1.2- Dithiolenes ligand structure

Figure 1. Dithiolene ligand structures, abbreviations, and nomenclature for common examples.

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an... 

The structures and redox properties of these complexes have been extensively reviewed 170,171 of interest here is the presence of an intense NIR transition in the neutral and mono-anionic forms, but not the dianionic forms, i.e., the complexes are polyelectrochromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. A large number of substituted dithiolene ligands has been prepared and used to prepare complexes of Ni, Pd, and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1,400 nm and extinction coefficients up to ca. 40,000 dm3 mol-1 cm-1 (see refs. 170,171 for an extensive listing). 

We are therefore faced here with radical complexes which easily distort depending on the structural arrangement and whose SOMO is different for every crystal structure associated with a given counter-ion, a very original feature in these series. The unfolded d1 complexes can be described as Mo(IV) complexes with a spin density essentially localized on the dithiolene ligand while the more folded complexes have a stronger metal character. This variable spin density delocalization is expected to influence strongly the amplitude and dimensionality of intermolecular interactions between radical species in the solid state, as detailed below in Sect. 3. 

The Mo(III) d3-d3 complexes [CpMo(dithiolene)]2 are characterized by a single Mo—Mo bond, further stabilized by interaction with the n system of the dithiolene ligands. Indeed, the analogous complexes where the two dithiolene are replaced by four thiolate groups were found to oxidize more easily and salts of the cationic d3-d2 [CpMo(SMe)4MoCp]+ were even isolated and structurally and magnetically characterized [50]. 

We have seen above several examples where [Cp2M(dt)]+ (M = Mo, W) complexes organize in the solid state into low dimensional structures, leading to characteristic magnetic behaviors such as spin chains (eventually alternated) or spin ladders. The extensive use in later years of dithiolene ligands such as dmit or dddt was aimed at... 

Dithiolene complexes was the name suggested by McCleverty47 for complexes of unsaturated 1,2-dithiols without implying any particular ligand structure or valence formalism. The dithiolene complexes described differ from the vanadium(III) dithiolates (Section 33.4.6) reported by three independent groups.48... 

Dithiolenes have been found to stabilize nickel(III) and a number of structural investigations have been performed on nickel(III) dithiolene complexes. Structural data and physical properties of selected compounds are collected in Table 120. The EPR spectra of the [NiS4] unit have been extensively studied in order to decide whether the unpaired electron resides mainly on the metal or on the ligand3202,3203,3210,3212-3217 giving rise to a true nickel(III) complex (422) or to a nickel(II)-stabilized ligand radical complex (423). 

The dithiolene ligands offer a variety of synthetic and structural possibilities and choices. The following description introduces in an intentionally general fashion (which some might consider to be superficial) the various classes and describes the methods used in their synthesis, while a more detailed discussion of their properties will be given in the subsequent parts of this section. 

Phosphines also are able to (a) add to a vacant coordination site in planar dithiolenes or split dimers such as [Fe(mnt)2]2 to form52 square pyramidal adducts of structure (36) and (b) displace certain dithiolene ligands to form dithiolato complexes of the general structure (37). These reactions also are briefly discussed in Section 16.5.3.8. 

Relating to the work of Heber and Hoyer42 (see Section 16.5.2.4.1) on methyl ethers of the dithiolene ligand, Richter et al % determined the structure of tris(S-methylethylenedithiolato)Rh and found an octahedral structure, a cis arrangement of the methyl groups and no evidence for interligand interactions of the S atoms. 

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

To underpin the syntheses, properties, and applications of dithiolene molecules, this chapter presents a comprehensive discussion of structurally characterized homoleptic dithiolene complexes. That is, the structural unit must contain dithiolene ligands as the only ligand type, and there must be only one type of dithiolene ligand. Emphasis is placed on structural aspects such as coordination geometries, bond distances and angles, and on identifying trends related to the specific dithiolene ligand and the identity and formal oxidation... 

Another characteristic measure of geometry is the bend between the SMS and SCCS planes of a dithiolene ligand chelate (r ) as shown in Scheme 4. Over 90% of bis(dithiolene) structures demonstrate average values of r <6°, indicative of the highly planar nature of the dithiolene-ligand chelate. 

There are roughly 50 homoleptic tris (dithiolene) complexes reported in the CSDC (5). The elemental distribution of these structures is outlined in Fig. 15. As opposed to bis(dithiolene) complexes, tris(dithiolene) complexes are based predominantly on early transition metal elements. Many of the tris(dithiolene) complexes are centered on V, Mo, and W. There are also complexes of Ti, Zr, Nb, Ta, Cr, Tc, Re, Ru, and Os. In addition, there are tris(dithiolene) complexes of Fe and Co, elements that also form homoleptic complexes with two dithiolene ligands. A detailed listing of the structural units along with references and geometrical parameters (to be discussed) is given in Table IV. 

It is not always the case that the dithiolene ligands are arranged in such a symmetrical fashion about the central metal atom. For example, the anionic portion of [AsPh4][Ta(bdt)3] (355) is shown in Fig. 21. The structure demonstrates two dithiolene ligands, defined by atoms S2 and S3, and S4 and S5, are closer to the TP extreme (0 = 16 and 16°, respectively). The third ligand, defined by atoms SI and S6, is twisted toward the OCT extreme (0 = 54°). This asymmetric distortion leads to large distortion parameters as outlined in Table IV (ASMStrans = 11.5° 8 = 11.8°). 

---