Fractional oxidation state compounds

Unfortunately, whatever the metallocenium and the metal bis-dithiolene moiety, no fractional oxidation state compound with formula (Cp 2M ) [M(M02] (with x < 1) has been reported in the literature. Concerning (Cp 2M )[Ni(dmit)2] (M = Fe, Cr, Mn), all our attempts to obtain chemically or electrochemically (Cp 2M )x[Ni(dmit)2] (with x < 1) have resulted in the synthesis of noncrystalline samples as sticky powders or mixture of fibers and grains, whose characterization was not possible. Therefore the combination between M(dmit)2 and metallocenium has been unsuccessful to obtain magnetic molecular conductors. 

Although combined with [Ni(dmit)2], none of the above-mentioned compounds exhibit electrical properties, since they are 1 1 materials without any charge transfer. One of the first attempts to obtain fractional oxidation state compound was performed by us in 2006 [101]. [Fe(sal2-trien)][Ni(dmit)2]3 has been obtained by electrocrystallization from an acetonitrile solution of [Fe(sal2-trien)][Ni(dmit)2], This compound behaves like an SC (ctrt = 0.1 S cm ) but does not exhibit any spin transition. This seems to be due to the statistical disorder of the whole Fem complexes, which prevents the occurrence of short contacts between cations. 

Chronologically, Sato and coworkers have been the first to obtain and to characterized unambiguously a fractional oxidation state compound containing the Ni (dmit)2 unit and an SCO cation, namely [Fe(qsal)2][Ni(dmit)2]3.CH3CN.H20 [106]. This complex has been obtained after electrocrystallization from an acetonitrile solution of [Fe(qsal)2][Ni(dmit)2].2CH3CN. 

The use of another SCO Fera complex [Fe(salEen)2]+ has afforded the fractional oxidation state compound [Fe(salEen)2]2[Ni(dmit)2]5.6CH3CN [104]. The latter is... 

Figure 4.18 Synthesis of the [dmit] ligand, its storable derivatives and [M(dmit)2] compounds. (FOSC = fractional oxidation state compound DA = donor-acceptor compound)...

A large number of C [M(dmit)2] complexes (n = 2, 1, 0) have been prepared. " The dianion salts [R4N]2[M(dmit)2] can be obtained by treating a solution of Na2dmit in methanol with the appropriate M metal salt and the appropriate tetraalkylammonium bromide (Figure 4.18). The corresponding monoanionic salt [R4N][M(dmit)2] is obtained from the dianionic salt by iodine oxidation. Further oxidation may lead to the neutral complex or to fractional oxidation state compounds (FOSCs) or donor-acceptor (DA) compounds. These oxidation steps are described in Section 4.3.2. 

To our knowledge, there are less than 30 compounds based on radical-cations and M(dmit)2 systems (Table 2). Most of them contain divalent or monovalent M(dmit)2 units, and only a few of them have been structurally and magnetically characterized. Since they are not in a fractional oxidation state, they behave as insulators with low room-temperature conductivity. 

Evidently, the most interesting materials are those in a fractional oxidation state, with general formula (cation)[M(dmit)2] (n > 1), since they can exhibit both electrical and magnetic properties. Only eight such complexes have been reported so far. All of them but (BDTA)[Ni(dmit)2]2 [89] have been obtained as powders. They have in general been poorly characterized, and their stoichiometries have been determined from elemental analysis. Among these powdered compounds, the... 

There are many important mixed valence and fractional formal oxidation state compounds (e.g., Fc304) and complexes of iron. Mixed valence trinuclear /.t-oxo-carboxylato-complexes are dealt with in Section 5.4.5.4.2, polynuclear complexes held together by sulfide bridges in Section 5.4.5.9.2. 

The three fluoro-acids HF, HS03F and CF3S03H have proved to be fruitful and essential media for preparation of many compounds of ionic species of transition metals in unusually high or low oxidation states and of cations of non-metallic elements in fractional oxidation states. 

