First transition series higher oxidation states

In the chemistry of nickel, we observe the continuing tendency for the higher oxidation states to decrease in stability along the first transition series unlike cobalt and iron, the -e3 state is rare and relatively unimportant for nickel and the +2 state is the only important one. 

The three elements to be treated in this chapter (Fe, Co, Ni) are the sixth, seventh, and eighth members of the first transition series. The first five members (Sc, Ti, V, Cr, Mn) have been treated in previous chapters (Chapters 12, 13, and 14).The ten elements of this first transition series (Sc through Zn) are characterized by electron activity in the 4s-3d levels. All elements in the 3d transition series are metals, and many of their compounds tend to be colored as a result of unpaired electrons. Most of the elements have a strong tendency to form complex ions due to participation of the d electrons in bonding. Unlike the previous three elements (V, Cr, Mn), these three do not show a variety of oxidation states. The higher oxidation states are almost absent in compounds, Fe showing principally the II and III, Co the II and III, and Ni only the II. The III states are less stable than the II states unless they are stabilized by complex formation. The resemblance of these three elements is notable, they being more like each other than they are to the elements below them. 

In this section the redox properties of the 10 elements of the first transition series are discussed. The lower oxidation states ( + 2 and + 3, + l of Cu) of these elements are treated together and the higher states are described and discussed separately. 

The available data (Bums, 1976a) show a trend towards the stabilization of progressively lower oxidation states at high pressures across the first transition series. Such observations indicate that higher oxidation states characteristic of the Earth s surface (Ti4, Cr34, Fe3+, Ni2+) may become unstable under the high P,T conditions of the Lower Mantle. Exotic oxidation states such as Ti(III), Cr(II), Fe(I), and Ni(I) could be prevalent towards the Core-Mantle boundary, particularly if hosted by sulphide phases. 

This is true not only for the first transition series but also for the heavier metals The ns electrons are lost before the (n — l) f or (a — 2)/ electrons. This gives a common + 2 oxidation state to transition metals, although in many cases there is a more stable higher or lower oxidation state. 

It is clear from the table that the maximum oxidation state of the group increases from left to right, peaks at Group 7 for the first row and at Group 8 for the second- and third-row transition metals, and then steadily decreases toward the end of the transition metal series. Within a group, the heavier elements can achieve higher oxidation states than first-row members (see, for example. Group 8). 

With the metals of the first transition series, the maximum coordination number of higher oxidation states is six, and this is so firmly fixed that in their fluoride complexes the oxidation state of the metal in question can be fixed by controlling the mol fraction of alkali metal present 26). Thus, the fluorination of a vanadium salt in the presence of a one, two, or three mol ratio of potassium ion, yields KV F6, KgWVFe, or KsV Fe. The same tendency is shown, but to a lesser degree, by metals of the second transition series, as exemplified by KRuFe and KgRuFe. For an unusual example in the third series, note that OsFe is known, OsFt is not stable, but heptavalent osmium is found as six-coordinated OsOFs (27). 

The first transition series or 3d-orbital series has a weak metallic character and less of atendancy for covalent bonding. This series also has strong oxidizing properties when in higher oxidation states. It forms many compounds with unpaired electrons, making it useful for catalysis and as magnetic materials. 

The first transition series, with the exception of scandium, tend to form 2+ cations. To do this, they must lose two 4s electrons from the valence shell. The 3d and 4s orbitals have similar energies so it s possible to lose a 3d electron too, giving rise to a cation with a 3+ oxidation state. A third electron can be removed also, but it requires much higher energy to do so. More electrons can be lost, too, and this is done by the sharing or loss of additional d-orbital electrons. 

The second and third transition series have trends in oxidation states similar to the first series. For higher oxidation states, the stability tends to increase as you go down the periodic table. For example, rust is quite stable (Fe203), but osmium tetraoxide (OsO ) is a volatile compound. It s so volatile that it can pass through the membrane material of an eye and bind to the back of the eye, causing blindness. 

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