Square-planar systems, isomerism

Reaction Mechanisms of Inorganic and Organometallic Systems 4.4 ISOMERISM IN Square-Planar Systems... 

Chemistry is stereochemistry. After development of the fundamental synthetic methods and the structural characterization of many important compounds in such an expanding field as organometallic chemistry during the past 25 years, the problems demanding solution become more subtle, so that more elaborate techniques, e.g., kinetic and spectroscopic measurements, as well as stereochemical methods, gain in importance. Stereochemical information can be obtained by a variety of techniques. Hence, the study of such stereochemical aspects as cis-trans isomerism in square-planar and octahedral transition-metal compounds was a consequence of the increasing availability of IR spectrometers, and temperature-dependent NMR spectroscopy proved to be an extremely valuable technique for the study of nonrigid systems. 

Isomerization of transition metal complexes is frequently observed to occur for octahedral d6 or square-planar d8 systems. Although the interconversions between isomers are commonly achieved thermally, they are also induced photochemically or by oxidation/reduction. In the latter case, the thermodynamically favored isomeric distribution is dependent upon the oxidation state of the metal center of the complex. 

For M"(aa)2 complexes with square planar geometry only geometric cis (14) and irons (15) isomers are possible. These isomers have been known for many years for the inert Pd" and Pt" ions. For the labile systems, e.g. Cu", deductions as to the major species in solution and their geometrical form has always been a problem because of their rapid interconversions. However, it has proved possible to isolate both geometrical isomers as crystalline solids in a number of cases, e.g. for [Cu(GlyO)2], [Cu(L-AlaO)2], and such isomerism probably accounts for the two solid forms for [Cu(DL-PheO)2] and [Cu(oL-TyrO)2]. 

Another interesting isomerization process can take place between square planar and octahedral systems in coordination compounds. 16.40 shows how the... 

In metal complexes, ligands may occupy different positions aroimd the central atom. Since the ligands in question are usually either next to one another cis) or opposite each other (trans), this type of isomerism is often also referred to as cis-trans isomerism. Such isomerism is not possible for complexes with coordination numbers of 2 or 3 or for tetrahedral complexes. In those systems, all coordination positions are adjacent to one another. However, cis-trans isomerism is very common for square planar and octahedral complexes, the only two types to be discussed here. Methods of preparation and reactions of some of these compounds are described in Chapter 4. 

Another interesting isomerization process can take place between square planar and octahedral systems in coordination compounds. 16.45 shows how the z lx -y separation changes as two trans ligands are brought closer to the square plane. When AE is small enough then a high spin d species is formed, as... 

The most common coordination number is six and such complexes have an octahedral structure. The next most common four-coordinated systems have either tetrahedral or square planar structures. Other complexes are known having different coordination numbers and structures. The stereochemistry of metal complexes is a fascinating subject. Several different types of isomeric structures are possible and have been demonstrated in these systems. For our purpose here it is sufficient to cite examples of geometrical (ds-trans) and optical isomerism. This can readily be iUustrated by the cis (III) and trans (IV) isomers of QCo(en)2Cl2]+. Note that the... 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

---