Coordination compounds polydentate chelating

Observations on the chelate effect and, in particular, the importance of ring size, enable predictions to be made concerning the formation of stable coordination compounds with polydentate ligands that contain a variety of individual functional groups. Work on the synthesis of organic ligands... 

Coordination compounds formed by hard and charged donors with lanthanides, like those of d- and s-block metal ions, are usually thermodynamically stable — especially if coordinated by chelating ligands. Table 9.1 shows a comparison of thermodynamic constants for some key polydentate ligand systems in common with lanthanides. 

A hither facet of research has involved the structural characterisation of aluminium complexes which incorporate polydentate salen-type ligands. These have been noted in both neutral and monocationic (ion-separated) contexts (the latter requiring that the metal centre be stabilised by an external Lewis base) [35]. While such charged systems are invariably mononuclear the same is only usually true of their neutral analogues by virtue of the sterically demanding bis(aryloxide), chelating ligand. In the context of these latter complexes, dimerisation has been noted [251] while, more recently, the employment of flexible alkyl chains between two salen-coordinated aluminium ions has enabled the observation of dinuclear compounds [160, 161]. 

Despite the multitude of well characterized boron compounds, the knowledge of azole derivatives of boron is still rather limited. For example, only three C-borylated pyrazoles are known. Although N-borylated pyrazole derivatives are considerably more abundant, until most recently these were restricted to compounds containing only four-coordinate boron. Of these, the poly(l-pyrazolyl)borate anions have been a bonanza for the coordination chemist, since the steric and electronic features of these ions render them as extremely useful (polydentate and chelating) ligands. However, recent studies of the chemistry of boron derivatives of pyrazoles have provided for some noteworthy developments and a survey of such compounds and their chemistry appears to be a timely subject. 

Overall, the formation of pentacoordinated complexes is favored by chelating and particularly by polydentate macrocyclic ligands. A search in the Cambridge Structural Database reveals that many of the structurally characterized five-coordinated Pd(II) compounds contain macrocyclic or rigid polydentate ligands, S, N, and P donor atoms being dominant. Complexes (23), (24) and (25) are typical examples. 

The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general - knitting themselves onto the metal ion stitch by stitch (or, more correctly, undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set, when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. 

Crown compounds - Macrocyclic polydentate compounds, usually uncharged, in which three or more coordinating ring atoms (usually oxygen or nitrogen) are or may become suitably close for easy formation of chelate complexes with metal ions or other cationic species. [5]... 

In simple complexes of lead with unidentate ligands, lead is usually in the Pb(IV) form. In chelated lead compounds with polydentate ligands, lead is usually in the Pb(II) form. These chelates, usually six-coordinate, are kinetically quite labile, although highly thermodynamically stable [73]. 

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