Geometric isomerism, in octahedral complexes

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... 

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... 

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. 

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

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. 

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. 

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. 

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... 

Geometric isomerism can also occur in chelated octahedral complexes (Figure 15.7, p. 416). Notice that an ethylenediamine molecule, here and indeed in all complexes, can only bridge cis positions. It is not long enough to connect to trans positions. 

Octahedral Having the symmetry of a regular octahedron. In an octahedral species, a central atom is surrounded by six other atoms, one above, one below, and four at the comers of a square, 176 complex in transitional metals, 418-420 geometric isomerism, 415 Octane number, 584... 

The most common type of geometrical isomerism involves cis and trans isomers in square planar and octahedral complexes. If the complex MX2Y2 is tetrahedral, only one isomer exists because all of the positions in a tetrahedron are equivalent. If the complex MX2Y2 is square planar, cis and trans isomers are possible. 

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. 

Fig. 2.15 Plot of AS (J K- mol ) vs AK (cm moK ) for racemization and geometrical isomerization of a variety of octahedral metal complexes. Only a few entries are selected from the 27 reactions tabulated in Ref. 180. The deviation of (four) Cr(IlI) complexes represented by Cr(phen) + (3) from the linear plot (best fit for 23 complexes) may indicate that these recemize by twist, and not dissociative, mechanisms. Racemization of Cr(C204)3 -(l), Co(Ph2dtc)3(2), Cr(phen)3 (3), Ni(phen)f+(4). Geometrical isomerization of trans-Cr(C204)2(H20)2 (5), trans-Co(en)2(H20) +(6), (3-Co(edda)en+(7).

There are two simple types of geometric isomerism possible for octahedral complexes. The first exists for complexes of the type MA2B4 in which the A ligands may be either next to each other (Fig. 12.18s) or on opposite apexes of the octahedron (Fig. 12.18b). Complexes of this type were studied by Wemer, who showed that the proiro and video complexes of tetraamminedichlorocobalt([Il) were of this type (see Chapter 11). A very large number of these complexes is known, and classically they provided a fertile area for the study of structural effects. More recently there has been renewed interest in them as indicators of the effects of lowered symmetry on electronic transition spectra. 

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. 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

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