Dithiolenes metal complexes

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. 

Scheme 4 Redox processes involving bis( 1,2-dithiolene) metal complexes...

Over the past decade a number of new covalently bonded TTF/ferrocene adducts have been reported [77, 78]. The crystal structure of the l,l -bis(l,3-dithiole-2-ylidine)-substituted ferrocene derivative has been published [77]. In this complex, ferrocene has essentially been incorporated as a molecular spacer between the two l,3-dithole-2-ylidene rings forming a stretched TTF molecule. This adduct, and its methyl-substituted derivative, have been combined with TCNQ to form charge-transfer complexes with room temperature powder conductivities of 0.2 S cm-1. Similar diferrocenyl complexes have been prepared with bis (dithiolene) metal complexes [79, 80]. 

Scheme 3 The main classes of heteroleptic Cp/dithiolene metal complexes...

Therefore the most common homoleptic bis-dithiolene metal complexes [Mn(S2C2R2)2]n" (M = Ni, Pd, Pt) can potentially take part in the redox sequence illustrated in Scheme 7. Once again, this has to be considered a simplified sequence in that the occurrence of internal metal-to-ligand electron transfer cannot be ruled out. 

In reality, different classes of derivatives contribute the general definition of bis-dithiolene metal complexes, Scheme 8.72... 

Bis(dithiolene)-metal complexes can also assume a tetrahedral geometry, for instance [Co(dmit)2]2-,74 [Cu(dmit)2]2-,75 and [Zn(dmit)2]2-.76 In this connection it is useful to note that [Co(edt)2]2- also possesses a tetrahedral geometry,76 whereas [Co(mnt)2]2- is planar.77,78... 

All crystals in this class are either insulators or semiconductors, and with a single exception (discussed in Sect. 3.3), they are ionic charge-transfer salts. The cations are generally either planar aromatics, such as TMPD+ and TTT+ (Sect. 3.2) or onium counter ions, such as Et4N+ and MePh3P+ (Sect. 3.3), although some cationic metal complexes are also known. For a majority of the systems, the anionic components are bis-1,2-dithiolene metal complexes, which have been studied and reviewed extensively81-84. ... 

Vibrational spectroscopy has not been extensively used in the characterization of tris(dithiolene) metal complexes. Moreover, complete assignments based on both IR and Raman spectra, isotope shifts, and normal mode calculations are not available for any individual complex. However, the available data suggest that the trends in M S and dithiolene ligand vibrational modes as a function of the metal, the charge on the complex, and dithiolene substituents, closely parallel those discussed above for square-plane bis(dithiolene) metal complexes. Accordingly, the vibrational data are consistent with highly delocalized complexes with predominantly ligand-based redox chemistry. 

The case for highly delocalized frontier orbitals and predominantly ligand-based redox chemistry in tris(dithiolene) metal complexes has been most convincingly made by the recent structural, vibrational, and DFT results for members of the related [M(CO)2(S2C2Me2)2]0,1 2 and [M(S2C2Me2)3]0 1 2 (M = Mo, W) series of complexes (39). The latter is the only set of tris(dithio-lene) complexes that has been isolated and structurally characterized in three oxidation states. While the vibrational studies focused on IR spectra of the [M(CO)2(S2C2Me2)2]° 1 2 series of complexes, the close parallel in... 

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). 

Another very important group of organic acceptors employed as components for organic metals during the last decade is presented by dithiolene metal complexes [e.g., l,3-dithiole-2-thione-4,5-dithiolate (dmit) ligand and... 

The expansion of the ir-conjugating system with outer heterorings, the idea presented by the ET molecule, can be applied to 1,2-dithiolene metal complexes with the donor character. For example, the M(dddt)2 (dddt=5,6-dihydro-l,4-dithiin-2,3-dithiol M = Ni, Pd, Pt, Au) molecule, where the central C = C double bond in ET is replaced by the transition metal, exhibits various molecular arrangements, some of which are similar to those found in the ET salts [29]. In the frontier molecular orbital of the 1,2-dithiolene complexes, the 3p orbitals of S atoms in the ligand show a significant contribution. In this sense, the molecular design for the 1,2-dithiolene complexes can be discussed in common with that for the organic ir molecules. 

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