Rearrangements involving changes in coordination polyhedra

Many four-coordinate complexes of nickel(II) undergo tetrahedral -square-planar rearrangement reactions. In the case of palladium(II) and 

The influence of steric factors on the position of tetrahedral - square-planar equilibria has been discussed in detail.38,39 jn general, two trends are apparent (i) the smaller the steric parameter, 0, of the phosphine ligand in a complex of the type [NiX2(PR3)2l, the greater the stabilization of the square-planar form, and in) the stabilization of the square-planar form is sensitive to the nature of the anionic ligand, X , and decreases in the order Cl Br P. 

The interconversion of square-planar and tetrahedral nickel(II) complexes typically occurs at a fast rate and such equilibria have been studied by NMR spectroscopy.48 a modified, extended-Hiickel method has been used to calculate electronic energy surfaces for the glycinate complex, [Pt(Gly)2l, in the transition from square-planar to tetrahedral configurations for both the ground and excited states. The calculations were carried out for various positions of the atomic nuclei during twisting of the molecule relative to the 

Ihara and Tsuchiya S have observed the rearrangement of square-planar, dicationic nickel(II) complexes to neutral, octahedral complexes by anion coordination. In the solid state, square-planar complexes of the type [Ni(Abi)4]X2 (Abi = 2-aminobenzimidazole X = Cl, Br, or NO3) are converted into octahedral complexes. With loss of water of crystallization upon heating, Eq. 11.2  

The same type of rearrangement has been observed for the square-planar benzimidazole complex, [Ni(Bimd)4][N03]2-2.5C2H50H. Upon heating, the ethanol of crystallization is lost and coordination of the nitrate counterions occurs to yield the octahedral complex, [Ni(N03)2(Bimd)4]. 

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