Geometrical isomerism tetrahedral complexes

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]. 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

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. 

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 ... 

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. 

Geometric isomerism is also possible in octahedral complexes when two or more different ligands are present, as in the cis and trans tetraamminedichlorocobalt(III) ion in Figure 23.7. Because all the corners of a tetrahedron are adjacent to one another, cis-trans isomerism is not observed in tetrahedral complexes. 

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-... 

Geometrical isomerism with respect to the central atom is impossible in tetrahedral complexes. The observation of geometrical isomers is, therefore, a way of deciding that a certain four-coordinate complex is square-planar rather than tetrahedral. 

MaJ, [Ma2b2], [Masb] and [Mabcd] can exist in only one geometrical form. They do not exhibit geometrical isomerism. Even polydentate ligands alone or in combination with monodentate ligands cannot alter the relative orientations of the bonds in tetrahedral complexes. Hence, tetrahedral complexes do not show geometrical isomerism. 

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... 

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