High-oxidation state transition-metal fluorides

High Oxidation State Transition Metal Fluorides... 

Some chemistry of high oxidation state transition metal fluorides... 

G. Lucier, C. Shen, W. J. Casteel, Jr., L. Chacon and N. Bartlett, Some Chemistry of High Oxidation State Transition Metal Fluorides in Anhydrous HF, J. Fluorine Chem. 72 (1995) 157-163. 

The highest halide of each metal is of course a fluoride Rep7 (the only thermally stable heptahalide of a transition metal), TcFg, and MnF4. This again indicates the diminished ability of manganese to attain high oxidation states when compared not only to Tc and Re but also to... 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

However, for very high oxidation states, which are formed notably with transition metals, for example, WF6 or OsF6, the energy available is quite insufficient to allow ionic crystals with, say, W6+ or Os6+ ions consequently such fluorides are gases, volatile liquids, or solids resembling closely the covalent fluorides of the nonmetals. It cannot be reliably predicted whether a metal fluoride will be ionic or molecular, and the distinction between the types is not always sharp. 

An extremely important class of fluoro-anions will be presented in Sec. 11.3.5 where it will be shown that it is possible to generate fluoro-anions of transition metals in unusually high oxidation states, e.g. MnIVF, AgmF4 and NilvFj and to isolate the corresponding binary fluorides from HF solutions of these anions. 

The syntheses cited in this section have all involved attainment of high oxidation states in transition metal fluoro-anions, and thence in binary fluorides and in cationic species, by oxidation with F2 or other fluoro-oxidants in HF of metals or of compounds in lower oxidation states. A couple of examples are offered of syntheses of new fluoro-oxo-compounds involving fluorination of oxides already in high oxidation states with rare gas fluorides in HF. The ratio of F 0 in the ligands of these new compounds is greater than in the related compounds already reported. 

Addition of complex alkali metal salts to a high temperature system leads to negative ion formation in the vapour. KCMS-IME can be applied to electron affinities and the estimation of negative ion enthalpies of formation. The EA of fluorides and oxides of transition metals in their higher oxidation states were determined. Almost all of these compounds have EA in the range 3.5-7.0 eV. Application of this method to fullerene vapours has yielded the electron affinities for the series of higher fuller-enes C (n=70, 72. 106). 

Shannon and PrewiLL [5] considered a very large collecLion of crysLallographic data for metal oxides and metal fluorides. Included in these data were transition metal compounds in which the bonding between cation and anion is both electrostatic and covalent in character. They noted that the eflective radius for a given ion depends on its coordination number in the crystal. In addition, for transition metal ions the radius depends on whether the d electrons in the ion are in a high or low spin state. By assigning a radius of 126 pm to the ion and 119 pm to the F ion when they are surrounded by six counter ions, they found that the eflective radii of the alkali metal ions and halide ions are very close to those obtained from electron density maps for the corresponding crystals. Their results for these ions are also summarized in table 3.1. 

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