Halides divalent states

Brown, I. D. and Duhlev, R. (1991). Divalent metal halide double salts in equilibrium with their aqueous solutions II. Factors determining their crystal structures. J. Solid State Chem. 95, 51-63. 

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 monosulfides of the rare earth elements behave differently from that discussed here since these are metal-like for all but those elements forming the most stable divalent states. Here a general proclivity towards forming tripositive ions seems more important (as in the metals themselves)—a property usually considered to result from a fortuitous balance between ionization and lattice or solvation energies. The sulfides have also been interpreted in terms of a degeneracy of the upper 4f levels with a 5d band (where applicable) (10). In contrast to the halides, there is little differentiation of the electrical properties among the monosulfides, monoselenides, and monotellurides (29). 

The novalac phenolic resins react under weakly acidic and anhydrous conditions using metal catalysts of the divalent state, e.g. Ca, Mg, Zn, Pb, Cd, Co, Ni and Cu acetals, halides or sulphonates. The mechanism of this ortho-hydroxymethylation reaction has been attributed to the formation of chelate complexes as intermediates as proposed occurs at the initial and subsequent condensation reaction of the phenolic alcoholics. It has been shown that electropositive bivalent metals work best when pH is between 4 and 7. This may be described as follows ... 

D.F.Evans prepared divalent organo ytterbium for the first time by the reaction of ytterbium metal with organic halides in THF at - 20 C. Magnetic susceptibility measurements showed that 75-92% of the ytterbium is present in the divalent state. Samarium reacts more slowly, giving a mixture of divalent and trivalent species, as expected from the greater instability of the divalent state when Sm is compared to Yb (30). These solutions give Grignard-like reactions for example, PhSmI reacts with benzophenone to afford Ph C(OH) in 72% yield. 

Homogeneous Catalysis Lanthanide Halides Organometallic Chemistry Fundamental Properties Tetravalent Chemisiry Inoiganic Tetravalent Chemistry Organometallic The Divalent State in Solid Rare Earth Metal Halides The Electronic Structure of the Lanthanides. 

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. 

The Divalent State in Solid Rare Earth Metal Halides... 

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... 

The pattern as seen in Figure 5 may be transferred to a periodic table of the rare earth elements, see Figure 6. Only elements underlaid in red form clusters. The lower I3 is, the easier it is to produce cluster complexes. Elements underlaid in blue form stable divalent compounds, for example EUCI2 the divalent state with the electronic configuration 4f 5d° (with n =7, 14, 6, 13 for R = Eu, Yb, Sm, Tm) has the highest stability and, thus, is the easiest to achieve when the third ionization potential is the highest. The divalent chemistry of these elements is alkaline-earth and saltlike this is described in The Divalent State in Solid Rare Earth Metal Halides. 

One of the main differences, therefore, between actinides and rare earths is the steeper change of the f-d energy difference along the actinide series (fig. 3). This first of all makes americium trivalent, but also implies that a series of elements at the end of the actinide series will be divalent. Indeed, the divalent state of nobelium is so relatively stable that, of the possible halide compounds, the only trivalent compound it will be able to form is the trifluoride (Johansson 1977a). 

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. ... 

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