Bis-dithiolene-transition metal

Figure 5. Ranges of average M S bond lengths for bis(dithiolene) transition metal structures.

Figure 7. Plot showing the inverse correlation between average MSC and SCC angles within homoleptic bis(dithiolene) transition metal complexes.

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Dichalcogenolene ligands form complexes with main group and d transition metal ions.103 Bis( 1,2-dithiolene) complexes have been obtained for metals, such as Cr, Mn, Ni, Cu, Zn, for the first row, Pd, Ag, and Cd for the second row, and Pt, Au, and Hg for the third row. Homoleptic /ra(l,2-dithiolcnc) complexes have been obtained for Ti, V, Cr, Zr, Nb, Mo, Tc, Ru, Ta, W, Re, and Os. Fe and Co have been found both in bis and /ra(l,2-dithiolene) complexes, although /ra(l,2-dithiolene) complexes containing these metal ions... 

The first metal bis-dithiolene complex with an SCO moiety as counter-ion was reported in 2005 [99], namely [Fe(sal2-trien)][Ni(dmit)2] (Fig. 8). Usually, [Fe(sal2-trien)]+ is a Fera complex which exhibits very modest magnetic properties a gradual and incomplete spin transition is observed when it is combined with (PF6) and (BPI14), and the complex remains in the LS state with Cl-, I-and (NO3)-. 

Coordinated transition metal redox-active macrocycles, 39 108-124 ammonium cation, 39 128-133 crown ether and bis crown ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithiolene complexes, 39 123-124 metalloporphyrin crown ether compounds, 39 108-109... 

There are roughly 421 reports of homoleptic bis(dithiolene) units based on transition metal elements. The approximate tally of the structures as a function of central metal atom is outlined in Fig. 2. The examples predominantly contain late transition metals. The majority of complexes are Ni based, partially because of interest in these complexes for materials applications. Other common central elements are Cu, Pd, Pt, Au, and Zn. There are also a few Fe and Co complexes and a small number of structures based on Cr, Mn, Ag, Cd, and Hg. 

The first-row transition metal elements, Cr, Mn, Fe, and Co, comprise a small number of monomeric structures. The only structural report based on Cr is [Cr(bdt)2]2- (Table IIA, Entry 18). The geometry is distorted tetrahedral (A = 83.9°). The Cr— S bond length of 2.364 A is rather long, the ninth longest of all metal bis(dithiolene) structures. 

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. 

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. 

Most of the late transition metals (such as Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Au, and Zn) have been found to form bis(dithiolene) complexes. A significant amount of work has been reported on the electronic structures and spectroscopy (32), redox properties (2), as well as the conductivity (33) of bis(dithiolene) complexes. Far less has been reported on their chemical reactivity. 

A great amount of research has focused on the electronic structure, spectroscopy, redox properties, and conductivity of homoleptic bis(l,2-dithiolene) complexes of c transition metal ions (51). The compounds are often highly... 

Although only rarely luminescent in ambient fluid solutions, square-planar transition metal bis(dithiolene) complexes do display significant and varied photochemical reactivity. Much of the photoreactivity described above for dianionic bis(dithiolene) complexes involves excited-state oxidation and often leads to radical formation. In addition, the excited states of these complexes are receiving attention for their potential as materials for optical (15), nonlinear optical (10-13), and electrooptical (16) devices. The relevance of this work to those applications is addressed in other parts of chapter 8 in this volume (87b). 

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