Nickel dithiolenes

Several transition metal ions form stable complexes with aliphatic 1,2-dithiols, which absorb in the near-lR. Known as dithiolenes, their nickel complexes in particular have been found to have valuable properties. The physical properties of dithiolenes can be readily tailored by variations on the substituents attached to the dithiols, see (4.13). Although they have low molar absorption coefQcients, when compared to cyanines etc., they do have one big advantage in that they show very little absorption in the visible region. Stracturally analogous dyes can be made from aromatic dithiols and oxothiols (4.14), and the much more bathochromic naphthalene derivatives (4.15), but they are much weaker absorbers. 

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The most characteristic feature of nickel dithiolene complexes is the existence of an electron transfer series whose members are interrelated by reversible one-electron steps. Three members I-III of the series, I and III being diamagnetic and II having an S= 1/2 ground state, are preparatively accessible [Ni(S2C2R2)2]2 (I) <- [Ni(S2C2R2)2]1 - (II) <- [Ni(S2C2R2)2] (HI). 

Infrared absorbing nickel dithiolene complexes (68) are also used in certain applications (see Chapter 9.13). However, they are far less durable than the phthalocyanine infrared absorbers.61... 

Light produced during welding is extremely harmful to the eyes, and to offer full protection to the worker, radiation from the near-IR needs to be eliminated, whilst still allowing the wearer to see clearly. Polymer compatible diiminium and nickel dithiolene near-IR absorbers have been developed for this outlet, and also for the protection of the eyes of users of higher-powered lasers. Wide attenuation of light... 

The possibility of cis-trans equilibria exists, but has not been explored in depth. One indication for such equilibria comes from the NMR investigation of the parent nickel dithiolene in a nematic solvent only a rapidly equilibrating compound could have produced the observed spectral pattern.35 In the preparation of some mesomorphic dithiolenes related to the one cited above, two products were initially obtained, which during further work-up converted into the trans isomer only.38 In Pt complexes of this type, the existence of two 195Pt- H couplings was taken as an indication36 that the cis and trans isomers coexisted. 

Influence of Ligand Structure on the Lowest Energy Transitions for Neutral Nickel Dithiolenes RL-S., S-, R4... 

Even an olefin may be sufficiently electrophilic to react with co-ordinated thiolate, and some nickel dithiolene complexes have been shown to react smoothly with norbornadiene (Fig. 5-84). Naturally, the dithiolene complexes also react with more conventional electrophiles, such as methyl iodide (Fig. 5-85). 

Figure 5-85. The alkylation and demetallation of a nickel dithiolene complex with methyl iodide.

Yields are diminished if the filiation is conducted prior to the addition of the metal, indicative of the thermal instability of these dithioamides. Furthermore, the yields are lower when Ni(II) salts are used in place of Ni powder, consistent with the oxidative character of the dithioamide. Nickel dithiolenes of the dithio-oxamides exhibit extraordinary extinction coefficients ( 80,000 dm3 mol em 1 at 1000 nm) (186, 187). 

Mixed-ligand complexes such as nickel dithiolene diimine can be prepared via ligand substitution reactions (Eq. 6) (52). 

Indium tin oxide Bi-julodinyl nickel dithiolene Tetra-julodinyl nickel dithiolene Kalium tetracyanoplatinat (German)... 

The nickel-dithiolene complex (Figure 8.100) is considered as a potential means of recovering ethene, because it is capable of binding ethene reversibly by a redox-switch... 

With a view to preparing polymerizable complexes, thiophene-substituted nickel-dithiolene complexes [Ni(L)(L )] have been synthesized and used to prepare films by electrochemical polymerization. The features of the complexes and of the polymers depend on the number of thiophene substituents. In particular, the complex with four thiophene substituents (L = L = thpdt, 12) shows a narrower HOMO-LUMO gap as compared to complexes with two thiophene and two phenyl groups or four phenyl groups [Xmax, nm (e, M- cm- ) 976 (38800) L = L = 12 931 (37700) L = 12, L = 9, R = Ph 866 (30900) L = L = 9, R = Ph] and gave a polymer whose electrochemical features are similar to those of poly[l,2-di(2,5-thienylene)ethane], suggesting that similar extended chains are formed"". 

The name dithiolenes was chosen to describe these compounds without prejudice towards one of the limiting structures. The equally descriptive name dithienes has been coined for the same reason, but it is now rarely used. The less fortunate description of dithiolenes as dithiolato complexes is found occasionally, but it does have a much more restricted meaning (see Section 16.5.2.4) and should be avoided for the neutral species. Nevertheless, Chemical Abstracts refers to dithiolenes as bis[l,2-ethenedithiolato(2—)] complexes of the respective central metal, for example the parent nickel complex (4) is listed as nickel, bis[l,2-ethenedithiolato(2—)-5,S ]- however, depending on the date of the CA issue, its tetraphenyl derivative will be found either under bis[a,a -stilbenedithiolato(2 — )]-nickel or as bis[ 1,2-diphenyl-1,2-ethenedithiolato(2 -)-S,S"]-nick-el. Even less appropriate are the CA names for the radical anions and dianions of the dithiolenes, which are referred to as metallates(—) and metallates(2 —) of the respective ligands the dianion of the parent nickel dithiolene thus is found as bis[l,2-ethenedithiolato(2—)]-nickelate(2—), a name which has little to do with the electronic structure of the compound. 

Electrochemical methods have also been used to polymerize nickel dithiolene complexes with thiophene substituents in order to form conductive materials [93]. 

The mesogenic nickel dithiolenes were found to have reduction potentials (against calomel) of +0.06V and 0.76 V and as such were expected to form charge-transfer complexes with the mesomorphic, substituted tetrathiafulvalene (TTF) derivatives (Figure 89), which had oxidation potentials (also against calomel) of +0.43 V and +0.88 V. 

Charge-transfer complexes were indeed formed and in a variety of stoichiometries mixtures containing the decyl derivative of the nickel dithiolenes (Figure 89 R = CioH2i) were always smectic. 

To conclude, if we are to perform reliable calculations for solid-state systems, we have to think about the likely complexity of the electronic band stmcture of the material we are trying to model. If the bands are rather flat, as in the case of the nickel dithiolene complex, then it is probably sufficient to model the variation in the electronic bands using one wavefunction, which is often situated at the F position if, however, the band stmcture is more complex, then multiple k points (and therefore multiple wavefunctions) will have to be calculated, and summed, to generate the overall wavefunction that provides a more accurate reflection of the complex electronic effects created by an infinite array of interacting molecules. 

The photochemistry of dithiolene complexes of selected second- and third-row transition metal ions have also been investigated. Like the complexes with nickel, dithiolene complexes of palladium, platinum, molybdenum, and tungsten catalyze the formation of hydrogen from water when aqueous solutions of the complexes are irradiated at wavelengths shorter than 290 Tetrahydrofiiran is... 

Analogous di- and tetrasubstituted nickel dithiolenes prepared by Strzelecka et al. 

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