Transition Metal Oxides and Salts

Scandates and titanates are colorless, since the lowest energy absorption is in the UV region. Vanadates are white-yellow, chromates orange-yellow, and permanganate violet. The intensity is high since transition is allowed. 

The d shell is filled from Sc + (3d ) to Zn + (3d °). Zinc oxide (ZnO) is colorless, but the others are at least weakly colored. The color is due to symmetry-forbidden transitions within the d shell, which become slightly allowed by vibrational coupling. If the complex is tetrahedral and missing a symmetry center, the color tends to be stronger since mixing of components in the states involved occurs, which allow the transition. Zn + has a filled 3d shell and thus no transitions are allowed within the 3d shell. 

CFT may be divided into weak field theory (WF-CFT) and strong field theory (SF-CFT). WF-CFT is a state interaction model of the type just mentioned. For example, the Ni ion with eight 3d electrons has a F ground state, consistent with Hund s rule. The crystal field is introduced as a perturbation and the F is split into three states. 

Reality is more complicated than this, however. By the same reasoning, CuO would be expected to be greenish-blue like Cu(H20) + in water, but is black. This is due to other types of excitations that are not included in the CFT. The black color of CuO may be due to intermetal coupling superposed on the CFT spectrum. Another possibility, also not within the CFT, is that the excitation is from the ligand levels of the 3d levels on the same metal complex. Some of these transitions, like the intermetal transitions, would have a high intensity. 

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Most of the transition metal oxides and salts may be treated as ionic crystals. The valence electrons of s symmetry (4s, 5s, or 6s) are stripped off. The metal forms an ion that is considerably smaller than the atom. The inner (n - l)d electrons (3d, 4d, or 5d) are degenerate in the central field approximation of the atom. In the next step, multiplets are formed (Section 2.4). In the third step, the transition metal ions interact with the crystal field, which is dominated by repulsion from the neighboring negative ions. This leads to a splitting of the 3d orbital energies. The theory describing the splitting of the electronic states in a crystal field is due to the American physicists Hans Bethe and John van Vleck and is called crystal field theory (CFT). 

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