Polydentate ligands, linear

Usually, there is a compensation effect, that is AHd —TAS11 so that AGd 0. Experimentally, AS1 is positive (especially for macrocyclic ligands), and so is very often A Hr (which means that usually, the Ln-L bonds are weaker than the Ln-OH2 ones) so that complexation reactions in water are entropy driven and, moreover, a linear relationship between AH and A. S 1 holds for the Ln(III) series of cations. One has, however, to be cautious when this approach is applied to polydentate ligands. The thermodynamic parameters may also reflect other factors such as the formation of stable 5-membered chelate rings. When another solvent is considered, the solvation enthalpy is much smaller than in water and the above considerations may no more hold. 

In cases where two or more identical ligands (or parts of a polydentate ligand) are involved, a superscript is used on k to indicate the number of such ligations. As mentioned above, any multiplicative prefixes for complex entities are presumed to operate on the K symbol as well. Thus, one uses the partial name ... bis(2-amino-KlV-ethyl)... and not ... bis(2-amino-K2iV-ethyl)... in Example 2 below. Examples 2 and 3 use tridentate chelation by the linear tetraamine ligand N,M-bis(2-aminoethyl)ethane- 1,2-diamine to illustrate these rules. 

Several methods have been used to distinguish between diastereoisomers in more complex systems. Thus, stereoisomers resulting from the coordination of linear tetradentate ligands have often been identified as trans, cis-a, or c/s-( S,10 and those resulting from coordination of macrocyclic tetradentate ligands have their own system.11 The scope of most of these nomenclatures is generally quite limited, but a proposal with wider application in the description of complexes of polydentate ligands has been made more recently.12... 

The technique also distinguishes between stereoisomers for complexes of higher polydentate ligands as indicated in Examples 4, 5 and 6 for linear tetradentate ligands such as N,A,-bis(2-aminoethyl)ethane- 1,2-diamine. In this case, the donor atom priority numbers in half of the tetradentate ligand have been primed. 

The metals which form square planar complexes belong to the Co(I), Ni(II), and Cu(III) triads. Although examples for each are known, the RW, Ir, Pd , Pt i, and Au form the most prevalent square planar complexes. Small linear or planar ligands are needed to achieve a close approach between molecules in a stack. Bulky ligands, for example, macroc lic polydentate ligands or substituted phosphines, amines, and sulfides, sterically hinder the close approach of molecules in a column. 

Figure 3. Chelate effect and size complementarity in linear polydentate ligands.

Besides the use of rigid bidentate ligands, linear chains can also be achieved by using chelating polydentate ligands. Pyrazine-2,6-dicarboxylic acid (L12) tends to promote... 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

Thus, in the next examples, we comment on supramolecular entities, including linear chains, two-dimensional sheets, and even three-dimensional networks. From them, perhaps the most common structural arrangement is that in which the metals form extended linear systems, usually built from mononuclear units, dinuclear, using polydentate donor ligands and polymetallic units. It is worth mentioning that in this part and also in the following we consider only structures built by gold-heterometal interactions, not bonds thus formal clusters will not be considered. 

Goedken, and Gritmon(J3) in 1977 to investigate the thermodynamic parameters of a variety of aminopolycarboxylate ligands across the lanthanide series. Variations in the enthalpy and entropy curves vs. Z were attributed to different degrees and patterns of dehydration as well as the increasing polydentate nature of the complexation. The linear relationship between log 6 and ZpKa of... 

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