Tetrahedral and Square Planar Complexes

Similar MO treatments are possible for tetrahedral and square planar complexes but are increasingly complicated. 

FIGURE 20.30 Energies of the d orbitals in tetrahedral and square planar complexes relative to their energy in the free metal ion. The crystal field splitting energy A is small in tetrahedral complexes but much larger in square planar complexes. 

Draw geometric isomers of octahedral, tetrahedral, and square-planar complexes (Section 8.4, Problems 17-20). 

We will begin our discussion of crystal field theory with the most straightforward case, namely, complex ions with octahedral geometry. Then we will see how it is applied to tetrahedral and square-planar complexes. 

We illustrate the applications and limitations of VB theory by considering octahedral complexes of Cr(III) d ) and Fe(III) d ) and octahedral, tetrahedral and square planar complexes of Ni(II) d ). The atomic orbitals required for... 

Crystal Field Splitting in Tetrahedral and Square Planar Complexes Four ligands around a metal ion also cause d-orbital splitting, but the magnitude and pattern of the splitting depend on whether the ligands are in a tetrahedral or a square planar arrangement. 

Populating d Orbitals in Tetrahedral and Square-Planar Complexes... 

Crystal-field theory also applies to tetrahedral and square-planar complexes, which leads to different d-orbital splitting patterns. In a tetrahedral crystal field, the splitting of the d orbitals results in a higher-energy t2 set and a lower-energy e set, the opposite of the octahedral case. The splitting by a tetrahedral crystal field is much smaller than that by an octahedral crystal field, so tetrahedral complexes are always high-spin complexes. 

The history of coordination chemistry may in a sense be said to have begun with the work of Werner. The early crystal-structure determinations by W. L. and W. H. Bragg showed that in crystals such as sphalerite, ZnS, there is tetrahedral coordination around both zinc and sulfur, and in crystals such as sodium chloride there is octahedral coordination about both the anion and the cation. The modem period may be said to have begun in 1921, with the determination of a cryst containing an octahedral complex by Wyckoff and of crystals containing tetrahedral and square planar complexes (1922) by Dickinson. Later developments include application of quantum mechanics, discussion of hybrid orbitals especially suited to bonding, and detailed interpretation of interatomic distances found by careful X-ray diffraction studies. 

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]. 

---