Pretransition Metals

Whitesides group provided a significant contribution on Grignard reagents and has continued its study on the mechanism of formation of these compounds. The rate of reaction of cyclopentyl bromide with magnesium in rotating disc experiments is mass-transfer controlled. 

Molle and Bauer have demonstrated that contrary to generally accepted opinion, the Barbier synthesis does not necessarily involve the formation in situ of a Grignard-type, organometallic compound. 

Et2AlCH2AlEt has been used as a probe to investigate the mechanism of the reaction of R3AI with a ketone in a 2 1 (as opposed to a 1 1) ratio. Evidence is obtained for the cyclic transition state containing a bridging R group, as in 

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Fig. 12. The relationship between the mean oceanic residence time, T, yr, and the seawater—cmstal rock partition ratio,, of the elements adapted from Ref. 29. , Pretransition metals I, transition metals , B-metals , nonmetals. Open symbols indicate T-values estimated from sedimentation rates. The sohd line indicates the linear regression fit, and the dashed curves show the Working-Hotelling confidence band at the 0.1% significance level. The horizontal broken line indicates the time required for one stirring revolution of the ocean, T. ...

The coordination chemistry of sea water represents a new and useful approach to understanding the chemical properties of sea water. The coordination chemistry of sea water differs from contemporary coordination chemistry in the following respects most complexes involve pretransition metals, most complexes are labile, the ligands are simpler (water, hydroxide, chloride, carbonate, sulfate), and time and space are important parameters. Principles of coordination chemistry are applied to contemporary research in marine science in four areas analysis of constituents of natural waters, the nature of metallic species in the oceans, the Red Tide problem, and carbonate geochemistry. 

Different Metals. Typically the coordination chemistry of the laboratory deals with the transition metals, usually of the d type, and the coordination chemistry of the metals of Groups I and II is commonly neglected. In contrast, the coordination chemistry of the ocean would, at first, seem to be the exclusive province of the pretransition metals this is seen clearly by looking at an average composition of an ocean (Table I) (36). 

A majority of the halides of the pretransition metals in groups 1 and 2 of the periodic table, as well as those of the lanthanides and actinides at the valence states of -1-2 and -1-3, are usually ionic compounds, whereas the nonmetals and metals with valency states >3 are usually covalent halides. 

Scope Non-transition metals include groups 1 and 2 of the. s-block elements, group 12, and /(-block elements in lower periods. Aluminum and the elements of groups 1 and 2 are classed as pretransition metals, the remaining ones as post-transition metals. 

Speaking very generally, the transition metals as a class form chelates that are more stable than those of the pretransition or post-transition metals (the same may, of course, be said about ordinary coordination compounds). In a large number of cases, a chelated complex is more stable than a complex that is similar but nonchelated thus the Zn(NH3) + complex is 8 percent dissociated in a 0.001 molar solution of Zn(NH .)4 (NOa)2 whereas the Zn(NH2CH2CH2NH2)2+ chelate is only 1 percent dissociated in a 0.001 molar solution of its nitrate. 

In order to discuss the physical meaning of a, we need to introduce the concept of early and late transition states. In the previous section we discussed in detail the transition state for CO dissociation over transition-metal surfaces and described the reaction as an example of a late transition state. The transition state is late along the reaction coordinate since the transition-state structure is close to the final dissociated state. Transition states which are early along the reaction coordinate are called early transition states and thus resemble the initial reaction states (see Chapters 4 and 7 for the definition of the pretransition state). The activation energies for the protonic zeolite reactions correlate with deprotonation energies (see Fig. 2.9) and are examples of intermediate transition states that also vary with the energies of the initial states . When a 0.5 (a 0), the transition state is early AS fv 0... 

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