Transition Metals Isomerization of NiXj

It is in transition metal chemistry that d orbitals acquire their overriding importance as is well known, their symmetry properties determine the structure of high-symmetry complex molecules and ions [6, Chap. 17]. The author is convinced that these properties can be used to advantage in the analysis of their reaction mechanisms as well. A single example will be cited, illustrating the interplay between orbital and spin symmetry that has to be taken into account. 

Divalent nickel (3d ) forms tetrahedral and square-planar complexes of comparable energy, their relative thermodynamic stability depending on the identity of the ligands [6, p. 751]. The discussion of their interconversion that follows is abstracted from the symmetry analysis published by Knorr and the author [23]. 

Unless retarded by repulsion between bulky substituents in the more crowded planar isomer, the tetrahedral-to-planar isomerization has a low enthalpy of activation, AH 10 4 kcal/mol [24]. Its entropy of activation is ordinarily quite negative for a unimolecular isomerization AS —10 kcal/mol K) but is substantially less so - occasionally approaching zero - in nickel(II) complexes with halogen atoms as coordinating ligands [25]. This pattern of Arrhenius parameters, characteristic of reactions that occur with spin inversion (see Chapter 9), is hardly surprising in view of the fact that the tetrahedral complex is high-spin (5 = 1) whereas the square-planar complex is low-spin S = 0). 

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- 

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