Dithiolene Ni-complex

Dithiolene Ligand Vibrational Frequencies (cm-1) as a Function of Charge in Square-Planar (Bis)dithiolene Ni Complexes... 

Selected Urey-Bradley Bond Stretching Force Constants (mdyn A-1) for Square-Planar Bis(dithiolene) Ni Complexes"... 

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. 

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. 

The two gold thiophenedithiolenes give ciystal structures with very similar structural features. The average Au-S bond lengths exhibited are 2.327 A for 8 and 2.303 A for lOc, " consistent with previous An dithiolenes. As with the Ni complexes, no significant intermolecular short contacts are... 

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). 

These experiments showed that the Ni dithiolene is highly delocalized while Pd and Pt dithiolenes show less delocalization, a trend which parallels the H chemical shifts and the observation that the latter two form dimers while the Ni complex does not. In the ptt series, all three complexes show a lower as well as a completely undifferentiated degree of delocalization. These data are given in Table 3. For obvious reasons, similar measurements cannot be made for 1,1-dithiolenes. 

A powerful route to dithiolene complexes employs alkenedithiolate dianions generated by the hydrolysis of cyclic unsaturated dithiocarbonates, which are formally called l,3-dithiole-2-ones. Representative of the many examples (60), the base hydrolysis route has been used to prepare the ferrocene-substituted dithiolene Ni[S2C2H(C5H4)FeCp]2 (61), the sulfur-rich dithiolene [Ni(S2C2S2-C2H4)2F (62), the cyano(dithiolenes) fra s- Ni[S2C2H(CN)]2 (n = 1, 2) (63), 2,3-thiophenedithiolates [Au(S2C,H2S)21 (64), and the tris(styryldithiolate)... 

Figure 26. Molecular axes appropriate for [Ni(dithiolene)2f complexes and a MO diagram for Ni(S2C2H2)2- [Adapted from (283).]...

In one seminal work on dithiolene compounds, Schrauzer and Mayweg (37) considered three possible structures for neutral dithiolene nickel complexes in which the oxidation state of nickel may be Ni(0), Ni(II), or Ni(IV) [Scheme 8 ... 

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. 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

The neutral Ni complex of an extended dithiolene ligand, (CgH6Sg)2-(Scheme 12) has been reported (180, 186). The Ni(C8H6Sg)2 complex may be... 

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). 

The rednctive electrochemistry of several Ni complexes of unsaturated dithiolate ligands has been examined. On the basis of the electrochemical redox potentials, EPR spectral evidence, and SCF calculations, these rednction products are best formulated as Ni complexes for dithiocarbamate and 1,2-dithiolene hgands, and as Ni stabihzed ligand-radical anions for dithiodiketonate species. It is often difficnlt to assign electron-density distributions within a molecnle, particnlarly with delocahzed hgands such as dithiolenes. 

The neutral dithiolene complexes [Ni(S2C2(CF3)2] and [Mo(S2C2(CF3)2] are known as strong oxidants whose anions are stable, but their synthesis from the metal carbonyls [220] is tedious and dangerous in the case of the Ni complex because of the very high toxicity of [Ni(CO)4]. Therefore, they have been little used [221] for this reason and also because some other oxidants with analogous oxidizing properties are commercial or easily accessible. 

---