Chemical reactivity and group trends

Chemical reactivity and group trends Table 10.2 Some physical properties of Group 14 elements 

Tin reacts readily with CI2 and Br2 in the cold and with F2 and I2 on warming to give 80X4. It reacts vigorously with heated S and Se, to form Sn and Sn chalcogenides depending on the proportions used, and with Te to form SnTe. 

Molten Pb reacts with the chalcogens to give PbS, PbSe and PbTe. 

The steady trend towards increasing stability of rather than M compounds in the sequence Ge, Sn, Pb is an example of the so-called inert-pair effect which is well established for the heavier post-transition metals. The discussion on p. 226 is relevant here. A notable exception is the organometallic chemistry of Sn and Pb which is almost entirely confined to the state 

The structural chemistry of the Group 14 elements affords abundant illustrations of the trends to be expected from increasing atomic size, increasing electropositivity and increasing tendency to form compounds, and these will become clear during the more detailed treatment of the chemistry in the succeeding sections. The often complicated stereochemistry of compounds (which arises from the presence of a nonbonding electron-pair on the metal) is 

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Chemical reactivity and trends Table 22.1 Some properties of Group 5 elements... 

Application of trends (1) and (2) can explain some obvious differences between chemical reactivity and structure in the main groups. 

The mathematical formulation and definition of GRDs provide information about the overall properties of the chemical systems and are also useful for analysing the thermodynamic aspects of chemical reactions. However, the GRDs caimot identify the reactive part or site of the molecule. Most of the chemical reactions are primarily concerned with the atoms, group of atoms or a specific site in a molecule. The important issues relate to the effect of charge or density fluctuations in chemical reactivity and the observed reactivity trends. The chemical reactivity for a system is determined by the sensitivity of electron density to the perturbations. Therefore, an appropriate definition was required in addition to the global descriptors [18]. A basic local descriptor is related to the change of the local electron density to the chemical potential of the system. This is known as local softness. 

Trends in chemical reactivity are also apparent, e.g. ease of hydrolysis tends to increase from the non-hydrolysing predominantly ionic halides, through the intermediate halides to the readily hydrolysable molecular halides. Reactivity depends both on the relative energies of M-X and M-0 bonds and also, frequently, on kinetic factors which may hinder or even prevent the occurrence of thermodynamically favourable reactions. Further trends become apparent within the various groups of halides and are discussed at appropriate points throughout the text. 

Periodic trends in ionization energy are linked to trends involving the reactivity of metals. In general, the chemical reactivity of metals increases down a group and decreases across a period. These trends, as well as a further trend from metallic to non-metallic properties across a period, and increasing metallic properties down a group, are shown in Table 3.1. 

Since this stereoelectronic effect is absent in the reactant minimum, it is unlikely that a general, intrinsic correlation between 51V chemical shifts and reactivities could exist when the substituents at vanadium are varied. Accordingly, a sharp increase in the barrier is computed when the second methoxy group is introduced (which must now be trans to the leaving methyl in the transition state), affording different trends for the barriers and the 8(51V) values upon successive methoxy substitution (12a). 

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