Observed Valence States

The respective term symbols X (electronic ground state) and A were first proposed [11, 12] in view of the Walsh [13] correlation diagram for molecular orbitals of the valence electrons of AHg molecules. The orbitals la (or lOg for the linear arrangement), Ibg (or lou), 2ai (or l7cj, and 1bi (or all for valence-electron numbering) then yielded the electronic configurations of the seven valence electrons of PHg as follows  

both states are correlated with 1o log IjcJ, of the hypothetical linear molecule [12]. The complete electronic configuration for the X ground state [14, 15] is based on an ab initio MO-SCF calculation of orbital energies [14]  

The positions of 1bi and 3ai, both correlated with lKu(core see below) were interchanged in another SCF calculation [16]. The corresponding expression for the linear case will be [15] 

The term symbols B, and were confirmed by rotational analyses, which yielded a type-C character of the observed electronic transition [8,17,18], see also a preliminary statement in [11]. 

The case of an asymmetric distortion (point group Cs) was considered in [19, 20] the term designations are then X A and A A.  

---

In addition to studies that have involved the determination of the thermodynamic states (potentiometric) and the observable valence states... 

According to the rotational constants X = A, B, C given in the preceding chapter, the asymmetry parameter k=(2B - A - C)/(A - C) takes values of about +0.6 and -0.8 for PHg in its observed valence states X and A A,. This shows the radical to be an asymmetric top in the ground state and a near-prolate symmetric top in the excited state. 

Approximate interbond angles a (for both observed valence states X and A A,), which were derived from earlier experimental work or from more qualitative theoretical concepts (e.g., the Walsh MO picture), are given In the chapter on the point group, p. 54. Results for the Internuclear distance and the Interbond angle, which were theoretically calculated by ab initio or semlempirical MO methods, are not reported explicitly. Ab initio calculations on r and a of a total of five valence states are in [8], of only the two observed valence states X and A in [9 to 11 ], of only the ground state in [12 to 20] and on a of both X and A in [21 ]. Semiempirical calculations were done on r and a of X [20, 22] and on a of both X and A [23] as well as the ground state X only [24]. 

The valence states of atoms will be classified as follows (cf. Sect. 3.5). Let be an ordered set of valence states of the atom with atomic number i> let be a subset of composed of those valence states that can be classified as "observable". For instance, the subset (Bg composed of "observable" valence states of carbon appears as follows... 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

The Re peak, present as a doublet in this catalyst, resembles the one obtained for Re20 on tape. This suggests, if rhenium hydroxides may be eliminated from consideration, that the calcined rhenium catalysts may have some rhenium in a valence state lower than 4-7. However, even reduction at 400 C of the uncalcined Pt-Re sample does not produce an observable amount of Re(0). 

Morrison and Hendrickson have reported that the IT band of biferro-cenium monocation has two peaks at 77 K (50), whereas the analysis of IT bands of biferrocene derivatives observed at room temperature is carried out assuming one broad peak, and it has been reported that Eq. (5) is applicable to biferrocene (41). As for the IT bands of higher unsubstituted oligoCferrocene-hl diylls in the mixed-valence states, only one report by Brown et al. on the IT bands of the monocation and dication of terferrocene and the dication of quaterferrocene (tetramer) has been published (64). 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Such highly ionized species have been detected for Cl-37 produced by the EC decay of Ar-37 in gaseous phase ((>). In solids, however, such anomalous states are not realized or their life time is much shorter than the half-life of the Mossbauer level (Fe-57 98 ns and Sn-119 17-8 ns) because of fast electron transfer, and usually species in ordinary valence states (2+, 3+ for Fe-57 and 2+, 4+ for Sn-119) are observed in emission Mossbauer spectra (7,8). The distribution of Fe-57 and Sn-119 between the two valence states depends on the physical and chemical environments of the decaying atom in a very complicated way, and detection of the counterparts of the redox reaction is generally very difficult. The recoil energy associated with the EC decays of Co-57 and Sb-119 is estimated to be insufficient to induce displacement of the atom in solids. 

Mixed valence aggregation, on the other hand, has never been observed — in low temperature matrices under conditions where dimeric aggregates do exist. Apparently, some minimum number of TTF molecules (greater than 2) appropriately oriented with respect to each other, are required for the stabilization of the mixed valence state. Both a crystalline matrix or a suitably constructed polymer could and seemingly does provide this environment. 

With regard to the latter point, the absence of a mixed valence transition in the oxidized low coverage polymer case is an important point. No mixed valence transtiion was observed over the whole range of oxidation (0-100%) studied. This indicates that whereas the D22+ aggregate is stable under these conditions, the mixed valent dimer analog, D2+, is not. At least in these polymeric matrices, therefore, the stoichiometric requirement for observation of the mixed valence state appears to involve (D2+)n where n > 2. 

Reductive elimination is simply the reverse reaction of oxidative addition the formal valence state of the metal is reduced by two (or one in a bimetallic reaction), and the total electron count of the complex is reduced by two. While oxidative addition can also be observed for main group elements, this reaction is more typical of the transition elements in particular the electronegative, noble metals. In a catalytic cycle the two reactions always occur pair-wise. In one step the oxidative addition occurs, followed for example by insertion reactions, and then the cycle is completed by a reductive elimination of the product. 

---