Planar, Octahedral

This equilibrium can be represented in general terms as the two steps (7.5) and (7.6), 

A related equilibrium involving the conversion of cw-Ni([13]aneN4)(H20)2 to -trans Ni([13]aneN4) + is believed to involve a planar isomer, (a-rran5 -Ni([13]aneN4) ), as an intermediate  

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Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

Fig. 7.3 Free energy profiles for pianar-octahedral equiiibria. In A, ligand or solvent exchange is more rapid than the planar, octahedral equilibrium as established in (7.7). In B, the formation and dissociation of the octahedral complex is rate-determining and the interconversion of the planar and five-coordinated species is more rapid.

The properties of nickel(II) complexes with unsaturated macrocycles which contain pyridyl groups are included in a very comprehensive review article.2626 The complexes are usually square planar with the exceptions of the trans octahedral NiX2(CR)2743 (CR = 2,12-dimethyl-3,7,ll,17-tetraazabicyclo[11.3.1]heptadeca-l(17),2,ll,13,l5-pentaene) and of the diamagnetic square pyramidal NiBr(CR)]Br-H20 (381).2747 The diamagnetic complexes Ni(CR)(C104)2 give rise to square planar octahedral equilibria in coordinating solvents,2744,2746 whereas... 

In the planar-octahedral equilibria of nickel(II) the d orbital population changes by transfer of one electron from the d2 orbital to the dx2-y2 a antibonding orbital. This results in a substantial increase in the nickel-nitrogen distances in the plane. Accompanying this is the formation of new metal-ligand bonds in the axial positions. 

Much of the focus of these studies has been on the relation between ligand substitution reaction mechanisms on octahedral nickel(II) and the dynamics of the planar-octahedral equilibria. For typical octa-... 

Fig. 7. Alternative reaction coordinate profiles for planar-octahedral equilibria of nickel(II).

In some studies an assumption has been made about which of B or C (Fig. 7) is the rate-determining step, based more on prejudice about the role of the spin state change than on other evidence. At present it appears that only mechanism B can be distinguished from A and C. This is because mechanism B provides a pathway for ligand (or solvent) exchange from the six-coordinate complex, which is more rapid than the planar-octahedral interconversion and which can be observed by NMR... 

Some diamagnetic planar nickel(II) complexes add only one ligand to form paramagnetic five-coordinate species. The dynamics of several of these equilibria have been examined by photoperturbation or NMR methods. The rate constants present in Table VII are of the order 106 sec 1 for the dissociation of the ligand from the five-coordinate species. These rates are comparable with those of the planar-octahedral equilibria and are consistent with the mechanistic interpretation presented above. 

The use of metal ions as kinetic synthetic templates is extremely widespread, and is an excellent way in which to bring about the organisation of a number of reacting components in order to direct the geometry of the product. Because some metal ions, such as the transition metals, often have preferred coordination geometries (e.g. tetrahedral, square planar, octahedral etc), changes in metal ion may have a profound effect on the nature of the templated product. Metal-ion-templated syntheses may be classified more generally as examples of self-assembly with covalent postmodification. For example, the synthesis of the artificial siderophore 10.2 is effected by the use of an octahedral Fe3+ template.8 In this case, the macrobicyclic product is obtained as the Fe3+ complex from which it is difficult to separate. 

Square planar Octahedral None NaCl, T1O2 (rutile)... 

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