Periodicity II Valencies and Oxidation States

Titanium is the first member of the 3d transition series and has four valence electrons, 3d24s2. The most stable and most common oxidation state, +4, involves the loss of all these electrons. However, the element may also exist in a range of lower oxidation states, most importantly as Ti(III), (II), (0) and —(I), Zirconium shows a similar range of oxidation states, but the tervalent state is much less stable relative to the quadrivalent state than is the case with titanium. The chemistry of hafnium closely resembles that of zirconium in fact, the two elements are amongst the most difficult to separate in the periodic table. 

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

Additional areas of lanthanide halide chemistry that have been reviewed include the synthesis, phase studies, and structures of complex lanthanide halides - compounds formed between one or more group 1 cation and lanthanide element halides (Meyer 1982). Halides in combination with lanthanide elements in the II, III, and IV oxidation states were considered with the chemistry of the heavier halides being emphasized. More recently the reduced ternary lanthanide halides (Meyer 1983) and the reduced halides of the lanthanide elements were reviewed (Meyer 1988). The latter review considered lanthanides in which the formal oxidation state of the cation was 2 and included hydride halides, oxide halides, mixed-valence ternary halides, and reduced halide clusters. Corbett et al. (1987) discussed the structures and some bonding aspects of highly reduced lanthanide halides and compounds stabilized by a second-period element bound within each cluster, e.g., SC7CIJ2B, SC5CI5B, YjCljC. 

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