Electron paramagnetic resonance spectra of some oxovanadium(IV) chelates

Abstract

X-Band e.p.r. spectra of magnetically dilute glasses at 77 K show that ethanol and pyridine form weak complexes with vanadyl chelates and that vanadium(IV) in solutions of these chelates is readily oxidised to vanadium(V). A method which enables spin Hamiltonian parameters to be extracted from systems which have C_2v symmetry is described, and the magnetic and optical properties of some five- and six-membered ring chelates of the VO²⁺ ion, and of their complexes with ethanol and with pyridine, are listed. The principal values of the hyperfine tensor components and the isotropic contribution to hyperfine coupling can be used to distinguish five-from six-membered ring chelates. Spin Hamiltonian parameters are equated to the atomic orbital coefficients in some of the molecular orbitals involved in bonding in these chelates. The weak C_2v component of the ligand field mixes the vanadium 3d_x²–y², 3d_z², and 4s orbitals and the magnetic resonance data is used to estimate the extent of this mixing. Mixing 3d_z² character into the orbital containing the unpaired electron accounts for ‘in-plane anisotropy’ in the g-tensor components. Mixing 4s character accounts for characteristic differences between hyperfine tensor components observed for five- and six-membered ring chelates and for the changes which occur in the magnetic properties of these compounds when additional complexing with solvent molecules takes place.