Position, group 4 metal positive charge substituents

La Mar (150) and Walker (156) have found a thermodynamic cis effect in the formation of hemichrome salts [Fe(TRP)L2]Cl (->-[27]) according to equilibrium (77) which was studied by 1H-NMR and optical spectroscopy for L = l-Melm (Table 22). As the electron-donating power of the para-phenyl-substituent of the porphyrin increases, the total formation constant, /J2, increases. This is because the product of the reaction contains a positively charged center which is stabilized by electron-donating groups. As a Hammet relation exists, the mesomeric part of the electronic transmission is also operative, and hence dative porphyrin-to-metal tr-bonding seems to be involved. 

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

Interaction of group 4 metal substituents with positive charge at the (3 position... 

Interaction of Group 4 Metal Substituents with Positive Charge at the (3... 

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