Lability of complexes

The most fundamental approach to assessing lability of complexes is by determination of the rate of isotopic exchange reactions. In the technetium-complex systems, no study of the exchange reaction on the central metal ion has been reported, but several reports have been published on isotopic exchange by ligand substitution. 

The greater lability of complex 146.C (compared to 145.c), as evinced by the much shorter reaction time, is typical of those that bear a carbomethoxy or acetyl substituent at the central carbon of an i73-allylic ligand. The temperature required for complete decarbonylation of complexes of type 146 and 148 increases with the size of the R-substituent, which suggests a mechanism involving hydride transfer.111 This would also explain the observed activating effect of the centrally located carbomethoxy group in 146.C, which would clearly labilize the methyl proton shown explicitly in 146. 

This analysis was made possible by our ability to measure CPL from racemic solutions of lanthanide complexes through use of circularly polarized excitation [12,45-47]. The varied lifetime of the lanthanide (III) ions allow for some insight concerning the lability of complexes of this type. For example, we have shown that although no CPL is detected in the luminescence from tris-terdentate complexes of Tb(III) and Eu(III) with oxydiacetate (ODA), CPL is observed from DytDPA -, indicating that, indeed, this complex is D3 in aqueous solution, but that only for the short lived Dy(III) ion (x = 20 psec) is the photoprepared enantiomerically-enriched excited state maintained throughout the emission lifetime [45]. 

Pearson, R.G. Crystal field theory and the lability of complexes of the first transition series. Fed. Proc. 20, 14-17 (1961)... 

The more the equilibrium Eq. (1.1) is shifted to the right, the more stable are the complexes. This concept oistability of complexes should not be confused with lability of complexes, which depends on the rate of direct and reverse reactions the larger the rate constants of these reactions are, the sooner the state of equilibrium is reached and the more labile the complex formed is. As values depend on the ratio of the rate constants only, complexes with the same may be both labile and inert. 

Table 1 Effect of some factors on lability of complexes Factor Comments...

Mention factors affecting the lability of complexes in addition to the electronic configuration of the central metal ion forming the complexes, giving one example of each. 

---