2 - Outline of crystal field theory

Summary

Crystal field theory gives a survey of the effects of electric fields of definite symmetries on an atom in a crystal structure.

– – A direct physical confirmation should be obtainable by analysis of the spectra of crystals.

H. A. Bethe, Annalen der Physik, 3, 206 (1929)

Introduction

Crystal field theory describes the origins and consequences of interactions of the surroundings on the orbital energy levels of a transition metal ion. These interactions are electrostatic fields originating from the negatively charged anions or dipolar groups, which are collectively termed ligands and are treated as point negative charges situated on a lattice about the transition metal ion. This is a gross simplification, of course, because sizes of anions or ligands such as O²⁻, OH⁻, H₂O, SO₄²⁻, etc., are much larger than corresponding ionic radii of cations (Appendix 3). Two effects of the crystalline field are the symmetry and the intensity of the electrostatic field produced by the ligands. The changes induced on the central transition metal ion depend on the type, positions and symmetry of the surrounding ligands.

Orbitals

The position and energy of each electron surrounding the nucleus of an atom are described by a wave function, which represents a solution to the Schrödinger wave equation. These wave functions express the spatial distribution of electron density about the nucleus, and are thus related to the probability of finding the electron at a particular point at an instant of time.