Geometrical isomerism four-coordinate complexes

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. 

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. 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

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). 

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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