Tetravalent Chemistry: Inorganic

This chapter gives an overview on the chemistry of tetravalent lanthanide compounds, especially those of tetravalent cerium. Following a brief Introduction it covers the tetrahalides, dioxides and other lanthanides(IV) salts. Coordination compounds of cerium in the oxidation state +4 include halogeno complexes and complexes of oxo acids, β-diketonates and related Schiff-base complexes as well as porphyrinates and related complexes. Introduction Besides the ubiquitous oxidation state Ln, the higher oxidation state Ln is also encountered with some lanthanoids, for example in the case of the ions Ce (f, orange-yellow), Pr (f, colorless), Nd (f, blue-violet), Tb (f, colorless), and Dy (f, orange-yellow) (Table 1). However, all three states Ln are never encountered for the same element. Thus the highly important mechanistic steps of oxidative addition and reductive elimination typical for the d-block metals cannot occur with the f-block metals as they would involve M or M transformations, respectively. Table 1. Possible oxidation states for rare earth metals. Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu +2 +2 +2 +2 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +4 +4 +4 +4 +4 Among the tetravalent lanthanide ions, only Ce is readily available in aqueous solution (E Ce/Ce = +1.44 V in 2M H2SO4, 1.61 V in 1M HNO3, 1.70 V in 1M HClO4). The different values for the reduction potentials indicate that stable complexes are formed in these acidic media. Thermodynamically, the oxidation of water by the Ce aqua ion should be possible, but the system is kinetically stable. In contrast, the much more positive normal potentials of the other tetravalent lanthanide ions Pr, Nd, Tb, and Dy (e.g. Pr: +2.86 V) make them very strong oxidizing agents which readily decompose water under evolution of O2. The oxidation of Ce to Ce with the use of strong oxidizing agents like MnO4 or S2O8 (Scheme 1) enables the selective separation of cerium