The Architecture of Coordination Compounds

These factors include parameters such as bonding angles, bond length, etc. Isomers are compounds with identical composition but with a different stereochemical arrangement. We distinguish essentially between geometrical and optical isomers. The latter may be considered as a special case of the former. They are characterized by the presence of elements of definite chirality. In chelate compounds, four types of chiral elements may be distinguished  

Diastereoisomerism is observed when two or more chiral elements appear in the same system. Isolation and characterization of enantiomers or diastereoisomers depend on the structural stability of the chiral element. Therefore the conformation of a chelate ring must generally be considered as labile. The structural stability of the configuration around the metal center and the stability of an asymmetric coordination atom depends on the nature of the central metal atom. In most cases an optically active ligand molecule can be considered as configurationally stable. The presence of a stable chiral element can induce definite chirality in another element which, taken separately, would be labile. 

The most striking feature in diastereoisomerism of metal complexes is the very rapidly growing number of isomers, by introducing different elements of chirality in the basic framework of a coordination compound. The complex formation with l,8-diamino-4-methyl-3,7-diazaoctane (5-Metrien) for example — a ligand with a single asymmetric center — leads not only to four geometric isomers. Three of these show a structure containing all four types of chiral elements mentioned, so that the system offers 28 possibilities of isomers. 

One should not only classify different types of diastereoisomers according to the combination of chiral elements, but also different types of diastereoselectivity. Usually such a distinction is not made, which might often lead to some confusion. The formation of [M(asp)2] (M = Ni2+, Cu2+) shows no or only little selectivity considering a difference in stability between the optically active forms [M(R-asp)2 ]-or [M(S-asp)2] and the meso-form [M(R-aspXS-asp)]-13 On the other hand we may assume that each aspartic acid molecule coordinates in a stereospecific manner in labile as well as in inert complexes14 The asymmetric carbon atom allows the coordination of aspartic acid as a tridentate ligand only in a way producing def- 

The most thoroughly investigated bidentate ligands are certainly the 1,2-diamines. 

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