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

The reaction of RS(CH2)(2)SR (R = Me, Et or Pr-i) with NbF5 produces [NbF4{RS(CH2)(2)SR}(2)][NbF6] which contain distorted eight-coordinate (dodecahedral) cations and octahedral anions, whereas RSe(CH2)(2)SeR (R = Me or Bu-n) form six-coordinate [(NbF5)(2)(mu-RSe(CH2)(2)SeR)]. 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)(2)SMe forms eight-coordinate cations in [NbX4{MeS(CH2)(2)SMe}(2)][NbX6] (X = Cl or Br), but other complexes of the heavier halides including [NbX5(L)] and [(NbX5)(2)(mu-L-L)] (L-L = RSe(CH2)(2)SeR; 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 (H-1, F-19, Nb-93, Se-77 or Te-125), IR and UV/visible spectroscopy, and X-ray crystal structures are reported for [NbF4{RS(CH2)(2)SR}(2)][NbF6] (R= Me, Pr-i), [NbF4(Me2S)(4)][NbF6], [NbCl5(Me2Se)], [NbBr5(Me2S)], [(NbCl5)(2){o-C6H4(CH2SMe)(2)}] and [(NbCl5)(2){MeSe(CH2)(2)SeMe}]. All the complexes are very moisture sensitive and the fluoride complexes decompose slowly with fluorination of the neutral ligand.