Octahedral complexes geometric isomerism

With 6-co-ordinate octahedral complexes, geometrical isomerism is also possible. Cis- and trans- isomerism is found in an ion such as [CoCl2(NH3)J ... 

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... 

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. 

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. 

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

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. 

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... 

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). 

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 There are two simple types of geometric isomerism possible for octahedral complexes. 

Geometric isomerism is possible for octahedral complexes. For example, complexes of the type MA4B2 can exist in two isomeric forms. Consider as an example the complex ion [Co(NH3)4Cl2]. The two like ligands (CC) can be either cis or trans to each other. These two complexes are different colors solutions and salts of the cis isomer are violet and those of the trans isomer are green. 

Octahedral complexes with the general formula MA3B3 can exhibit another type of geometric isomerism, called mer-fac isomerism. This can be illustrated with the complex ion [Pt(NH3)3Cl3]+ (see Table lS-1). In one isomer the three similar ligands (e.g., the CC ligands) lie at the corners of a triangular face of the octahedron this is called the r isomer (for facial). 

Fig. 1.5 Possible geometric isomerism around a metal core in a square planar (A) and two octahedral complexes (B, C).

The number of possible diastereomers depends on the variety of ligands and sometimes requires use of the one-letter code (cis/trans is noted c/t). This nomenclature may be applied to square planar complexes and to square planar pyramidal and octahedral complexes, but not to tetrahedral complexes where a given position is equivalent to any other. Moreover, geometric isomerism often implies the existence of optical isomerism. 

Syn/anti nomenclature is mainly employed for octahedral complexes when geometric isomerism arises from the presence of a fused ring. Therefore, the syn isomer has adjacent fused rings whereas the anti isomer has opposite fused rings [5]. 

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