Crystal field theory tetrahedral

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

Figure 5.6 Energy level diagram of the splitting of the J-orbitals of a transition metal ion as a result of (a) octahedral co-ordination and (b) tetrahedral coordination, according to the crystal field theory. (From Cotton and Wilkinson, 1976 Figure 23-4. Copyright 1976 John Wiley Sons, Inc. Reprinted by permission of the publisher.)...

In crystal field theory each ligand is approximated as a point charge or a point dipole and each metal-ligand interaction is taken to be purely electrostatic. So our problem is reduced to one of investigating the effect of point charges (or point dipoles) arranged tetrahedrally or octahedrally about an electron in a dai y or a dXI orbital. 

Crystal field theory d-orbital splitting in octahedral and tetrahedral complexes... 

The crystal field theory as applied to octahedral, tetragonal, tetrahedral and sejuare planar metal complexes... 

Almost any theory correctly predicts the octahedral and tetrahedral structures of 6- and 4-coordinated molecules. Thus, a simple ionic model in which a central positive ion attracts a number of negative ions gives these structures because, for a given central atom to ligand distance, they maximize the distance between the ligands and hence minimize their repulsive interactions. For this reason, the octahedron is more stable than the trigonal prism, and the tetrahedron is more stable than the square plane. An extension of this model, namely crystal field theory, explains... 

We use the vibrational frequencies and degeneracies estimated by Hildenbrand (3) and adopted by Brewer ( ). These are in reasonable agreement with the estimates of Tumanov and Galkin ( ). The structure is assumed to be tetrahedral and a Mo-F distance of 1.82 A from MoPg(g) is used. The electronic contributions are those estimated by Brewer ( ) based on crystal field theory. 

We have assumed implicitly that Cr3+ ions will not occur in tetrahedral holes in an oxide lattice. Cr + is a species. In the crystal field theory, such ions lead to particularly good stabilization of octahedral coordination. For example, the coordinate bond energy of Cr(H20) + is 120 kcal, of which 9 represent crystal field stabilization energy (CFSE) (35). In fact, there are no known chromias with crystal struc-... 

However, with the application in the 19,80s of crystal field theory to transition-metal chemistry it was realized that CFSEs were unfavourable to the tbrmation of tetrahedral d complexes, and previous assignments were re-examined. A typical ca.se was INitacacfil, which had often been cited as an example of a tetrahedral nickel complex, but which was shown"- in 19.86 to be trimeric and octahedral. The over-zealous were then inclined to regard tetrahedral d as non-existent until first L. M. Venanz,i" and then N., S. Gill and R. S. Nyholm" " demonstrated the existence of discrete tetrahedral species which in some cases were also rather easily prepared. 

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. 

Eg( D) state by approximately 1.1 1.5 eV (Westre et al. 1997) (3) for " Fe the states reverse in energy, as expected, on going from the octahedral to the tetrahedral coordination, with a smaller separation of about 0.6 eV (Westre et al. 1997), as predicted by crystal field theory. 

---