Monoanionic molybdenum and tungsten tris(dithiolene) complexes: a multifrequency EPR study.

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: M(S(2)C(2)R(2))(3) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is Mo(IV)((A)L(3)(5-•)) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as Mo(V)((B)L(3)(6-)) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound W(bdt)(3) exhibits spin-Hamiltonian parameters similar to those of Mo(bdt)(3) and thus is formulated as W(V)((B)L(3)(6-)). The EPR spectra of W((A)L(3)) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {W(IV)((A)L(3)(5-•)) ↔ W(V)((A)L(3)(6-))}. The contrast with the corresponding Mo(IV)((A)L(3)(5-•)) complexes highlights tungsten's preference for higher oxidation states.