Comparisons multiple bonds

Compared to the sum of covalent radii, metal-silicon single bonds are significantly shortened. This phenomenon is explained by a partial multiple bonding between the metal and silicon [62]. A comparison of several metal complexes throughout the periodic table shows that the largest effects occur with the heaviest metals. However, conclusions drawn concerning the thermodynamic stability of the respective M —Si bonds should be considered with some reservation [146], since in most cases the compared metals show neither the same coordination geometries nor the same oxidation states. 

During this, the electrons of the partial X—Z multiple bond are used. Experiments show that the ester can be further active in the polymerization. Its reactivity, however, is reduced in comparison with ion pairs. From a mechanistical point of view, the chain propagation should proceed in the manner of a SN2 reaction, that is with the monomer as nucleophile and the ester as substrate. With the assistance of quantum chemical calculations using the CNDO/2 method, the differences between covalent species and free ions should be examined. The following contains the three types of anions used ... 

Comparison of the Domain and Orbital Models of Multiple Bonds... 

Today two directions of research are of interest On the one hand, investigations on the reactivity of basic systems are important to elucidate the typical" Si=E-multiple bond properties, in particular with respect to their use as synthons in organo silicon chemistry without being hampered in their synthetical potential by bulky substituents in this context, a comparison on their reactivity with the carbon analogues is still attractive. On the other hand, the isolation of new stable unsaturated silicon compounds and their structure determination continues to be of interest for quite a number of research groups worldwide. 

The first-row homonuclear diatomic molecules A2 of main-group elements (A = B, C, N, O, F) exhibit a well-known diversity of ground-state multiplicities, bond lengths, and bond energies. Calculated potential-energy curves for low-lying singlet and triplet states of these species are pictured in Fig. 3.27 and summarized in Table 3.13 (with comparison experimental values). Because these homonuclear... 

Table 3.13. Calculated spin multiplicity, bond length Re, and dissociation energy De of first-row homonuclear diatomic molecules, with comparison experimental valued1 in parentheses...

Although addition of organoiridium compounds to carbonyls and CN multiple bonds has been little investigated in comparison with those of organorhodium compounds, several characteristic reactions have been disclosed recently. 

Bond Lengths Bond lengths and multiple bonding were discussed in Chapter 5, and a comparison of... 

A detailed account of the descriptive chemistry of the heavier transition metals is beyond the scope of this book. Many aspects of the chemistry of these elements such as metal-metal multiple bonds, metal clusters, organometallic chemistry, and coordination chemistry are discussed in other chapters. The present discussion will be limited to a comparison of the similarities and differences of the heavier metals and their lighter congeners. 

The decrease in bonding order arising from delocalization of multiple bonds in conjugated systems results in a shielding of the central carbon atoms. This can be clearly seen by comparison of the pairs 1-butene/1,3-butadiene and ris-3-octene/ri.s-ris-3,5-octadiene ... 

A useful comparison of the reactivities of boranes and metal hydrides toward various types of multiple bonds is given in Table 16-6. 

The comparison between C and Si in the previous paragraph also holds good for N and P, and for O and S. However, a greater tendency to form multiple bonds can be discerned as we move towards the right-hand side of the Period Na— Cl. As the atoms contract in size across the Period, p -p overlap is improved. C=S bonds are far more common than C=Si or C=P. Table 6.2 shows that C=S is about twice as strong as C—S. 

The carbon-heteroatom (C-E) bonds of the heavier heterobenzenes are quite long in comparison to the C-C bond distances. See Table 1. Nevertheless, the C-E bonds are shorter than normal C-E single bonds. The E-P bond of phosphabenzene (1.73 A) is 0.14 A shorter than the sum of its covalent radii, while in both arsabenzene (1.85 A) and stibabenzene (2.05 A), the heteroatom-carbon bonds are 0.13 A shorter. Extrapolation to bismabenzene suggests that the Bi-C bond distance is close to 2.16 A. Obviously the C-E bonds of the heterobenzenes have multiple bond character. 

In summary, it is noted that multiple bonding between the heavier Group 14 elements E (Ge, Sn, Pb) differs in nature in comparison with the conventional a and 7T covalent bonds in alkenes and alkynes. In an E=Ebond, both components are of the donor-acceptor type, and a formal E=E bond involves two donor-acceptor components plus a p-p n bond. There is also the complication that the bond order may be lowered when each E atom bears an unpaired electron or a lone pair. The simple bonding models provide a reasonable rationale for the marked difference in molecular geometries, as well as the gradation of bond properties in formally single, double and triple bonds, in compounds of carbon versus those of its heavier congeners. 

TABLE 13. Comparison of the base-strengthening effects of nitrogen and oxygen atoms attached to multiple bonds (H20, 25 °C)a b... 

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