Charge Density Studies of Transition Metal Compounds

The electron density in transition metal complexes is of unusual interest. The chemistry of transition metal compounds is of relevance for catalysis, for solid-state properties, and for a large number of key biological processes. The importance of transition-metal-based materials needs no further mention after the discovery of the high-Tc superconducting cuprates, the properties of which depend critically on the electronic structure in the CuO2 planes. The results of theoretical calculations of systems with a large number of electrons can be ambiguous because of the approximations involved and the frequent occurrence of low-lying excited states. The X-ray charge densities provide independent evidence from a technique with very different strengths and weaknesses, and thus can make significant contributions to our understanding of the properties of transition-metal-containing molecules and solids. In inorganic and organometallic solids, the average electron concentration tends to be high. This means that absorption and extinction effects can be severe, and that the use of hard radiation and very small crystals is frequently essential. Needless to say that the advent of synchrotron radiation has been most helpful in this respect. The weaker contribution of valence electrons compared with the scattering of first-row-atom-only solids implies that great care must be taken during data collection in order to obtain reliable information on the valence electron distribution. When the field exerted by the atomic environment is not spherically symmetric, as is the case in any crystal, the degeneracy of the d-electron orbitals is lifted. In the electrostatic crystal field theory, originally developed by Bethe (1929) and Van Vleck (1932), all interactions between the transition metal atom and its ligands are treated electrostatically, and covalent bonding is neglected. Since the ligands are almost always negatively charged, electrons in orbitals pointing towards the ligands are repelled more strongly, and the corresponding orbitals will be higher in energy. The discussion is the simplest for the one d-electron case, in which d-d electron repulsions are absent.