Lower oxidation states halides

We also investigated solutions of quinones and substituted catechols, in the latter case using mixtures of ammonia and an organic solvent to achieve sufficient solubility, and here the electrochemical efficiency shows that indium goes to the +III oxidation state. In the presence of an o-quinone, oxidation of the lower oxidation state halide leads to InX(catecholate), and hence by substitution to InX3> When a 1,2-diol is used, there are again questions as to the solute species which are generated at the cathode, but the overall reaction can be written as... 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Lower oxidation states are rather sparsely represented for Zr and Hf. Even for Ti they are readily oxidized to +4 but they are undoubtedly well defined and, whatever arguments may be advanced against applying the description to Sc, there is no doubt that Ti is a transition metal . In aqueous solution Ti can be prepared by reduction of Ti, either with Zn and dilute acid or electrolytically, and it exists in dilute acids as the violet, octahedral [Ti(H20)6] + ion (p. 970). Although this is subject to a certain amount of hydrolysis, normal salts such as halides and sulfates can be separated. Zr and are known mainly as the trihalides or their derivatives and have no aqueous chemistry since they reduce water. Table 21.2 (p. 960) gives the oxidation states and stereochemistries found in the complexes of Ti, Zr and Hf along with illustrative examples. (See also pp. 1281-2.)... 

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. 

Several oxohalides are also known, mostly of the types An OaXa, An OaX, An OXa and An "OX, but they have been less thoroughly studied than the halides. They are commonly prepared by oxygenation of the halide with O2 or Sb203, or in case of AnOX by hydrolysis (sometimes accidental) of AnX3. As is to be expected, the higher oxidation states are formed more readily by the lighter actinides thus An02X2, apart from the fluoro compounds, are confined to An = U. Conversely the lower oxidation states are favoured by the heavier actinides (from Am onwards). 

The short-lived [MH2(Cp)2] and [TaH4(dmpe)2] have been obtained from the Mv hydrides using photogenerated r-butoxy radicals, and were characterized by low temperature ESR.575 On the other hand, thermally stable, well-defined dinuclear or mononuclear MIV hydrides have been prepared by oxidative addition of H2 to dinuclear Mm or mononuclear Mn halide phosphine adducts, respectively. They constitute attractive entries to lower oxidation state compounds, and will be reviewed in Sections 34.4.3.l.i and 34.6.1.2.i. 

The chemistry of niobium and tantalum in their lower oxidation states is expanding rapidly. The first structurally characterized molecular Nb111 derivative was reported in 1970,525 while Nb111 and Tam halide adducts were described in 1973580 and 1978, respectively.581... 

VO(OR)3 are very convenient and easily available precursors for the preparation of a wide range of V(V) derivatives and via their reduction by matalla-lkyls even for the derivatives of the lower oxidation states. The complete or partial substitution of OR-groups has led to halides, carboxylales, P-diketo-nates, alkyls, sulfides, azids, complexes with Shiffbases, phenylisocyanate, and so on. Hydrolysis of VO(OR)3 is discussed in Chapter 9. 

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 ... 

Halides and oxides corresponding to each of the states are known but will not be described here. The lower oxidation states of vanadium are summarized in Table 26-2. 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

Both of these elements show a marked tendency to form metal atom cluster compounds in their lower oxidation states. The best known are oxo and halide cluster complexes. 

Like the halides, the oxohalides are hydrolyzed readily those of MVI1 go to M04, while those in lower oxidation states disproportionate to give M04 and lower oxides. 

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