Enhancement of Ligand Polarisation - Intrinsic Effects

This phenomenon has been known since the investigations of Werner and is the basis for the formation of dated and oxolated complexes. The formation of basic salts and the acidity of aqueous iron(m) solutions provide further examples (Fig. 2-14). 

In all the cases above, the ligand is water and we are looking at the effect of co-ordination upon the first and second ionisation processes (Fig. 2-15). 

In the case of tripositive ions, this may be a significant effect and the pKa values of [A1(H20)6]3+ and [Fe(H20)6]3+ are 4.97 and 2.20, respectively. The biological, geological and technological importance of this phenomenon cannot be over-emphasised it is the 

Initial explanations in terms of an associative SN2-type reaction proved untenable, and the reaction is now thought to involve deprotonation of a co-ordinated amine (Fig. 2-17). This is the SN1 cb or Deb mechanism. The key step is the formation of the amide intermediate, [Co(NFl3)4(NH2)Cl]+, which undergoes halide loss to generate the reactive five-co-ordinate intermediate [Co(NH3)4(NH2)]2+ (2.2) (Fig. 2-18). 

Effects such as these may be transmitted a considerable distance through the molecule. Thus, the cation derived by the protonation of pyrazine (2.3) has a pKa of 0.6, and hence it is a relatively strong acid (Fig. 2-19). 

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