Polarity of M-C bonds

Simply stated, metals (M), as defined above from the viewpoint of organic synthesis, can induce polarization of M +-C bonds to provide carbanionic species or carbon nucleophiles. Less well appreciated, but equally or perhaps even more important is their ability to readily and conveniently provide Lewis acidic sites for inducing a wide variety of synthetically useful reactions. Furthermore, it has been increasingly well recognized that the d-block transition metals represent a couple of dozen of elements that collectively exhibit some ultimately desirable chemical reactivities. In addition to providing (i) bonded nucleophiles and... 

Highlights in the chemistry of cyclopentadienyl compounds have been reviewed.65 Trends in the metallation energies of the gas-phase cyclopentadienyl and methyl compounds of the alkali metals have been studied by ab initio pseudopotential calculations. Whereas there is a smooth increase in polarity of M-(C5H5) bonds from Li to Cs, lithium appears to be less electronegative than sodium in methyl derivatives. The difference between C5H5 and CH3 derivatives is attributed to differences in covalent contributions to the M-C bonds. In solution or in the solid state these trends may be masked by the effects of solvation or crystal packing.66 The interaction between the alkali metal ions Li+-K+ and benzene has also been discussed.67... 

According to Pauling the polarity p of a chemical bond is the measure of its ionicity. It is related to the dipole moment /x by the equation p = iXi/d, where d is the interatomic distance. The polarity of M—X bonds in MX4 molecules is illustrated in Table 4. The polarity of a specific M—X bond increases significantly as M changes from C to Si, and it diminishes slightly on going from Si to Ge, Sn and Pb. At the same time, the polarity of a specific M—X bond decreases sharply as the atomic number of the halogen X increases. 

The variation of M-Cl distances in the trichlorides is very similar to the variation of M-C bond distances. To the extent that M-Cl bonds are shortened by polarity effects, the shortening appears to be nearly constant down the group. 

The organometaUic compounds can be functionaUy classified on the basis of the nature of M—C bond [1]. The extent of polarity in the bond can explain the structure and reactivity of the organometaUic compounds. 

The bond dipoles m Table 1 3 depend on the difference m electronegativity of the bonded atoms and on the bond distance The polarity of a C—H bond is relatively low substantially less than a C—O bond for example Don t lose sight of an even more important difference between a C—H bond and a C—O bond and that is the direction of the dipole moment In a C—H bond the electrons are drawn away from H toward C In a C—O bond electrons are drawn from C toward O As we 11 see m later chap ters the kinds of reactions that a substance undergoes can often be related to the size and direction of key bond dipoles... 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

If the starting material contains M-H or M-C bonds a further complication can arise due to the possibility of a CO2 insertion reaction. Thus, both [Ru(H)2(N2)(PPh3)3] and [Ru(H)2(PPh3)4] react to give the formate [Ru(H)(OOCH)(PPh3)3], and similar CO2 insertions into M-H are known for M = Co, Fe, Os, Ir, Pt. These normal insertion reactions are consistent with the expected bond polarities M +-H and 0 =C +=0, but occasionally abnormal insertion occurs to give metal carboxylic acids... 

Olefin polymerization by catalysts based on transition metal halogenides is usually designated as coordinated anionic, after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+ - C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds ( polar-type inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminum-polymer bonds. 

Perhaps dramatization of the cos 6 term by preparation of a system in which a positive charge resulted in shielding of a proton of appropriate geometry as in [1] or [2] would call attention to its importance. In each of these geometries the polarization of the C—H bond should result in an increase in the electron density around the proton, an increase in Oj and an upfield shift of about T3 p.p.m. at a distance of 3 A. ... 

The PMR spectra of the M(R2Dtc)3 complexes (M = As, Sb, Bi) in benzene show solvent shifts induced by the ring current of benzene, which are interpreted in terms of 1 1 van der Waals benzene--M(R2 Dtc) 3 adducts. The association constants for these adducts appear to parallel the polarity of the C-N bond in the R2Dtc ligands (422). 

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