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The niobium(III) molecular derivatives Nb 2 Cl 4 (acac) 2 (acacH) 2 (PhPMe 2 ) 2 (acacH = acetylacetone), Nb 2 Cl 5 (dpm)(PhPMe 2 ) 2 and Nb 2 Cl 3 (dpm) 3 (dpmH = dipivaloylmethane) were isolated in high yields by reacting the corresponding β-diketone with Nb 2 Cl 6 (PhPMe 2 ) 4 or Nb 2 Cl 6 (SMe 2 ) 3 . These products were characterized by elemental analysis, mass spectra, infra-red and 1 H and 31 P NMR spectroscopy. Unlike dipivaloylmethanato derivatives, extensive dissociation reactions were observed for Nb 2 Cl 4 (acac) 2 (acacH) 2 (PhPMe 2 ) 2 in solution, which therefore contains mainly Nb 2 Cl 4 (acac) 2 (PhPMe 2 ) 2 species.

The reduction of niobium(V) chloride by magnesium in CH 2 Cl 2 and in the presence of various ethers and sulfides has been investigated. New niobium(III) species - molecular: Nb 2 Cl 6 (p-dioxane) 2 , [Nbcl 2 (OC 6 H 4 CHO)(THF) 2 ] 2 and ‘Nb 2 Cl 6 (THF 4 ’; and cluster: Nb 3 Cl 8 (SMe 2 ) 2 (Et 2 ) - have been isolated in good yields. The access to a previously reported niobiun(III) molecular compound, Nb 2 Cl 6 (SMe 2 ) 3 , has been improved. All these compounds were, characterized by elemental analysis, infrared and proton NMR spectroscopy. Dimeric species, with formally a double bond between the metallic centers, account for the diamagnetism of all the molecular compounds.

The coordination ability of about 40 potential monodentate and bidentate ligands with niobium and tantalum pentamethoxides has been investigated and is discussed.

The niobium(V) oxyyalkoxide derivatives [NbO(OCH2CF3)3 · MeCN]2, NbO(OCH2CF3)2(OCHCF2) (CF3CH2OH)0.5, [NbO(OEt)3]2, [NbOCl2(OCH3)· CH3OH]2 and NbO(OCH3)(OC6H4CHO)2 have been obtained by direct alkoxylation of niobium oxychloride, and characterised. The molecular constitution of the solutions of [NbO(OEt)3]2 and [NbO(OCH2CF3)3· MeCN]2 in non-polar solvents was tentatively interpreted as a dynamic equilibrium between various μ-alkoxo and μ-oxo isomers, the latter being favoured by dilution. In polar media, dimeric unit are retained for [NbO(OEt)3]2, while the triflouroethoxide derivative is split into monomeric species.

The niobium(V) and tantalum(V) oxyalkoxide derivatives NbO(OMe)2(OC6H4CHO), NbO(OMe)2[HB(pz)3] (pz = C3H3N2), NbOCl(OCH2CCl3)2· 2DMSO and TaOCl2(OMe)·2DMSO have been obtained from the pentaalkoxides or from the alkoxychlorides and characterized by microanalysis, molecular weight measurements, IR and NMR spectroscopy. The structure in solution of these oxo derivatives — all monomeric — which was mainly investigated through low-temperature 1H NMR studies, is discussed.

The ability of various crown-ethers L (dibenzo-18-crown-6, 18-crown-6 and 15-crown-5) to coordinate and reduce niobium and tantalum pentachlorides has been explored. Compounds of different stoichiometries, such as (NbCl5)2·L (L = DBC, 18-CRW-6, 15-CRW-5), NbCl5·L (L = DBC, 18-CRW-6), (NbCl4)2(15-CRW-5), (NbCl4)3·L2 (L = 18-CRW-6, 15-CRW-5) and TaCl5)2(DBC); TaCl5(15-CRW-5), (TaCl4)2(18-CRW-6), were isolated and characterized by elemental analysis, magnetic susceptibility measurements, infra-red and nmr spectroscopy. The proton nmr data were tentatively interpreted to mean that disproportionation reactions, leading to mono and polynuclear species having various ligand to metal ratios, occur in solution.