1.2- Dithiolenes Dithiolene-based compounds

It is the only series of dithiolene-based compounds (and the only series of molecular metal complex-based compounds) exhibiting superconducting behavior. 

TABLE 6. Structural and physical properties of novel metal bi(l,2-dithiolene)-based compounds... 

In this section, we will focus only on compounds based on M(dmit)2 anions and (Cp 2Mn)+, since Gama and Almeida also report on metal-dithiolene containing compounds with metalloceniums in Chapter (see pp. 97-140). 

Solid-State Properties (Electronic, Magnetic, Optical) of Dithiolene Complex-Based Compounds... 

Only in recent decades has it become clear that inorganic molecule-based compounds can possess remarkable physical properties. These properties can be electrical, magnetic, or optical. One of the first typical examples is KCP (kalium tetracyanoplatinat, in German), K2[Pt(CN)4X0.3] /1H2O X = Cl, Br), which was prepared as early as 1842 (1, 2). This complex was shown to be a metal-like conductor more than a century later (3, 4). Likewise, but within a shorter period of time, the use of metal complexes of 1,2-dithiolene ligands (Scheme 1), first... 

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS 401... 

Information on the synthesis of the dithiolene ligands and their complexes may be found in Chapter 1 of this volume (39). Likewise, information on the preparation of the dithiolene complex-based compounds described in this chapter may be found in previous articles and reviews (9, 10, 40-46) and will not be discussed here. 

In spite of the large number of dithiolene palladium complex-based compounds with tetraalkylammonium cations that have been prepared, only a few... 

Very few dithiolene platinum complex-based compounds have been reported and most of them exhibit poor electrical conductivity (9, 122, 123). The complex (Me4N)[Pt(dmit)2]2 does show metal-like behavior (RT conductivity = 10 S cm 1) but only >220 K (123). Another exception is (NHMe3)[Pt(dmit)2]3. This compound, which has the unusual 1 3 stoichiometry, behaves as a metal down to 180 K, with a room temperature conductivity of 140 S cm-1 (91). The Pt(dmit)2 units form pairs with an eclipsed overlap mode. Unlike the analogue Ni derivatives (see above) substituting one hydrogen atom for one methyl group in the (Me4N)+ cation leads to an improvement in the electrical properties. 

The superconducting properties of these dmit-based salts have prompted extensive studies of related dithiolene-based systems obtained by changing the nature of the metal, the cation in NIOS salts, or the donor in D-A compounds, and by using other ligands resembling dmit2. To date, no superconductor has been characterized in any of these related series. 

The spin-Peierls transition (18, 19) is a peculiar magnetoelastic transition that may take place in a limited series of quasi ID insulating systems (20, 286). It is interesting to note that, for once, this behavior had been theoretically predicted for organic free radicals (18, 19) long before its experimental observation. It happened that the first experimental observation of a spin-Peierls system was reported for dithiolene systems, namely, (TTF)[M(tfd)2], where M is Cu or Au (25, 26). However, since bis(l,2-dithiolene) complex-based compounds have a tendency to form ID stacks (see above), this priority is not truely amazing. 

A large number of papers have appeared on this research reporting additional theoretical and experimental studies on the (TTF)[M(tdf)2] (M = Cu, Au) compounds, and detailed reviews are available (20, 286). Note, however, that these dithiolene complex-based compounds were the first members of the quite limited series of spin-Peierls systems. Among these, another related, selenium analogue dithiolene complex-based compound, (TTF) Cu[Se2C2(CF3)2]2, also undergoes a spin-Peierls transition at 6 K (290). 

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