Thomas Wolfram

Concepts of Surface States and Chemisorption on d-Band Perovskites

This chapter is principally concerned with the surface properties of the transition metal oxides with cubic perovskite structure. The topics discussed include: five-and sixfold coordinated transition metal-ion clusters and the bonding of molecules to such clusters, the bulk and surface electronic structure of the d-band perovskites and the nature of molecular bonding to the solid surface including band effects, the character of oxygen adsorption and the role of oxygen vacancies. Throughout the chapter we have attempted to show relationships between the electronic structure and simple, but general, concepts of importance in chemisorption and catalysis.

Applications of Group Theory to Atoms, Molecules, and Solids: Crystal-field theory

Splitting of d -orbital degeneracy by a crystal field As we saw in the previous chapter the s - and p -orbital degeneracies are unaffected when an atom or ion is placed in a site of octahedral symmetry. On the other hand, the d -and f -orbital degeneracies are changed. Transition metal ions of Ti, Fe, Ni, and Co, for example, have 3 d electrons in their outer unfilled shells and exist as positively charged ions in solids and molecular complexes. Most frequently, the transition metal ion is coordinated with six neighboring ligands at a site of octahedral symmetry. The second most common situation is tetrahedral coordination with four neighboring ligands. Many of these transition metal solids and molecular complexes are colored and many are magnetic. The colors are attributed to vibronic (electronic plus vibration) transitions between the d -orbital groups that are split in energy by the non-spherical potential of the ligands. When the ligand orbitals are included in determining the splitting, the procedure is called ligand-field theory. Splitting due to adjacent ligands is discussed in Chapter 6. Crystal-field theory was developed by Bethe [4.1] and Van Vleck [4.2] to explain the optical spectra of transition metal complexes and to understand their magnetic properties. In its simplest form the crystal-field model represents the ligands surrounding a metal ion as point charges that interact with the transition metal ion only through an electrostatic potential.