Oxidation states down a group

Key points: In Groups 4-10, the highest oxidation state of an element becomes more stable on descending a group, with the greatest change in stability occurring between the first two rows of the d block; ease of oxidation of the metal does not correlate with the highest available oxidation state. The stability of an oxidation state of an element change on descending a group. Figure 19.5 is the Frost diagram for the chromium group: note that Mo (VI) and W(VI) lie below Cr (VI) in H₂Cr₂O₇, indicating that the maximum oxidation state is more stable for Mo and W, with the greatest change in stability occurring between Cr and Mo. This pattern is repeated

Figure 19.5 A Frost diagram for the chromium group in the d block (Group 6) in acidic solution (pH=0).

across the d block: for Groups 4-10 the highest oxidation state of an element becomes more stable on descending a group, with the greatest change in stability occurring between the first two series of the block. The increasing stability of high oxidation states for the heavier d metals can be seen in the formulas of their halides (Table 19.3), and the limiting formulas MnF₄, TcF₆, and ReF7 show a greater ease of oxidizing the 4d- and 5d-series metals than the 3d-series. The hexa- fluorides of the heavier d metals (as in PtF₆) have been prepared from Group 6 through to Group 10 except for Pd. In keeping with the stability of high oxidation states for the heavier metals, WF6 is not a significant oxidizing agent. However, the oxidizing character of the hexafluorides increases to the right, and PtF₆ is so potent that it can oxidize O₂ to O⁺₂:

Even Xe can be oxidized by PtF6 (Section 18.5). The ability to achieve the highest oxidation state does not correlate with the ease of oxidation of the bulk metal to an intermediate oxidation state. For example, although elemental iron is susceptible to oxidation by H(aq) under standard conditions,

no chemical oxidizing agent has been found that will take it to its group oxidation state of 8 in solution. By contrast, although the two heavier metals in Group +8 (Ru and Os) are not oxidized by H⁺ ions in acidic aqueous solution:

they can be oxidized by oxygen to the +8 state, as RuO₄ and OsO₄:

We see from Fig. 19.5 that the Frost diagrams for the positive oxidation states of Mo and (especially) W are quite flat. This flatness indicates that neither element exhibits the marked tendency to form M(III) that is so characteristic of chromium. Mononuclear complexes of Mo and W in oxidation states 3, 4, 5, and 6 are common.

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مواضيع ذات صلة

Thermochemical correlations

Magnetic measurements

Octahedral complexes

Sulfide complexes

Disulfides

Polyoxometallates

Mononuclear oxido complexes

Metal oxides

Structural trends

Intermediate oxidation states in the 4d and 5d series

Intermediate oxidation states in the 3d series

High oxidation states