Square planar complex geometrical isomerization

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. 

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... 

Finally, let us assume that the ligands are distinguishable say that 1 and 2 are Cl whereas 3 and 4 are Br. There is only one isomer of the tetrahedral complex but the square-planar complex has two cis and trans. Both Sa and Sf, thus offer geometrically convenient intramolecular pathways for cis trans isomerization, but imply the occurrence of two successive spin-flips along the pathway, as the low-spin complex is converted to the high-spin tetrahedral com-... 

Ffg.22.S. Geometric isomerism in a square-planar complex, MAjBt. 

A clear sign that P chemical shifts are a composite from several sources, comes from a consideration of the effects of geometric isomerism on the phosphorus resonance position. Consider the square planar complexes ais and trans-MCI2 (PR Ph ) 2 as shown... 

The term cis describes geometric isomers in which two groups are attached on the same side of a double bond in an organic molecule, or along the same edge of a square in a square-planar complex, or at two adjacent vertices of an octahedral complex. (See also geometric isomerism.) cis-trans isomerism is a type of stereoisomerism. 

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... 

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. 

The trans compound melts at approximately 90 °C, and continued heating leads to isomerization to the cis structure. Geometrical isomerizations can also lead to a change in structure of the complex. For example, a change from square planar to tetrahedral structure has been observed for the complex [Ni(P(C2H5)(C6H5)2)2Br2]. 

The unsymmetrical nature of / -mercaptoethylamine should lead to geometric isomerism among its metal complexes, cis and trans isomers might be expected with the square planar nickel (II) and palladium (II) derivatives and facial and peripheral isomers with cobalt (III). However, during the course of the preparation of various samples in which the procedure and experimental conditions were varied, no evidence of such isomerism was apparent (6, 15). This is particularly evident in the case of the cobalt (III) complex, CoL3. Samples prepared by the addition of cobalt (II) chloride 6-hydrate to strongly basic aqueous solution of the ligand and by displacement of ammonia and (ethylenedinitrilo)-... 

Complexes with coordination numbers of 4 are typically either tetrahedral or square planar. The tetrahedral geometry (Fig. 8.18a) predominates for four-coordinate complexes of the early transition metals (those toward the left side of the d block of elements in the periodic table). Geometric isomerism is not possible for tetrahedral complexes of the general form MA2B2, because all four tetrahedral sites are completely equivalent. 

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

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.

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]. 

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