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(S2C2R2)(3)](z) [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 (Theta) and dithiolene fold angles (alpha) 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(AL(3)(5-center dot))](1-) (L-A = 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(BL36-)](1-) (L-B = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z2))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [w(v)(L-B(3)6-)](r-). The EPR spectra of [W(L-A(3))(1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W-IV(L-A(3)5-*)](1-) <-> [W-V(L-A(3) (6-))](1-)}. The contrast with the corresponding [Mo-IV(L-A(3)5-)](1-) complexes highlights tungsten's preference for higher oxidation states.