Octahedral geometric isomers

Geometrical isomerism in octahedral compounds is very closely related to that in square planar complexes. Among the most familiar examples of octahedral geometrical isomers are the violet (cis) and green (trans) forms of the tetraamminedichlorocobalt(iii) and chromium(iii) cations, which have structures XII and XIII. The largest number of geometrical isomers would exist for... 

A similar type of isomerism occurs for [Ma3b3] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (/hcial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

Where unidentate ligands are present, the ability to effect the resolution of an octahedral complex (i.e. to separate 2 optical isomers) is proof that the 2 ligands are cis to each other. Resolution of [PtCl2(en)2] therefore shows it to be cis while of the 2 known geometrical isomers of [CrCl2en(NH3)2] the one which can be resolved must have the cis-cis structure since the trans form would give a superimposable, and therefore identical, mirror image ... 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

Which of the following octahedral complexes show geometric isomerism If geometric isomers are possible, draw their structures. 

Geometric isomers for square and octahedral coordination with two different ligands. Top right designation of ligand positions in an octahedral complex... 

FIGU RE 16.2 Geometrical isomers for MX4Y2 complexes having planar hexagonal, trigonal prism, and octahedral structures. 

As happens for other physico-chemical techniques, one must first ask if an electrochemical investigation is able to distinguish geometric isomers of the type cisjtrans or facjmer metal complexes. In principle, this is possible since, as mentioned previously, the redox potential of an electron transfer is influenced also by steric effects. For instance, we have seen in Chapter 5 that some octahedral complexes of the scorpiand diammac display different electrochemical responses, depending on whether the two outer amino groups assume cis or trans arrangements. One must keep in mind, however, that the differences in the electrochemical response of isomeric complexes can sometimes be quite small, so may escape a first examination. 

Geometric isomers for square and octahedral coordination with two different ligands. 

Figure 1 Geometric isomers of octahedral complexes with linear tetradentate ligands...

Dichloro-bis (2,4-pentanedionato) zirconium(IV) is monomeric and a weak electrolyte in nitrobenzene solution n.m.r. chemical shifts,7 infrared and Raman spectra,8 and dipole-moment studies21 indicate that this compound exists in solution as the octahedral cis geometrical isomer. Chloro- and bromotris(2,4-pentanedionato) zirconium (IV) are seven-coordinate complexes which are monomeric in benzene and only slightly dissociated in nitrobenzene and 1,2-dichloroethane. Iodotris(2,4-pen-tanedionato) zirconium (IV), however, is appreciably dissociated both in nitrobenzene and in 1,2-dichloroethane.7... 

Diorganoantimony(V) halides form monomeric covalent adducts, R2SbX3 L, with various oxygen donor ligands (e.g. DMSO, HMPA, TPPO, PyO)91,97). An octahedral geometry has been proposed on the basis of infrared and H NMR spectral data. Out of the three possible geometrical isomers, the existence of the following isomer has been proposed ... 

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... 

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