Transition metal compounds lattice energies

One of the most successful applications of crystal field theory to transition metal chemistry, and the one that heralded the re-discovery of the theory by Orgel in 1952, has been the rationalization of observed thermodynamic properties of transition metal ions. Examples include explanations of trends in heats of hydration and lattice energies of transition metal compounds. These and other thermodynamic properties which are influenced by crystal field stabilization energies, including ideal solid-solution behaviour and distribution coefficients of transition metals between coexisting phases, are described in this chapter. 

A consideration of the lattice energies of transition metal compounds affords an explanation of the trends in the ionic radii of these elements as well as estimates of crystal field stabilization energies. This treatment offers a bridge between the sections (Chapters 2 through 6) on coordination compounds and those (Chapters 7 and 8) involving solid-state structures and energetics. 

There are several other types of thermodynamic data that reflect the ligand field stabilization caused by splitting the d orbitals. For example, the lattice energies of the MC12 (where M is a +2 transition metal ion) compounds also show a double humped shape when plotted as shown in Figure 19.8. However, these types of data will not be discussed because the trends follow naturally from the principles that have already been presented. 

Salts of the bases MOH are crystalline, ionic solids, colorless except where the anion is colored. For the alkali metal ions the energies required to excite electrons to the lowest available empty orbitals could be supplied only by quanta far out in the vacuum ultraviolet (the transition 5p6 —5p56s in Cs+ occurs at 1000 A). However, colored crystals of compounds such as NaCl are sometimes encountered. Color arises from the presence in the lattice of holes and free electrons, called color centers, and such chromophoric disturbances can be produced by irradiation of the crystals with X rays and nuclear radiation. The color results from transitions of the electrons between energy levels in the holes in which they are trapped. These electrons behave in principle similarly to those in solvent cages in the liquid ammonia solutions, but the energy levels are differently spaced and consequently the colors are different and variable. Small excesses of metal atoms produce similar effects, since these atoms form M+ ions and electrons that occupy holes where anions would be in a perfect crystal. 

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