Magnetic circular dichroism spectroscopy of the vanadyl ion

Abstract

Magnetic circular dichroism (m.c.d.) spectra of a series of vanadyl complexes with tetragonal symmetry have been measured in order to confirm the assignment of the d–d spectrum and to provide an estimate of the angular momentum of the excited ²E state. The spectra of the ions [VO(H₂O)₅]²⁺ and [VO(NCS)₅]^3– show the presence of the expected A term with the correct sign in their lowest-energy band, thus confirming the original assignment of Ballhausen and Gray. At low temperature the spectrum of the species [VO(H₂O)₅]²⁺ is dominated by C terms whose signs under the lowest-energy band again support the assignment of the lowest-energy band to the transition ²B₂→²E; in addition a C term is found under the shoulder on the higher-energy side of this band showing that the transition is ²B₂→²B₁, which obtains its intensity through second-order spin–orbit coupling to the ²E state. An estimate is made of the orbital-reduction factor in the excited ²E state and some molecular-orbital (MO) calculations are reported in an attempt to account for the reduction. The results suggest that previous MO treatments of the vanadyl ion have overemphasized π-bonding to the axial oxygen atom and neglected π-bonding to the equatorial ligands in the antibonding orbitals. Low-temperature m.c.d. spectra of the vanadyl ion doped into K₃TlCl₆,2H₂O are also reported.