Chelate rings trends

A parallel trend is observed for MgBr2-promoted additions of cis- and trans-crotyl tributylstannanes to a-benzyloxy aldehydes but the effect is much smaller (Table 9) [18], In such reactions the orientation of the allylic stannane and the chelated aldehyde is governed by steric effects in which the vinylic y-hydrogen orients over the five-membered chelate (Fig. 4). Support for this picture is provided by competition experiments in which y3,)8-dimethylallyl tributyltin was found to be markedly slower than the crotyl or allyl derivatives in additions to a-benzyloxypropanal. The observed rate decrease was attributed to the disfavored relationship of a vinylic methyl substituent with the chelate ring resulting in unfavorable steric interactions. 

What we find is that there is a preferred chelate ring size as the ring size rises, there is a rise in stability of the assembled complex, and then a fall as the ring continues to grow. This trend depends on a number of factors, such as what metal ion, what donor groups, and what ligand framework is involved. Nevertheless, for the common lighter metals (first row of the periodic d block) the trend is fairly consistent ... 

Overall, the five-membered chelate ring is preferred. We can actually measure this trend experimentally, such as for the series of O-donors in Figure 2.10 below. This is seen experimentally in terms of the stability of metal complexes (you can consider this as... 

This means the same trend in K values is seen regardless of ligand (unless some other special effect overrides it). This trend identifies copper(II) as the metal ion that forms the most stable complexes, irrespective of ligand. The series predicts a fall in K values in each direction from Cu2+, irrespective of ligand. The trend is preserved for the series of metal ions with didentate chelates even when the type of donor atom or the size of the chelate ring is varied (Figure 5.2). 

The enhanced stability of the five-membered chelate rings relative to those of larger size is also apparent in the trend of the thermodynamic parameters for the complexation of trivalent cations by a homologous series of polyaminocarboxylate ligands (Rizkalla et al. 1988,1989, Choppin and Muscatello 1985). These systems were analyzed, as described in section 3.2, in terms of dAH where... 

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