Tetrahedral fields metal ions

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

Although the effect on the d orbitals produced by a field of octahedral symmetry has been described, we must remember that not all complexes are octahedral or even have six ligands bonded to the metal ion. For example, many complexes have tetrahedral symmetry, so we need to determine the effect of a tetrahedral field on the d orbitals. Figure 17.5 shows a tetrahedral complex that is circumscribed in a cube. Also shown are lobes of the dz- orbital and two lobes (those lying along the x-axis) of the dx> y> orbital. 

For metal ions having d2, d3, d7, and d8 configurations, the ground state is an F state, but there is an excited P state that has the same multiplicity. For d1 and d7 ions in an octahedral field, the spectroscopic states are the same (except for the multiplicity) as they are for d3 and d8 ions in tetrahedral fields. Therefore, the expected spectral transitions will also be the same for the two types of complexes. The three spectral bands are assigned as follows (Tlg(F) means the Tis state arising from the F spectroscopic state) ... 

I shall take the simple view that most metal oxide structures are derivatives of a closest packed 02 lattice with the metal ions occupying tetrahedral or octahedral holes in a manner which is principally determined by size, charge (and hence stoichiometry) and d configuration (Jj). The presence of d electrons can lead to pronounced crystal field effects or metal-metal bonding. The latter can lead to clustering of metal atoms within the lattice with large distortions from idealized (ionic) geometries. 

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.)...

A splitting of magnitude A6 is produced and it depends on the nature of both the metal ion and the ligand. In the case of the octahedral field each electron placed in one of the t2g orbitals is stabilized by a total of -2/5A, while electrons placed in the higher energy eg orbitals are destabilized by a total of 3/5A. The splitting for a tetrahedral complex, Atet is less than that for an octahedral one and algebraic analysis shows that Atet is about 4/9A. ... 

Tetrahedral complexes arc favored by steric requirements, either simple electrostatic repulsions of charged ligands or van dcr Wauls repulsions of large ones. A valence bond (VB) point oT view ascribes tetrahedral structures to p% hybridization From a crystal field (CF) or molecular orbital (MO) viewpoint we have seer that, in general, tetrahedral structures are not stabilized by large LFSE. Tetrahedral complexes are thus favored by large ligands like Cl-. Br. and 1 and small metal ions of three types ... 

Transition mela] ions in zeolites behave much as expected for ions m a weak oxide field, but often the metal ions are found in trigonal sites, so their spectra and magnetic properties are somewhat different from those of the more common octahedral and tetrahedral fields.31... 

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