4-2 oxidation state dithiolene complexes

In the Cp2M(dithiolene) series, d° complexes were investigated essentially with Ti, and to a lesser extent with Zr and Hf, in their IV oxidation state. These complexes can be reversibly reduced to the d1, Tim anionic species but they were never isolated in the solid state. Attempts to oxidize these d° complexes were also unsuccessful, as electrochemical oxidation leads to their decomposition [23, 24]. The essential structural characteristic of these d° complexes is the strong folding of... 

Several reviews presenting various aspects of dithiolene chemistry have appeared over the years,1015-1026 so that this summary will only focus on some selected findings. Also, the photodissociations of Ni dithiolene complexes, which lead to long-lived Ni complex radicals, have been reviewed.1027 Since many studies in the field of Ni-dithiolene chemistry deal with different oxidation states of the metal, this chapter will also cover much work related to Ni111 and NiIv. 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

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). 

Matsubayashi et al. revealed donor abilities of the unsymmetrical diimine-dithiolene complexes [11-14]. The unsymmetrical complexes provided cation radical salts with various anions including I3, Br3 and TCNQ by use of chemical oxidation [11-14]. The electrical resistivities of the cation radical salts measured with their compressed pellets at room temperature are summarized in Table 1. The electrical resistivities of the dmit complexes were very high. The cation radical salts of the CgH4Sg-complexes, which have the BEDT-TTF moiety [22, 23], exhibited lower resistivity than those of dmit complexes, except for [(Bu-pia)Pt(CgH4Sg)] salts. However, crystal structures of these salts were not reported, and details of their electrical properties and electronic states were not discussed based on their crystal structures. 

Chapter 2- Transition metal dithiolenes are versatile complexes eapable of a wide range of oxidation states, coordination geometries, and magnetie moments. As a eonsequence, these complexes have been widely studied as building bloeks for erystalline molecttlar materials. Particularly successful are the square-planar metal dithiolenes (Chart 1), from which materials have been produeed that exhibit condueting, magnetic, and nonlinear optieal properties, as well as supereonduetivity in some cases. " In their application to molecular- based... 

Evidence of the existence of oxidation state +IV is not yet convincing. Some complexes with dithiolene or dithiolate systems have been reported, such as [Au(5,6-dihydro-l,4-dithiin-2,3-dithiolate)2] (Figure 1.76) [350], or [Au(2,3-dithiophe-nedithiolate)2] [351]. They were prepared by chemical or electrochemical oxidation of... 

A further class of dinitrosyl complex is that containing dithiolene and related ligands. Because of electronic delocalization, there are problems of assignment of oxidation state in... 

The electron transfer properties of nickel 1,2-dithiolenes Ni(R2C2S2)" have been extensively studied in recent years, but there is still much controversy concerning the nature of the bonding in these complexes. On the basis of a simple VB model neutral and binegative species (87) may be assumed to contain the metal in the oxidation state +2, mononegative species may contain either nickel(II) or nickel (III), and trinegative species are assumed to contain nickel(I). 

A square planar bis-chelate complex with dithiotropolonate monoanion has been isolated in the solid state (295).2142 This complex does not exhibit the electrochemical properties of the bis(l,2-dithiolene) complexes and undergoes ring alkylation and oxidation reactions (Scheme... 

A qualitative description of the bonding in dithiolenes, as exemplified by the neutral d complex Ni(S2C2H2)2, involves the structures (1)—(3) in which the metal assumes formal oxidation states of 0, 2+ and 4 +, while the ligands are either neutral dithiodiketones or dinegative dithiolates . 

Multifunctional dithiolene ligands have been used to prepare binuclear as well as polymeric complexes. Some examples of this type of complex were already mentioned above (25 and 41) and involve the ligands benzenetetrathiol (51) or its alkyl ethers,78 the tetrathiosquaric acid (52), ethylenetetrathiol (53) and tetrathiooxalic acid (54) and its Se analog. The two-electron difference in oxidation state between (53) and (54) is much the same as that discussed for the parent dithiolene ligand. Crystallographic results (see Section 16.5.3.1) point out the equivalence of the latter two ligands. 

---