Dithiolenes electron delocalization

A further class of dinitrosyl complex is that containing dithiolene and related ligands. Because of electronic delocalization, there are problems of assignment of oxidation state in... 

Dithiolenes are best considered to be a resonance hybrid of the limiting structures (1)—(3). In both bis- and tris-dithiolenes the electron delocalization is not limited to the ligand, but includes the metals to give rise to cyclic delocalization ( aromaticity ). To symbolize this electron delocalization in dithiolenes, they can be represented, in a manner similar to that used for benzene, by formulas containing a ring inside the framework given by the metal, sulfur and carbon atoms. We will use this notation, shown in (4), throughout this chapter. 

Although the ptt complexes are deeply colored,150 they lack the intense absorption at low energy found in the class of dithiolenes and which is assigned to a n-% transition of the delocalized system. The absence of such a transition in the ptt complexes is not a contradiction of the above results on the cyclic electron delocalization. It rather points out that the dithiolenes show delocalization throughout the entire complex unit as shown in (81), but the delocalization in the ptt complexes apparently involves the two ring systems independently, as shown in (82) the cyclic electron delocalization does involve the metal, but there is no crossover of delocalization from one half of the molecule to the other. 

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

The extensive electron delocalization in bis(dithiolene) complexes makes it possible for them to exist in a range of charge levels. It also makes oxidation state assignment of the metal and ligands potentially ambiguous. Mononuclear bis(dithiolene) complexes can undergo one, two, or even three reversible... 

Many of the general characteristics of dithiolene ligands and their metal complexes described in other chapters of this volume are also useful to Nature. A recurring theme of dithiolene coordination chemistry is their ability to stabilize multiple redox states for a wide variety of metals. The access to multiple redox states combined with a propensity for substantial electronic delocalization likely influenced Nature s choice of dithiolene as a chelating ligand over other potential bidentate sulfur donors. 

Interplay of metal/ligand electronic delocalization and solid-state magnetic behavior of paramagnetic Cp/dithiolene complexes as molecular hinges 04ACR179. 

Dithiolene complexes with the maleonitriledithiolate (mnt) ligand form highly delocalized systems and are widespread in studies of conducting and magnetic materials. The electronic properties have been extensively studied with various computational methods including Hiickel and extended Hiickel approaches to identify the nature of the orbitals involved in intramolecular and intermolecular interactions. These structural properties allow the complexes to interact in the solid state via short stacking S, S and short interstack S---S contacts.10 4-1048... 

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