Isomerism in octahedral complexes

Compounds with bidentate ligands and monodentate increase the potential for isomerism in octahedral complexes. Figure 23.16 shows the isomers of the dichloro-bis(ethylenediamine)cobalt (III) ion, CoCl2(en)2. Isomer A is the trans isomer (it has a green color). Both B and C are cis isomers (both have a violet color). Yet isomers B and C are not identical molecules. They are enantiomers, or optical isomers that is, they are isomers that are nonsuperimposable mirror images of one another. 

Cis-trans isomerism in octahedral complexes is shown in structures [5-7] and [5-8], Norbornadiene molybdenum tetacarbonyl can assume only the cis form, as seen in [5-9]. Arene 71-complexes also may exhibit cis-trans isomerism. X-ray diffraction data have shown that of the two structures [5-10] and [5-11], which could exist for ditoluenechromium(O), the thermodynamically more stable structure [5-11] exists under ordinary conditions. Dibi-phenylchromium(O) [5-12] and [5-13] may also exist in at least two forms. 

The previous examples demonstrate optical isomerism in octahedral complexes. Tetrahedral complexes can also exhibit optical isomerism, but only if aU four coordination sites are occupied by different ligands. Square planar complexes do not normally exhibit optical isomerism as they are superimposable on their mirror images. 

Octahedral To understand how geometric isomerism can arise in octahedral complexes, refer back to Figure 15.4. Notice that for any given position of a ligand, four other positions are at the same distance from that ligand, and a fifth is at a greater distance. 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

We conclude that in octahedral alkyliridium(III) complexes the presence of tertiary phosphines favors exclusively the n -alkyl over the corresponding secondary alkyl, irrespective of the size or basicity of the phosphine. This preference is probably largely electronic in origin, but steric factors cannot be ruled out. A key step that generates a vacant coordination site for both alkyl-group migration and isomerization in octahedral tertiary phosphine complexes of rhodium(III) and iridium(III) is dissociation of halide ion. 

The ammonia and amine complexes are the most numerous chromium derivatives and the most extensively studied. They include the pure ammine [CrAm6]3+, the mixed ammine-aqua types, that is, [CrAm4 (H20) ]3+ (n = 0-4, 6), the mixed ammine-acido types, that is, [CrAm6 X ](3" )+ (n = 1-4, 6), and mixed ammine-aqua-acido types, for example, [CrAm6 m(H20) Xm](3 m7+ (here Am represents the monodentate ligand NH3 or half of a polydentate amine such as ethylenediamine, and X an acido ligand such as halide, nitrite, or sulfate ion). These complexes provide examples of virtually all kinds of isomerism possible in octahedral complexes. 

Geometrical isomerism also occurs in octahedral complex ions. For example, the compound [Co(NH3)4Cl2]Cl has cis and trans isomers (Fig. 20.12). 

This kind of isomerism occurs when a compound can be represented by two asymmetrical structures, one of which is the mirror image of the other. It is common in octahedral complexes involving bidentate groups for, unless bidentate groups are present, there is always a plane of symmetry in complexes of this shape. The cation of [CoCl2(en)2]Cl has two optically active cis-forms (Fig. 289(a) and (b)) in addition to an inactive trans-form (Fig. 289(c)). [Co(cn)3]Br3 has also been resolved Fig. 290(a) and (b) show the two enantiomorphs of the terpositive cation. 

Most other inorganic reactions have been carried out using ETC catalysis isomerization of octahedral complexes [39 1], disproportionation [42], metal-metal bond cleavage and formation [43, 44], CO extrusion in formyl complexes [11]. Although many studies involve electrochemical initiation, the use of a chemical oxidant is also often shown to work. It is possible to use a photoexcited state as the initiator given its enhanced redox power [45]. 

Geometric isomerism is also possible in octahedral complexes when two or more different ligands are present, as in the cis and trans tetraamminedichlorocobalt(III) ion in Figure 23.7. Because all the corners of a tetrahedron are adjacent to one another, cis-trans isomerism is not observed in tetrahedral complexes. 

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... 

Geometric isomerism is possible also in octahedral complexes when two or more different ligands are present. The cis and trans isomers of the tetraam-minedichlorocobalt(lll) ion were shown in Figure 24.1. As noted in Section 24.1 and Table 24.1, these two isomers have different colors. Their salts also possess different solubilities in water. 

In complexes of chelates there are a number of types of isomerism which may occur. In a tris(ethylenediamine) octahedral complex two optically active isomers occur (often denoted A and A). 

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