Dithiolene, structure

Figure 2. Distribution and frequency of homoleptic bis(dithiolene) structures.

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. 

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. 

An additional structural characteristic found in some tris(dithiolene) structures is the bend (p) between the SMS and SCCS planes in the five-membered... 

There are nine homoleptic tris(dithiolene) structures reported in the CSDC based on main group elements, specifically Sn, Sb, and In. The elemental distribution is shown in Fig. 15 and a detailed listing of the structures is given in Table V. 

There are only three types of crystallographically solved homoleptic mono-(dithiolene) structures [Tl(bdt)]2 2 (326) two examples of Ag(mnt) 4 4 with different cations (15) and [Pd(S2C2(COOMe)2)]6 n, where n 0 and —2 (378). The structures are all comprised of distinct mono(dithiolene) units that satisfy the coordination sphere of the central atoms through S atoms that form additional bridging bonds. 

Figure 26. Atom positions and bonds for the dimeric mono(dithiolene) structure [Tl (bdt)]2 2 (326). Selected distances and angles are reported. Significant ligand bends are observed (r = 37.2 and opposite, 50.5°).

Figure 27. Bonds and atom positions for the tetrameric mono(dithiolene) structure [Ag(mnt)]4 4 (15). There are two distinct, yet similar, tetramers. Only one is shown here. Selected average distances and angles are reported. See original reference for esd values.

Examination of structurally characterized dithiolene complexes identifies a number of important features of dithiolene ligand systems. The ligands largely maintain their shape, bond distances, and angles over a wide range of metal-based complexes. Even additional bridging bonds found in bis(dithiolene) dimers and multimeric mono(dithiolene) structures do not greatly alter the metal-dithiolene chelate. 

The structural information collected here will hopefully provide helpful knowledge and potential insight into the syntheses, characterization, and properties of dithiolene structures as a whole, including heteroleptic coordination environments where dithiolene ligands are found. 

The dithiolene structural unit that is common and required by all Mo and W enzymes was only recently identified definitively. After several decades of active research on molybdenum enzymes, conclusive structural evidence for the dithiolene piece was available as recently as 1995 (42). 

Average bond length and angle parameters for the homoleptic tris(dithiolene) complexes are less sensitive to the identity of the transition metal or dithiolene ligand than those of bis(dithiolene) structures. Ranges of values are summarized in Fig. 22. Average M—S bond lengths cluster between 2.263 and 2.543 A, with values for two [Fe(mnt)3]2 units (2.263 and 2.269 A) (18), Co CAlCO),, 3... 

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