Six- and eight-coordinate thio- and seleno-ether complexes of NbF₅ and some comparisons with NbCl₅ and NbBr₅ adducts

The reaction of RS(CH₂)₂SR (R = Me, Et or ⁱPr) with NbF₅ produces [NbF₄{RS(CH₂)₂SR}₂][NbF₆] which contain distorted eight-coordinate (dodecahedral) cations and octahedral anions, whereas RSe(CH₂)₂SeR (R = Me or Buⁿ) form six-coordinate [(NbF₅)₂(μ-RSe(CH₂)₂SeR)]. Et₂S and Me₂Se (L) also form six-coordinate [NbF₅(L)], but Me₂S forms both [NbF₅(Me₂S)] and an eight-coordinate cation in [NbF₄(Me₂S)₄][NbF₆]. MeS(CH₂)₂SMe forms eight-coordinate cations in [NbX₄{MeS(CH₂)₂SMe}₂][NbX₆] (X = Cl or Br), but other complexes of the heavier halides including [NbX₅(L)] and [(NbX₅)₂(μ-L–L)] (L–L = RSe(CH₂)₂SeR; o-C₆H₄(CH₂SMe)₂ and o-C₆H₄(CH₂SeMe)₂) contain six-coordinate niobium. The very unstable [NbCl₅(Me₂Te)] was characterised spectroscopically, but all other attempts to form telluroether complexes resulted in decomposition, and NbI₅ was reduced even by thioethers. The complexes have been characterised by multinuclear NMR (¹H, ¹⁹F, ⁹³Nb, ⁷⁷Se or ¹²⁵Te), IR and UV/visible spectroscopy, and X-ray crystal structures are reported for [NbF₄{RS(CH₂)₂SR}₂][NbF₆] (R = Me, ⁱPr), [NbF₄(Me₂S)₄][NbF₆], [NbCl₅(Me₂Se)], [NbBr₅(Me₂S)], [(NbCl₅)₂{o-C₆H₄(CH₂SMe)₂}] and [(NbCl₅)₂{MeSe(CH₂)₂SeMe}]. All the complexes are very moisture sensitive and the fluoride complexes decompose slowly with fluorination of the neutral ligand.