Header Ads

Header ADS

Crystal-Field Theory (Exercises )

Although the ability to form complexes is common to all metal ions, the most numerous and interesting complexes are formed by the transition elements. Scientists have long recognized that the magnetic properties and colors of transition-metal complexes are related to the presence of d electrons in metal orbitals. In this section we will consider a model for bonding in transition-metal complexes, called the crystal-field theory, that accounts for many of the observed properties of these substances. (The name crystal field arose because the theory was first developed to explain the properties of solid, crystalline materials, such as ruby. The same theoretical model applies to complexes in solution.)

We have already noted that the ability of a metal ion to attract ligands such as water around itself can be viewed as a Lewis acid-base interaction. The base—that is, the ligand—can be considered to donate a pair of electrons into a suitable empty orbital on the metal, as shown in Figure 24.27. However, we can assume that much of the attractive interaction between the metal ion and the surrounding ligands is due to the electrostatic forces between the positive charge on the metal and negative charges on the ligands. If the ligand is ionic, as in the case of Cl or SCN, the electrostatic interaction occurs between the positive charge on the metal center and the negative charge on the ligand. When the ligand is neutral, as in the case of H2O or NH3, the negative ends of these polar molecules, containing an unshared electron pair, are directed toward the metal. In this case the attractive interaction is of the ion-dipole type. In either case the result is the same; the ligands are attracted strongly toward the metal center. The assembly of metal ion and ligands is lower in energy than the fully separated charges, as illustrated on the left side of Figure 24.28.



Figure 24.27 Representation of the metal-ligand bond in a complex as a Lewis acid-base interaction. The ligand, which acts as a Lewis base, donates charge to the metal via a metal hybrid orbital. The bond that results is strongly polar, with some covalent character. It is often sufficient to assume that the metal-ligand interaction is entirely electrostatic in character, as is done in the crystal-field model.

To continue click on the link below:

http://wps.prenhall.com/wps/media/objects/3313/3393071/blb2405.html

No comments

Powered by Blogger.