Dithiolene metal complexes neutral

Potentially coordinatively unsaturated dithiolene-metal complexes are rare,298-306 and 1 1 dithiolene-transition-metal complexes with no other ligands are, to our knowledge, unprecedented.307 The neutral complex [PdS2C2(COOMe)2]6,308 is homoleptic containing one dithiolene unit for each palladium atom and no other ligands. Electrochemical reduction of the compound depicted in Figue 21 proceeds in four reversible steps. 

Since this chapter was written and during the editing procedures of this volume devoted to the dithiolene metal complexes, a review on metal 1,2-bisdithiolene complexes has been published by N. Robertson and L. Cronin, in Coord. Chem. Rev., 227, 93 (2002). Another point which deserves to be noted is the publication of the first neutral metal 1,2-bisdithiolene compound, namely Ni(tmdt)2, (tmdt2 = trimethylenetetrathifulvalenedithiolate), which exhibits a metallic character down to 0.6 K (H. Tanaka, Y. Okano, H. Kobayashi, W. Suzuki and A. Kobayashi, Science, 291, 285 (2001). 

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. 

As has been noted previously81, the structural parameters of metal dithiolene complexes are relatively insensitive to the overall charge on the complex, and the monoanionic dithiolene complexes generally retain the mmm (D2h) molecular symmetry found in both neutral and dianionic dithiolene complexes. The only notable structural trend is a lengthening of the M-S bond as the overall charge on the transition-metal complex increases. 

These complexes are redox active. The two one-electron reductions resemble more closely the reduction of the corresponding bis(diimine) complex than those of the corresponding bis(dithiolene) complex. The redox potentials are more sensitive to diimine ligand variation than to dithiolene variation. One-electron oxidation is relatively insensitive to diimine ligand variation. However, the dependence of one-electron oxidation on dithiolene variation has not been assessed directly due to the limited amount of data available on the oxidation of the corresponding dithiolene complexes. It has been proposed that the LUMO of the neutral mixed-dithiolene diimine complexes possesses more diimine than dithiolene character and that the HOMO is mainly metal d orbital in nature (52). 

Possible oxidation states of the metal in neutral tris(dithiolene) complexes have been considered. Upon complexation, the ethylenic bond in the dithiolene ligand has been observed to increase in length, and the C-S ... 

The electronic structure of dithiolene complexes is of crucial importance in determining the electron transfer in the derived compounds exhibiting interesting physical properties. Bis(l,2-dithiolene) metal(II) complexes, for example, may be isolated as dianions, monoanions, and neutral species... 

A close structural relationship with I IF derivatives, especially BEDT-TTF, is exhibited by dddt metal complexes [dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate (63)]. The most interesting feature of this dithiolene ligand is the ability of its metal complexes to form not only anionic salts like dmit, but also cationic salts like TTF derivatives [89], to afford non-stoichiometric IR salts of type [M(dddt)2]mX . Thus the cyclic voltam-mogram of [Bu4N][Ni(dddt)2], after its initial oxidation, exhibits the reduction of neutral [Ni(ddt)2]° to anion [Ni(ddt)2]" at 0 V, and its further reduction to the dianion [Ni(dddt)2]2 , as well as the oxidation of [Ni(dddt)2]° to the cation [Ni(dddt)2] + at 0.8 V (MeCN versus Ag/Ag/Cl). The feasible synthesis of conducting donor-acceptor complexes involving dddt metal derivatives as donors and dmit metal derivatives as acceptors has also been demonstrated [90]. 

Enedithiolate anions are redox-active ligands (see equation 1), capable of coordinating with a variety of metals as neutral dithioketones (1), thioketone radical thiolate monoanions (2) and ene-l,2-dithiolate dianions (3). The complexes are named dithiolenes irrespective of their oxidation state to remind one that they are noninnocent ligands . 

In complexes of the dithiolene type ligands the oxidation number of the metal cannot unambiguously be determined, due to the uncertainty of the charge on the ligand. R2C2S2 can be considered either as a dinegative dithiolato anion or as a neutral dithio-keto ligand. 

This makes it difficult to assign clearly separated formal charges to the metal and to the ligand, which would not be meaningful as a consequence of the noninnocent nature of 1,2-dithiolene ligands.53,54 In Scheme 5, the case of neutral diamagnetic square-planar Zw(l,2-dithiolene) complexes is illustrated. 

Scheme 5 Lewis structures for neutral bis( 1,2-dithiolene) complexes of d8 metal dications...

Synthesis. The first known thiophene-based metal dithiolenes were reported by Gol dfarb and Kalik in 1968." These consisted of the nickel and copper complexes of 2,3-thiophenedithiolate produced from ethyl 3-thienyl sulfide as shown in Scheme 1. Both complexes were reported as black, high melting solids and were thought to be the neutral bis(2,3-thiophenedithiolate) species. 

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). 

Square planar 1,3-dithioketonato complexes Ni(R2C3S2)2 are reminiscent of the 1,2-dithiolene complexes and their electrochemical behaviour is comparable. The neutral complexes have been found to undergo a one-electron reduction to the unstable monoanions [NifRjQS J- (89) in the range —0.9 to —1.1 V vs. SCE in MeCN or DMF at a Pt electrode.349 There is also some controversy as to whether the reduction of these complexes is metal- or ligand-based.349,350... 

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