Oxides halides, divalent states

The rare earth chlorides can be separated through sublimation but a very high temperature and good vacuum are required. Recently [46] Eu2+ has been obtained pure by the distillation of its halides using the fact that Eu2+-halides are less volatile than the halides of trivalent rare earths. Sm, Eu and Yb oxides can be reduced to the divalent state by carbon and volatilized selectively from a mixture with other rare earth oxides [47]. 

The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX2L] (with L = (203), R,R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.551 The redox behaviour of these compounds was of interest because it was considered that the potentially -acceptor macrocycle (203 R = R = Me) may promote the formation of Mn° or Mn1 species or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation it was also of interest to study whether this was the case for the present systems. 

The II State. All the elements Ti to Cu inclusive form well-defined binary compounds in the divalent state, such as oxides and halides, which are essentially ionic. Except for Ti, they form well-defined aqua ions [M(H20)6]2+ the potentials are summarized in Table 17-1. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

The enthalpy of vaporization of the metals refers to the process Ln s) — LU(g). This has an influence in the stability of oxidation states of the lanthanides (see Variable Valency, The Divalent State in Solid Rare Earth Metal Halides, and Tetravalent Chemistry Inorganic) and the variation of AHvap across the series is shown in Figure 3. 

In order to fully understand the crystal chemistry of the anhydrous LnXs and their solvates ([LnXj(solv) ]), the Ln atomic properties of these species must be considered. The predominant oxidation state for LnX species is the +3 state however, for a number of these cations, tiie +2 (see The Divalent State in Solid Rare Earth Metal Halides) and +4 (see Tetravalent Chemistry Inorganic) states are available. Since the bonding in these compounds is mainly ionic, the cation size and sterics of the binding solvent play a significant role in determining the final crystal structures isolated. The ionic nature of the LnX complexes makes... 

Oxide-halides of the alkaline-earth-hke divalent lanthanides, OlGlXe (R = Eu, Yb, Sm) are discussed in The Divalent State in Solid Rare Earth Metal Halides. Although, these have the topology of cluster complexes with isolated... 

Up until the last 25 years, the known oxidation states for molecular rare earth compounds were those of the final colunrn of Table 1.1 not in parentheses. Thus, all rare earths form compounds in very stable -i-III state, three Sm, Eu, Yb give +II compounds and Ce, Pr, Tb have compounds in the +IV state. As shown by the data in parentheses in the final column of Table 1.1, new oxidation state molecular compounds have been a recent research highlight, " and the divalent state is now known for all rare earth elements except Pm. On the other hand, ionic divalent halides LnX (X=Cl, Br, I) have been known for elements (Ln=Nd, Dy, Tm) other than Ln= Sm, Eu, Yb for many years from the work of soUd state chemists. However, the diiodides, Lnl (Ln=La, Ce, Pr, Gd) do not contain Ln but Ln " and are to be formulated La (e )(T)j. They show metallic properties. ... 

Another example of a divalent metal of this group, but which in fact is probably entirely analogous to the dihydiides, is LaL. However, the most extensive set of examples of these metals in low formal oxidation states is provided by the binary and ternary halides produced by... 

The dihalides of Si and Ge are polymeric solids that are relatively unimportant compared to those of Sn and Pb. The latter elements are metallic in character and have well-defined +2 oxidation states. Physical data for the divalent halides are shown in Table 11.1. The compounds of Si(II) are relatively unstable because the reaction... 

A variety of halide and haloamine complexes have been prepared which show evidence for one-dimensional structures in the solid state. The materials can be broken down into (1) complexes containing metals in the same oxidation state and (2) complexes comprised of metals in different oxidation states. Type (1) complexes may be dianions and dications, for example Magnus Green Salts or alternatively chains of neutral molecules, for example, Pt(en)Cl2. Type (2) complexes are comprised of alternating square planar rf metal complexes (metal = Pdii,Pt ,Aui i) and octahedral / metal complexes (metal = Pd, Pt ), or linear platinum complexes are formally trivalent while the gold complexes are formally divalent. 

All divalent, trivalent, and tetravalent chlorides, bromides, and iodides are hygroscopic and an appreciable solution chemistry has been characterized, but only aspects of solution chemistry that relate to hydrates are considered here. The solid-state chemistry of the lower oxidation states (to IV) of cations in combination with the F, Cl, Br, and I atoms, and combinations of these with main-group cations is considered. The numerous reactions some of these halides undergo with organic reagents and solvents are considered only to the extent they relate to solid-state chemistry. 

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