonds of the (p-p)7i type. For instance, in contrast to the C=0, C=C and C=C bonds, the analogous Si=0, Si=C and Si=C bonds are not stable under ordinary conditions. 

Changing the heteroatom to oxygen (the element between nitrogen and fluorine), it was intriguing to observe that it exhibited reactivity patterns of both its neighbors [4, 8]. Conducting the reaction of methoxytris(trimethylsilyl)silane with potassium te/t-butoxide in benzene in the presence of 18-crown-6 led to the formation of the a-alkoxysilyl anion 2. The compound was meta-stable at room temperature and displayed slow self-condensation. But it could be derivatized with electrophiles like ethyl bromide and trimethylchlorosilane to give the respective methoxysilanes. 

Scientists have found patterns in the reactivity of elements. Metals are more reactive the farther down the column, or group, you go. In Group 1, for example, potassium is more reactive than sodium and lithium. Nonmetals get less reactive as you move down the group. For example, fluorine is the most reactive of the halogens. Chlorine, which is below it, is less reactive. 

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