Solids divalent states

Cyanide complexes of platinum occur most commonly in the divalent state, although there has been increasing interest in the complexes formed with platinum in a higher oxidation state. Among the complexes most recently studied have been the mixed valent complexes where platinum cyanides in the divalent state are partially oxidized. These complexes form one-dimensional stacks with Pt-Pt interactions. In the solid state these materials show interesting electrical conductivity properties, and these compounds are discussed by Underhill in Chapter 60. In this section the preparative procedures and spectroscopy of the complexes will be covered, but for solid state properties the reader is referred to Chapter 60. 

The solid is not affected in the presence of air for short periods, but the solution is easily oxidized. However, in the absence of air, the solution is stable indefinitely. The observed magnetic moment (fiet = 1.85 fiB) and four equally spaced, equally intense lines in the ESR spectrum clearly establish the divalent state for Au. The conductivity, electronic spectra, and cyclovoltammetry results corroborate this conclusion. The structure is suggested to be square planar (2). The same compound is also formed (121) by reacting equimolar amounts of Au(I) and Au(III) with the ligand (mnt)2- [Eq. (5)]. 

The divalent state, as M2 +, is well-established for both solutions and solid compounds of Sm, Eu and Yb (Table 27-4). Less well-established are Tm2+ and Nd2 +, but the +2 ions of all the lanthanides can be prepared and stabilized in CaF2, SrF2 or BaF2 lattices by reduction of, e.g., MF3 in CaF2 with Ca. 

Silver sulfide, when pure, conducts electricity like a metal of high specific resistance, yet it has a zero temperature coefficient. This metallic conduction is believed to result from a few silver ions existing in the divalent state, and thus providing free electrons to transport current. The use of silver sulfide as a solid electrolyte in batteries has been described (57). 

Among the many rare earths that can form solid-state compoimds existing in the divalent state, such as for instance RX2, X being a halogen, three of them can be classified as "common" europium, ytterbium and samarium. The chemistry of these species has been widely studied, and there are several reasons for this (a) they are not overly reactive in the sense that their redox potentials are not too negative so... 

Solid-state divalent scandium compounds are well documented (Meyer and Jongen, 2006 Poeppelmeier et al., 1980) and there are also some monovalent scandium compounds such as ScCl in which Sc-Sc bonds are present (Poeppelmeier and Corbett, 1977). So scandium opened the possibility to find monovalent molecular compounds, similarly to the well-known Al coordination chemistry, and since scandium looks like aluminium in some respects. Indeed, low-valent scandium molecular compounds have been found both in the monovalent and in the divalent state. 

The physico-chemical properties of Co ions in solid solutions of MgO have received much attention. In particular, the species formed upon interaction of CO have been intensively studied by i.r. " The observed bands are very similar in all cases, but the interpretation of the valence state of the Co in the surface carbonylic species is debated. Extensive surface reduction is infact hypothesized with formation of low-valent Co complexes, while the Co centre is still considered to be in its original divalent state. ... 

Removal of the templates from MgAPOs by calcination in oxygen can result in bridging hydroxyl groups or in the generation of Lewis acid sites which are thought to be Mg " cations that are not fully tetrahedrally coordinated within the framework. In the case of transition metals such as Mn, Fe " and Co " , calcination in oxygen results in the oxidation of some or all of the cations to the trivalent state. These can then be reduced back to the divalent state and protons introduced (Chapters 3 and 9). For substituted metal cations that show no redox behaviour, such as calcination results in solid acid catalysts,... 

In a vast majority of these applications, these ions are introduced into crystalline solids in their stable trivalent state. However, there are a few lanthanide ions whose divalent state is also stable, e.g., Eu ", Tm ". One of the main differences between the trivalent and divalent lanthanides (apart from the electrical charge) is that the charge-transfer transitions and the parity allowed 5d-4f transitions of the divalent ions are located at lower energies than those of the trivalent ions because of this they can more easily studied experimentally. Regarding Tm " ions, it should be kept in mind that the 4f electron shell has only two energy levels the ground state p7/2 and the excited state located at approximately 9500 cm [2]. As a result, the 4f-4f and 4f-5d transitions of the Tm ions do not overlap and can be easily studied separately. 

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... 

A note about the bonding and electron requirements is that the Be octahedron has been calculated to be two-electron deficient when bonding in solids, namely, it lacks two electrons to satisfy the bonding orbitals. Therefore, for trivalent rare earth atoms, which supply three electrons, there is one extra electron in the conduction band, and RBg for trivalent rare earths can be well understood to be good metals. When the metal atom is divalent, such as for europium in the divalent state and calcium, the hexaboride becomes semiconducting or similar to a semimetal. 

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. 

Only Eu and Yb, among the rare earths, form immiscibility gaps with Sc in the liquid and solid. The difference in valence of these two rare earths and Sc is, perhaps, the main reason of this difference of interaction. The divalent state is more typical for Eu and Yb, whereas the other rare earths are usually trivalent. The same divalent state is characteristic for aUcaline earths which are immiscible with Sc in the liquid and solid too. Therefore, it is possible to predict the same type of binary phase diagrams of Sc with alkaline metals. These elements are even more different from Sc in the valence state and other characteristics (melting temperature, electronegativity, etc.) than are the alkaline-earth metals. 

Americium, unlike europium, does not have a divalent state in aqueous solution this was long greeted with some surprise. However, Am(n) has been prepared in the solid compounds AmQj, AmBr2, and Aml2 and demonstrated as a dilute solution in Cap2 [99-102,105]. Reduction conditions used to prepare Eu and Sm " do not reduce Am (aq) to Am (aq) [229]. Myasoedov et al. claim electrochemical evidence for unstable Am(ii) in acetonitrile, instantly oxidized by water in the solvent [230]. Sullivan et al. [356] formed transient Am(ii) by pulse radiolysis, with an absorption maximum at 313 nm, and tj/2 5 x 10 s for disappearance. 

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