Isomerization octahedral complexes

Black and McLean pointed out that a hexadentate ligand can yield either of two isomeric octahedral complexes of meso or racemic stereochemistry (Fig. 9) [52, 53]. These differ primarily in that the former consists entirely of facially coordinating tridentate loops the latter comprises two meridional loops (i.e., those in which three adjacent donor atoms, as well as the metal ion, lie in a plane). In all encapsulated complexes reported to date, 18S6 coordinates in the centrosymmetric meso form. This behavior contrasts with the racemic coordination found for corresponding hexaaza macro-cycles [144-147]. 

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). 

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. 

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. 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

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... 

This is by far the most studied of the isomerisms, with those of square planar and octahedral complexes receiving the major attention. As we shall see, developments in the preparation and separation of isomeric forms and the clever use of nmr techniques have played a major part in the increased understanding of the field. 

Although outside the main thrust of the book, attention should be drawn to the important use that octahedral complexes are receiving as molecular probes. Metal complexes can sometimes recognize specific sites on biological material. The array of isomeric forms possible with some metal complexes (previous section) adds a dimension to their use as probes. 

Group IV Donors. A H n.m.r. study of the cis-trans isomerization of [OslCO) -(SiMe3)2] indicates that the process is non-dissociative. These species may therefore provide a rare example of stereochemical non-rigidity in nonchelate octahedral complexes. Similar non-rigidity was observed for [Os(CO)4-(SiMeCl ) ] and [Os(CO)4(SnMe3)2]. ... 

The Pickett s model and extensions thereof have the potential to deal with such situations, namely involving cis/trans or mer/fac octahedral complexes, provided the Eg and p parameters are known for the corresponding isomeric metal centers. Although this application still remains virtually unexplored, the following case is illustrative. The trans- and cis-(ReCl(dppe)2 centers display quite different values of those parameters Eg = 0.68 V, p = 3.4 [21] for the former, and Eg = 0.41 V, P = 1.88 [24] for the latter. Hence, for example, for the carbonyl (Pi = 0) complexes [ReCl(CO)(dppe)2], the cis isomer is oxidized at a lower potential than the trans (0.41 vs. 0.68 V), whereas for the nitrile [ReCl(NCR)(dppe)2] complexes, with Pi (NCR) in the range from —0.23 to —0.33 V [24] (Table 4), the reverse occurs. 

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

It is found on analysis of the problem that when only two d orbitals are available for combination with the s and p orbitals six equivalent bond orbital- of strength 2.923 (nearly as great as the maximum 3 for the best spd hybrid) can be formed, and that these six orbitals have their bond directions toward the corners of a regular octahedron. We accordingly conclude that complexes such as [CoCNHs ], [PdClc]—, and [PtCle] — should be octahedral in configuration. This conclusion is of course identical with the postulate made by Werner to account for isomerism in complexes with different substituent groups,1 and verified also by the x-ray examination of Co(NH8)el3, (NHOaPdCU, (NH4)2 PtCl , and other crystals (see Fig. 5-1). 

There is only one form of a monosubstituted octahedral complex MAiB. A disubstituted complex MA4B2 can exist in two isomeric forms, cis and trans ... 

This is by for the most common coordination number. With certain ions si -coordinate complexes are predominant. For example. chromhim(lll) and cobah(MI) are almost exclusively octahedral in their complexes. It was this large series of octahedral Cr(III) and Co(III) complexes which led Werner to formulate his theories of coordination chemistry and which, with square planar plalinum(II) complexes, formed the basis for almost all of the classic work on complex compounds. Before discussing the various isomeric possibilities for octahedral complexes, it is convenient to dispose of the few nonoctahedral geometries. 

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). 

Most tridentate triamine ligands 165 and 166 command a high level of interest [285e,286,293-295,297,298]. In general, octahedral complexes have been obtained in which the existence of the three isomeric compounds 167—169 has been established by x-ray diffraction [28 5e] ... 

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