Thus, disproportionation occurs when a highly electrophilic polyatomic non-metal cation interacts with a nucleophilic base to form, in the first instance, a polyatomic cation of lower charge per non-metal atom (lower fractional oxidation state) and a compound, essentially covalent, formed between the base and the non-metal of the cation, with that non-metal now in a higher oxidation state than in the parent cation. For example It disproportionates to IJ and IF5. With greater availability of base, the polyatomic cation first formed will disproportionate to a lower-charge polyatomic cation and ultimately to the free element from which the parent cation was formed. Similar patterns differing in detail, are observed for metallic polyatomic cations, monatomic transition metal cations in low oxidation states and for all naked electrophilic cations. 

III. Compounds of Gold in Fractional Oxidation States Gold Clusters. 243... 

In the case of DA compounds, both donor and acceptor molecules may undergo oxidation towards fractional oxidation states. In general, their half wave potentials are similar and formation of the DA compound may compete with crystal growth of FOSC phases. This feature was illustrated in the case of TTF[Ni(dmit)2]2. Single crystals of this phase were electrocrystallized using TTF and TBA[Ni(dmit)2]. The cyclic voltammogram of each compound shows that [Ni(dmit)2] is oxidized at a potential 120 mV lower than that of TTF. However, addition of TTF to a solution of (Bun)4[Ni(dmit)2] inhibits the formation of the (Bun)4 o.29[Nl(dmit)2] FOSC phase in favor of the TTF[Ni(dmit)2]2 one.76... 

Metal bis-dithiolene complexes exhibiting conducting or superconducting properties can be neutral or in a fractional oxidation state. In FOSCs, the associated cation is of the closed-shell type, while in DA compounds, the metal bis-dithiolene is an acceptor and the donor is an open-shell cation. As already mentioned in Section 4.2.2.3, the most encountered... 

In most cases, oxidation states are positive or negative integers however, on occasion an atom within a compound can have a fractional oxidation state. Consider KO2. The oxidation states are assigned as follows ... 

Dihalodicarbonyl iridium compounds constitute an important class of chain compounds. These compounds are synthesized by treating K2lrXg with CO under pressure or heating in formic acid. Single crystal study of Pr(CO)2Cl2] revealed a chain structure. Alkah metal counterions compensate the charge. The fractional oxidation states of iridium in this series vary in the small range of -1-1.39 to -1-1.44. " These compounds exhibit metallic copper luster and metal-like conductivity. 

Despite the pancity of well-characterized gold(0) compounds, there are numerous cluster complexes that contain gold in a mixed or fractional oxidation state that lies between Au(0) and Au(I). These clnsters are described in a later section on clusters and chains with fractional oxidation states. 

The +3 oxidation state is exhibited by bismuth in the vast majority of its compounds. A few inorganic and a variety of organic compounds, however, contain the element in the +5 state. Other rarer oxidation states reported for bismuth include +2, +1, and —3. Bismuth also forms polynuclear ionic species with oxidation states that ate usually fractional and range from —1 to +1. 

Uranium dioxide, UQ2, can be further oxidized to give a nonstoichiometric compound U024v, where 0 < x < 0.25. See Exercise 5.77 for a description of nonstoichiometric compounds, (a) What is the average oxidation state of uranium in a compound with composition UO,j-> (b) If we assume that the uranium exists in either the +4 or the +5 oxidation state, what is the fraction of uranium ions in each ... 

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

In fact, one of the peculiar properties of the title class of compounds is the ability of the molecular entity to carry a charge which can vary considerably, also assuming fractional values in non-integral oxidation state (NIOS) salts. The different molecular oxidation states are reversibly accessible by chemical or electrochemical means. A good example is the case of fe(l,2-dithiolene) complexes of ds metal ions [such as Ni(II), Pd(II), Pt(II), Au(III)], whose charge can assume values typically ranging between —2 and 0 (see Scheme 4). 

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