Fluorides of Lower Oxidation States

The permanganate oxidation of phenols is complicated by the intervention of lower oxidation states of manganese, (c/. the oxidation of toluene, p. 298). For example, the oxidation of 2,6-dinitrophenol in weakly acidic solution displays an induction period, following second-order kinetics thereafter. However, addition of potassium fluoride inhibits reaction almost completely, but manganous ions strongly accelerate it. 

Actinides Generally present in fully oxidized form, but salts such as nitrates, fluorides, and chlorides are common in solution Fully oxidized forms are insoluble. Oxides of lower oxidation states may be slightly soluble. Actinide salts are soluble... 

At least in compounds with metals of lower oxidation states up to three or four, the metal-fluorine bond is almost purely ionic and the small, least polarizable fluoride ion behaves as a small hard sphere with an ioitic radius which varies, depending on the coordination number, between 1.285 (CN 2) and 1.320 A (CN 6). The structures of fluorides are therefore determined mainly by simple geometrical and electrostatic principles, such as sphere packing, the rigid sphere concept, which allows derivation of the coordination number around... 

Figure 1.3 NAO/NLMO p/s hybridization ratios for bonds and lone pairs of second- and third-period hydrides and fluorides in lower oxidation state (B3LYP/def2-TZVP results).

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

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 ... 

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. 

Table 10.5 lists some properties of these compounds. As usual in the lower oxidation states, the fluorides have significantly lower volatilities. This can alternatively be explained... 

Several trends are noticeable from the data in the table. The bond strengths of the interhalogens are clearly related to the dift erence in electronegativity between tbe component halogen atoms, as expected on the basis of Pauling s ideas on ionic character (Chapter 5). Furthermore, the tendency to form the higher fluorides and chlorides depends upon the initial eiectronegativiiy of the central atom.- Only iodine forms a heptafluoride or a trichloride. Not shown in Table 17.2 (except indirectly by computation from the values) is the instability of certain lower oxidation states to disproportionation ... 

In the early sixties, there was still much disarray in our knowledge of the binary fluorides of the noble metals. B. Weinstock and his coworkers had done much to clarify the pattern of highest oxidation-states, but there was still, at that time, much confusion concerning lower oxidation state fluorides. Our preparation of PtFs (see Ref. 17) and the similarity of its physical properties to those of the supposed... 

---