Electronic absorption spectra of some oxovanadium(IV) compounds

Abstract

Single-crystal polarised electronic absorption spectra at 298 and 5 K, in the range 5 000–30 000 cm^–1, of [VOX₂(tmu)₂](tmu = tetramethylurea, X = Cl or Br), [NEt₄]₂[VOCl₄], and [AsPh₄]₂[VO(NCS)₄], and similar spectra of poly(dimethylsiloxane) mulls of [VOCl₂(tmtu)₂], [VOCl₂(tppo)₂], [VOCl₂(hmpa)₂], K₃[VO(CN)₅], [NH₄]₃[VOF₅], and [NMe₄]₂[VO(NCS)₄(OH₂)](tmtu = tetramethylthiourea, tppo = triphenylphosphine oxide, and hmpa = hexamethylphosphoramide) are reported. The absorption bands have been assigned based on a combination of their observed polarisation, relative intensities, vibrational fine structure, and changes in positions with changes in the in-plane ligands. For the five-co-ordinate complexes all of the expected d–d type transitions occur below 30 000 cm^–1, the electronic bands at 5 K for (3d_xy*)→(3d_xz*,3d_yx*) being in the range 12 860–15 100 cm^–1, for (3d_xy*)→(3d_z²*) in the range 21 000–22 000 cm^–1, whilst the (3d_xy*)→(3d_x²–y²*) transition varies in energy as expected from the changes in the in-place ligands. For the six-co-ordinate complexes only the (3d_xy*)→(3d_xz*, 3d_yz*) and (3d_xy*)→(3d_x²y²*) transitions are observed below 30 000 cm^–1. It is suggested that in these latter complexes the (3d_xy*)→(3d_z²*) transition lies above 30 000 cm^–1 its energy being increased due to an increase in the ligand-field strength along the molecular z axis.