The Square Planar-Tetrahedral ML4 Interconversion

In the first section of tills chapter wc built up the orbitals of a square planar ML complex. An alternative geometry would be a tetrahedral species. There arc two basic ways to convert a square planar complex into a tetrahedral one. The 16.29 to 16.30 interconversion involves twisting one pair of cis ligands about an axis shown in 16.29. That will conserve D2 symmetry along all points that interconnect 16.29 with 16.30. In the other path the two trans L—M—L angles arc decreased, as shown in 16.31, ultimately yielding the tetrahedron 16.32. This conserves D2J symmetry. The elements of this latter pathway have actually been developed in Section 15.4, so we shall briefly explore this distortion. A Walsh 

FIGURE 16.3. Distortion of a square planar to tetrahedral MJL4 complex maintaining synimelry. 

Another interesting isomerization process can take place between square planar and octahedral systems in coordination compounds. 16.40 shows how the 

especially since it involves a spin-state change and two-electron energy terms as w ell. 

Sinictural Inorganic Chemistry, fourth edition. Clarendon Press, Oxford (1975), p. 964-967. 

In the first section of this chapter, v/e built up the orbitals of a square planar ML4 complex. An alternative geometry v/ould be a tetrahedral species. There are two basic ways to convert a square planar complex into a tetrahedral one. The 16.33 to 16.34 interconversion involves twisting one pair of cis ligands about an axis 

The 2o g level at the square planar geometry is mainly z with some antibonding from the lone pairs (see 16.1). This distortion moves the ligands into the nodal planes of z. Therefore, 20 g is stabilized it becomes one partner of an e set, along with xy that is unperturbed along the rearrangement path. 

The e set at the tetrahedron has the same role as Cg and big in the square plane and tig in the octahedron, namely they will participate in n bonding to the 

Walsh diagram at the extended Huckel level for the square planar to tetrahedron distortion in NiH4. 

16 SQUARE PLANAR, TETRAHEDRAL ML COMPLEXES AND ELECTRON COUlsTHNC 

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THE SQUARE PLANAR-TETRAHEDRAL ML4 INTERCONVERSION 307 TABLE 16.1. Mean Nickel-Ligand Distances (A) ... 

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