Magnetic properties and fluxional behaviour of heteroleptic cyclopentadienyl-dithiolene d(1) and d(0) niobium complexes

Paramagnetic d(1) niobium complexes of general formula [Nb(eta-C(5)H(4)R)(2)(dithiolene)] [R = SiMe(3) or Bu(t); dithiolene = C3S52- (4,5-disulfanyl-1,3-dithiole-2-thionate), C3OS42- (4,5-disulfanyl-1,3-dithiol-2-onate) or dddt(2-) (5,6-dihydro-1,4-dithiine-2,3-dithiolate)] have been prepared and their redox properties studied. The crystal structure determination of [Nb(eta-C(5)H(4)SiMe(3))(2)(C3S5)] showed that the folding angle of the NbS2C2 plane along the S-S axis is 34(2)degrees, an intermediate value compared with those of analogous d(0) (45-50 degrees) and d(2) (0-10 degrees) complexes, as rationalized by extended-Huckel calculations. The Curie-type temperature dependence of the spin susceptibility of [Nb(cp)(2)(C3S5)], [Nb(cp)(2)(C3OS4)] and [Nb(eta-C(5)H(4)R)(2)(C3S5)] (cp = eta-C5H5) demonstrates that the paramagnetic complexes do not interact with each other in the solid state. Cationic d(0) complexes were obtained either by chemical oxidation (tetracyanoquinodimethane) of the d(1) molecules or from the direct reaction of dddt(2-) with d(0) [Nb(eta-C(5)H(4)R)(2)Cl-2][PF6]. The fluxional behaviour of these d(0) complexes has been investigated by temperature-dependent NMR studies and shown to be comparable with that of isoelectronic d(0) [Ti(cp)(2)(dithiolene)] complexes.