Hafnium lower 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. 

In the lower oxidation states the chemistry of titanium has little or no counterpart in the chemistries of the group IVB elements. The only lower oxidation state of these elements is two, for which the stability order is Ge < Sn < Pb. However,, both zirconium(III) and hafnium(III) are similar to if less stable (towards oxidation) than titanium(III) and have comparable although less extensively investigated chemistries. 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

The organometallic chemistry of titanium is dominated by complexes in the +IV oxidation state and in comparison there are relatively few examples of titanium complexes in the +III oxidation state. For information on organotitanium(iv) see Chapter 4.05. However, examples of titanium(lll) complexes are more common than examples of titanium complexes in lower oxidation states (for information on organotitanium in oxidation states 0 to II see Chapter 4.03) and titanium(m) chemistry is considerably more advanced than the chemistry of the heavier group 4 metals, zirconium and hafnium in the +m oxidation state. For information on organozirconium(m) and organohafnium(m) see Chapter 4.07. 

The lanthanide contraction, however, has also effects for the rest of the transition metals in the lower part of the periodic system. The lanthanide contraction is of sufficient magnitude to cause the elements which follow in the third transition series to have sizes very similar to those of the second row of transition elements. Due to this, for instance hafnium (Hf ) has a 4" -ionic radius similar to that of zirconium, leading to similar behavior of these elements. Likewise, the elements Nb and Ta and the elements Mo and W have nearly identical sizes. Ruthenium, rhodium and palladium have similar sizes to osmium iridium and platinum. They also have similar chemical properties and they are difficult to separate. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it no longer noticeable due to the so-called Inert Pair Effect (Encyclopedia Britannica 2015). The inert pair effect describes the preference of post-transition metals to form ions whose oxidation state is 2 less than the group valence. 

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