High oxidation states

Key point: The group oxidation state can be achieved by elements that lie towards the left of the d block but not by elements on the right. Oxygen is usually more effective than fluorine at bringing out the highest oxidation states because less crowding is involved. The group oxidation state (in which the oxidation number is equal to the Group number) can be achieved by elements that lie towards the left of the d block but not by elements on the right (Section 9.5). For example, Sc, Y, and La in Group 3, with configurations nd1(n+1)s², are found in aqueous solution only in oxidation state 3, which corresponds to the loss of all their outermost electrons, and the majority of their complexes contain the elements in this state. The group oxidation state is never achieved after Group 8 (Fe, Ru, and Os). This limit on the maximum oxidation state correlates with the increase in ionization energy and hence noble character from left to right across each series in the d block. The trend in thermodynamic stability of the group oxidation states of the 3d-series elements is illustrated in Fig. 19.3, which shows the Frost diagram for species in aqueous acidic solution. We see that the group oxidation states of Sc, Ti, and V fall in the lower part of the diagram. This location indicates that the element and any species in intermediate oxidation states are readily oxidized to the group oxidation state. By contrast, species in the group oxidation state for Cr and Mn (+6 and+7, respectively) lie in the upper part of the diagram. This location indicates that they are very susceptible to reduction. The Frost diagram shows that the group oxidation state is not achieved in Groups 8-12 of the 3d series (Fe, Co, Ni, Cu, and Zn), and also shows the oxidation states that are most stable under acid conditions; namely Ti³⁺, V³⁺, Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, and Ni²⁺.

The binary compounds of the 3d-series elements with the halogens and with oxygen also illustrate the trend in stabilities of the group oxidation states. The earliest metals can achieve their group oxidation states in compounds with chlorine (for example, ScCl₃ and TiCl₄), but the more strongly oxidizing halogen fluorine is necessary to achieve the group oxidation state of V (Group 5) and Cr (Group 6), which form VF5 and CrF₆, respectively. Beyond Group 6 in the 3d series, even fluorine cannot produce the group oxidation state, and MnF₇ and FeF8 have not been prepared. Oxygen brings out the group oxidation state for many elements more readily than does fluorine because fewer O atoms than F atoms are needed to achieve the same oxidation number, thus decreasing steric crowding. For example, the group oxidation state of +7 for Mn is achieved in manganate (VII) salts, such as potassium permanganate, KMnO₄. As can be inferred from the Frost diagram in Fig. 19.3, chromate (VI) CrO₄^-2, manganate (VII) MnO₄, and ferrate (VI) FeO_4^-2 are strong oxidizing agents and become stronger from CrO4 2 to FeO₄^-2. This trend is another illustration of the decreasing stability of the maximum attainable oxidation state for Groups 6, 7, and 8. Yet another example of the greater difficulty of oxidizing an element to the right of Cr to its group oxidation state is that the air oxidation of MnO₂ in molten potassium hydroxide does not take Mn to its group oxidation state but instead yields the deep green compound potassium manganate (VI), K₂MnO₄ . The disproportionation of MnO₄^-2 in acidic aqueous solution yields manganese (IV) oxide (MnO₂) and the deep purple manganate (VII) ion MnO₄:

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قسم الشؤون الفكرية ينظم برنامجًا دينيًّا لكشافة الكفيل في النجف الأشرف

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دراسة: كوب ميلك شيك واحد قد يكون "قنبلة دماغية"

هل يعد "الباراسيتامول" خيارا آمنا للنساء الحوامل؟

دراسة تكشف الرابط بين العلاقات العاطفية والرضا عن الحياة

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كارثة صامتة.. "الهواء الملوث" يسرق نور عيون الأطفال

دراسة: رصد أول دليل على وجود "إشعاع هوكينغ"

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المزيد

مواضيع ذات صلة

Thermochemical correlations

Magnetic measurements

Octahedral complexes

Sulfide complexes

Disulfides

Polyoxometallates

Mononuclear oxido complexes

Metal oxides

Structural trends

Oxidation states down a group

Intermediate oxidation states in the 4d and 5d series

Intermediate oxidation states in the 3d series