Bis(?5-cyclopentadienyl) dihalides of niobium(IV) and tantalum(IV), [MX2(cp)2] (M = Nb or Ta; X = Cl, Br, or I; cp = ?5-C5H5) have been prepared by known methods or via metathetical displacement of chloride ligands in [MCl2(cp)2] by bromide or iodide ions. The new, characterised bis(monothiobenzoate) complexes, [M(SCOPh)2(cp)2] (M = Nb or Ta), have been synthesised from [MCl2(cp)2] and TI(SCOPh) in acetone. The molecular structure of [Ta(SCOPh)2(cp)2], determined by X-ray diffraction of a single crystal [space group P21212, a = 7.8458(22), b = 19.903(4), c = 7.028(3) Å, Z = 2, R = 0.031, R' = 0.037] comprises a normal, bent sandwich geometry with monodentate, S-bonded monothiobenzoate ligands. The angle S-Ta-S, 79.4(8)°, is relatively acute for a complex of type [MX2(cp)2] with a d1 electronic configuration. Comparative cyclic voltammetric studies have been performed for [MX2(cp)2] (M = Nb or Ta; X = Cl, Br, I, or SCOPh) in dichloromethane solutions. Oxidation to [MX2(cp)2]+ is essentially reversible for these systems and ease of oxidation decreases with ligand X in the order SCOPh > Cl > Br > I. Reduction is irreversible in dichloromethane at room temperature for the dihalide complexes, whereas a primary, reversible reduction of [M(SCOPh)2(cp)2] may be attributed to the formation of the respective monoanions. All complexes [MX2(cp)2] have been investigated by e.s.r. spectroscopy in solutions and in frozen glasses of 2-methyltetrahydrofuran-dichloromethane. Solution spectra yield isotropic parameters gisoand Aiso(M) (M = 93Nb or 181Ta) and the variation of these parameters with the nature of X in [MX2(cp)2] is discussed: values of gisoor Aiso(M) increase or decrease, respectively, with change of X in the sequence Cl, Br, I. Glass spectra of [MX2(cp)2] have been analysed in terms of the anisotropic e.s.r. parameters.
|Number of pages||9|
|Journal||Journal of the Chemical Society, Dalton Transactions|
|Publication status||Published - 1990|