Complex ions geometric isomerism

Low temperature and low concentration of Cl ions favor the formation of Co(H20)6 ions. Adding HCl (more Cl ions) favors the formation of C0CI4 This reaction decreases [Cl ], so the pink color is restored. 15.48 Fe is nearly colorless, so it must be light yellow in color. 15.50 The Be complex exhibits optical isomerism. The Cu complex exhibits geometric isomerism. 

Definitions. Define and illustrate the following terms (a) complex ion, (b) ligand, (c) strong-field complex, (d) weak-field complex, (e) geometrical isomerism, (f) kinetic stability, (g) thermodynamic stability. 

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

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... 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

Lanthanide(III) ions have been found to catalyze the geometric isomerization of trans-bis(oxalato)diaquochromate(III) and frany-bis(malonato)diaquochromate(III) to yield the corresponding cz.v-isomers. The proposed mechanism for the uncatalyzed and catalyzed pathway for the oxalato complex is shown in Fig. 7.29. 

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

Geometrical isomers are not necessarily optical isomers. For instance, the trans isomer of [Co(en)2Cl2]+ shown in Fig. 20.17 is identical to its mirror image. Since this isomer is superimposable on its mirror image, it does not exhibit optical isomerism and is therefore not chiral. On the other hand, cis-[Co(en)2Cl2]+ is not superimposable on its mirror image thus a pair of enantiomers exists for the complex ion, making the cis isomer chiral. 

Does the complex ion fCo(NH3)Br(en)2] exhibit geometrical isomerism Does it exhibit optical isomerism ... 

The complex ion exhibits geometrical isomerism, since the ethylenediamine ligands can be across from or next to each other ... 

NMR was used to assign the geometrical isomerism the fac isomers show only two methyl signals, whereas the mer isomers display a complex spectrum. The crystal structure of the A fac isomer was claimed to confirm the assignment (50) however, the complex ion was disordered and the structure refinement was not completed (51). The structure determination of the mer complex has recently been completed (52) The chelate bite angle is 84.3(9)°. 

The steric structures presented explain the early detection of isomery (isomerism) of the complex compounds of Co, Pt, etc., as well as predict the number of geometric(al) isomers of the complex ion. 

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 ... 

Cobalt(II) forms more tetrahedral complexes than any other ion except zinc(II). Draw the structure(s) of the tetrahedral complex [CoCl2(en)]. Could this complex exhibit geometric or optical isomerism If one of the CD ligands is replaced by Br, what kinds of isomerism, if any, are possible in the resulting compound ... 

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). 

Characteristically, metal ions form complexes that can exist as several isomers. This is a consequence of the stereochemistry, resulting from high numbers of ligands, adopted by most metal complexes. The best known examples of isomerism in complexes are geometrical isomers (such as cis and trans isomers), but these are not the sole type. We can, following a traditional approach, divide the area into two classes constitutional (or structural) isomerism and stereoisomerism. 

Figure 22.10 Geometric (cis-trans) isomerism. A, The c/s and trans isomers of the square planar coordination compound [Pt(NH3)2Cl2]. B, The c/s and trans isomers of the octahedral complex Ion [Co(NH3)4Cl2]. The colored shapes represent the actual colors of the species.

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