1.2- diaminoethane chelate complex

Diaminoethane (en), H2NCH2CH2NH2, forms tris- and bis-chelate complexes. The tris-chelate [Ni(en)3]2+ cation (94) has been known since 1899.658 It is considerably more stable than the six-coordinate complexes formed with monodentate amines as shown in Table 39 where the stability constants of some six-coordinate complexes are reported. 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Romeo et al. (1978) clearly indicate that complexes of divalent metal ions with 1,2-diaminoethane are more stable than those with 1,3-diaminopro-pane. Moreover, in a thorough discussion of the relations between the chelate effect and the ring size, Anderegg (1980) has listed thermodynamic data of complex formation between divalent metal ions and ligand [45], showing that almost invariably the stability of chelate rings decreases with increasing n in the order 5 > 6 > 7. 

A series of optically active linear tetradentate ligands, which have been stereospeeifically synthesized, have been used to prepare complexes with a trans-[CoN Cl2] core and both five- and six-membered chelate rings, in an attempt to correlate the sign of the Cotton effect with the known chiralities of compounds. How ever, a complete correlation was not possible. The synthesis, resolution, and properties of some oxalato, malonato, and diacido complexes of Co " with the stereospecific flexible tetramine ligands 5-Me-3,2,3-tet and NA -bis-(2-picoyl)-l-methyl-1,2-diaminoethane (picpn) have been reported. The stereospecificity is demonstrated by comparison of the optical rotation of the ligand prepared via an asymmetric synthesis with that of the ligand isolated from a resolved complex. The stereochemistry of the complexes has been deduced. ... 

One of the most spectacular and useful template reactions is the Curtis reaction , in which a new chelate ring is formed as the result of an aldol condensation between a methylene ketone or inline and an imine salt. The initial example of this reaction was the formation of a macrocyclic nickel(II) complex from tris(l,2-diaminoethane)nickel(II) perchlorate and acetone (equation 53).182 The reaction has been developed by Curtis and numerous other workers and has been reviewed.183 In mechanistic terms there is some circumstantial evidence to suggest that the nucleophile is an uncoordinated aoetonyl carbanion which adds to a coordinated imine to yield a coordinated amino ketone (equation 54). If such a mechanism operates then the template effect is largely, if not wholly, thermodynamic in nature, as described for imine formation. Such a view is supported by the fact that the free macrocycle salts can be produced by acid catalysis alone. However, this fact does not... 

The acetylacetone carbanion undergoes addition to the imine carbon atom of complex (92) but subsequent cyclization and deacylation processes occur (Scheme 42).212 The apical ammonia group is the most acidic and consequently is favoured to cyclize on to the carbonyl group. In the bis-(1,2-diaminoethane) complex related to the imine chelate (92), the two apical nitrogen atoms are no longer geometrically equivalent. Thus two products are formed when hydrogen cyanide is added to the complex and subsequent cyclization takes place (Scheme 43).213 However, the cyclization reaction is stereoselective. 

The chelate effect is well known in coordination chemistry and relates to the observation that metal complexes of bidentate ligands (such as 1,2-diaminoethane, en) are significantly more stable than closely... 

The stabilisation afforded by the chelate effect is highly dependent on the size of the chelate ring (Figure 1.8). Five-membered rings, as in metal complexes of 1,2-diaminoethane, are often the most... 

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