Chelate complex, geometrical isomers

Table 9.3 Number of possible geometric isomers depending on the number of different ligands (designated by A, B, C,. ..) for some coordination polyhedra (excluding chelate complexes). Of every pair of enantiomers only one representative was counted...

The chelated square-planar metabolite [Pt(Met)2)l has been detected in urine of patients treated with cisplatin (89). It is a relatively unreactive complex, existing in solution as a mixture of three diaster-eoisomers of each of the two geometrical isomers (90). The cis isomer predominates over the trans isomer by 87 13, and interconversion is slow (half-lives for conversion of cis to trans 22.4 h and of trans to cis 3.2 h at 310 K). 

Fig. 6. Structures of the chelating amide ligands and the irans, cis-a, and cis- Q geometric isomers of their complexes.

Tris-chelate complexes exist in enantiomeric configuration A and D about the metal atom, and when the chelating ligand is unsymmetrical, there are also geometrical isomers, cis and trans. Each geometrical isomer exists in enantiomeric forms thus there are four different molecules. In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is important. When the metal ions are of the inert type, it is often possible to resolve the complex then the process of racemisation can be followed by measurement of optical rotation as a function of time. Possible pathways for racemisation fall into two broad classes those without bond rupture and those with bond rupture. 

Both the cis and trans geometrical isomers of chromic ferrioxamine B isomerize to equilibrium solutions with half-lives of several days at room temperature. This is considerably slower than that found for the simple tris hydroxamate complexes such as Cr(men)3 and is caused by the steric constraints of the ferrioxamine B ligand and its hexadentate chelation. 

Among the commonest types of diketo complex are those with the stoichiometries M(dike)3 and M(dike)2. The former all have structures based on an octahedral disposition of the six oxygen atoms. The tris(chelate) molecules then actually have >3 symmetry and exist as enantiomers. When there are unsymmetrical diketo ligands (i.e., those with R R"), geometrical isomers also exist, as indicated in (11-XXVI). Such compounds have been of value in investigations of the mechanism of racemiza-tion of tris(chelate) complexes. Chiral diketonate complexes have found many... 

It was mentioned above that tris(chelate) complexes of the type (Co(en) ] lack an improper axis of rotation. As a result, such complexes can exist in either of two enantiomeric forms (or a racemic mixtire the two). Figure 12.20 illustrates the complex iems (Colen),] and (Cr(ox)3p". each of which is chiral irifh >, symmetry. It is not necessary to have three chelate rings present. The cation dichloro-bis(ethylenediamine)coball(I () exists as two geometric isomers, cis and trans The trans isomer has approximate D f, symmetry (Hg. 12.21b). Because it has three internal miiror planes, it is achiral. The cis isomer has symmetry and is chiral (Fig. I2.2la>. Since the two chlmde ions rq)Jace two nitrogen atoms from an eth-... 

Because the two regioisomeric products 8a and 8b have almost the same molecular dimensions, it is difficult to discriminate between the two isomers with the geometric constraints imposed by the zeolite pores. Considering that calcium ions are apt to form mainly five-membered chelate complexes with polyhydroxy compounds (Fig. 4b) 32,33) and that calcium zeolites have also been employed as sorbents in carbohydrate separations (ii), it is possible to speculate that in the CaY-supported NaN3 system the epoxy alcohol first forms a coordinated structure around a calcium ion, as shown in Fig. 4a, followed by ring opening with an azide anion at the C-3 position of the epoxy alcohol, giving a stable, five-membered chelate complex with the calcium ion. 

Carboxamides containing alkylamine arms like (34), synthesized from dialkylmalonate and ethylene-diamine, act as chelating dianionic N4-donors,44,45 whereas the geometric isomer (35) was prepared from 1,3-diaminopropane and chloroacetyl chloride.46 The hexadentate bis(pyrazine)-carboxamides (36) and (37) can be obtained by the addition of 1,3-propanediamine to the appropriate carboxylic acid.47 Compound (36), in the dianionic form, can be used to obtain homo- and hetero-metal bridged complexes, whereas (37) has been shown to yield tetranuclear copper(II) complexes involving opened-up dianionic ligand molecules.47... 

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