Octahedral complexes stereoisomers

In describing a stereoisomer, it is perhaps most important initially to define whether or not it is chiral. The origins of chirality (optical activity) in coordination compounds and important experimental results have been recently reviewed.112,113,121,122 The classical example of chirality or enantiomerism in coordination chemistry is that of octahedral complexes of the type [M-(bidentate)3]. These exist in the propeller-like,123 non-superimposable, mirror-image forms (13a) and (13b). Synthesis of this type of complex from M and the bidentate ligand in an achiral environment such as water results in an equimolar mixture of the two stereoisomers. The product... 

Optical isomers are special kinds of stereoisomers they are non-superimposable mirror images of each other (Fig. 16.27). Both geometrical and optical isomerism can occur in an octahedral complex, as in [CoCI2(en)2]+ the trans isomer is green (14a) and the two alternative cis isomers (14b) and (14c), which are optical isomers of one another, are violet. 

Although there are formally six possible stereoisomers of an octahedral complex of general formula Ma2b2c2, for dichlorido(diazane)bis(triphenyl-phosphane)cobalt(l+) only four isomers occur, since an arrangement of the two nitrogen atoms of the hydrazine trans to each other in the complex is not possible. The two OC-6-32 isomers are chiral, they are enantiomers and have the A and C configuration, respectively. Both the other two isomers are achiral diastereomers. 

OC-6 complexes. Generally, both forms (the A and the A) of the octahedral complex Pt(th-4,5ppy)2(R)(X) are produced (only one stereoisomer, the C,C-cw N,N -ds, C(chelate), X-trans isomer is formed), but interestingly, they have very different solubilities, making separation an easy task. By this pathway, a new synthetic route to enantiomerically pure bis(bidentale) OC-6 complexes from chiral SP-4 complexes has been established... 

On first view, the situation with respect to the it-allyl complexes appears simple. However, this is deceptive. There are no less than 16 possibilities, which are listed in Figure 11.2 [6]. This is a demonstration how complicated the situation is when one has to deal with octahedral complexes. According to density functional theory (DFT) calculations and using cyclooctatetraene as ancillary ligand [6] and mechanistic investigations [11], there is a kinetic as well as thermodynamic preference for the formation of the experimentally observed stereoisomer. 

A process that has been studied widely in relation to phenomena such as chiral symmetry breaking, spontaneous resolution and chiral amplification is the reaction (Fig. 18) of [Co(H20)2 (OH)2Co(en)2 2](S04)2 (denoted 1) with NH4Br to give the chiral complex cA-[CoBr(NH3)(en)2]Br2 (denoted 2). This reaction is historically important, as 2 was one of the first octahedral metal complexes to be resolved into A and A stereoisomers, some years after Werner predicted that octahedral ions M(en)2XY should exist as enantiomeric pairs. 

Optically active stereoisomers can be obtained of a complex such as MfCsO s, containing oxalate groups which occupy two adjacent octahedral corners ... 

Because of the presence of two identical unsymmetric bidentate ligands and two identical monodentate ligands in the chelate complexes 3-5, five stereoisomers have to be considered for these compounds (Fig. 5). As shown by the crystal structure analyses, all ctxnpounds crystallized as the (OC-6-12)-isomer (for the nomenclature system, see Ref. [14]), with slightly distorted octahedral Si-coordination polyhedra. 

Although the resolution into their enantiomeric forms of octahedral metal complexes bearing bidentate ligands was accomplished at an early stage,19 the application of such materials for the preparation even of dinuclear complexes was greatly delayed.20 Only recently has the preparation of a particular stereoisomer of a diruthenium complex been accomplished by the resolution of the bis-o-phenanthroline complex (9). The subsequent use of the optically active material in a stereoselective reaction with 2,5-bis(2-pyridyl)pyrazine (10) yielded the dinuclear com-... 

Werner s researches provided an overwhelming body of evidence that the hexacoordinate complexes must be octahedral. By counting the number of stereoisomers observed when a central ion is joined to various numbers of monodentate and bidentate ligands in hexacoordinate complexes, Werner... 

Stereoisomerism is commonly shown by octahedral (six co-ordinate) complexes associated with bidentate ligands. An example is the complex containing nickel as the transition metal and 1,2-diaminoethane (NH2CH2CH2NH2) as the bidentate ligand (Figure 24.9). The two isomers are stereoisomers because the two different molecules are mirror images of each other and cannot be superimposed. 

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